Jump to content
33andrain

Message added by 33andrain

Recommended Posts

9 hours ago, Yaakov said:

 

The interesting thing that I have found in my research, is that significant (cat 2+) Long Island/New England hurricanes have only occurred in hurricane seasons with near to below normal activity.  These include 1869, 1938, 1944, 1954 (Carol, Edna), 1960 (Donna), 1985 (Gloria), 1991 (Bob).

 

Another important aspect is that the Jun water temperatures in those years featured slightly below normal SSTs in the eastern/central tropical Atlantic, and above normal SSTs off the US East Coast.  The Aug/Sep water temperatures were similar, but the above normal SSTs off the East Coast were weaker.  See attached SST composites.

 

Furthermore, I created an index of SST in the subtropical western Atlantic (from 22.5 to 42.5N and 80 to 50W):
https://www.esrl.noaa.gov/psd/cgi-bin/data/timeseries/timeseries.pl?ntype=4&lat1=23&lat2=42.5&lon1=280&lon2=310&iseas=0&mon1=4&mon2=5&iarea=0&typeout=1&Submit=Create+Timeseries

 

In this region, the SST anomaly this May/Jun is the third highest since 1948, behind only 2016 and 2012.  Warm SSTs in the subtropical western Atlantic in May/Jun, and even more so in Aug/Sep/Oct, are correlated with increased hurricane strikes along the east coast from GA northward to New England.  This is shown in the chart below, which I annotated the names of east coast hurricanes.

 

 

SNIPPED (charts not copied to save space)

 

Yaakov, this is really fascinating and some excellent work that you've done. I see that you are a professional meteorologist in Israel - while I'm just a keen life time amateur (over 55 years of weather interests and UK based). Just to be clear - I assume that you used those NOAA figures to create your SST index or do you do work for NOAA?   It will be interesting to see a parallel set of figures for the eastern tropical Atlantic. Your SST charts do show that to some extent with the colder values in the east. Then we need to build in the phases of the AMO.  The last -ve phase (and previous -ve phases as far as is known) was associated with generally weaker Atlantic hurricane seasons.  However, last time around, relative SSTs were probably not as high in the west Atlantic as they are now (some sst warming probably due at least in part to climate change) while the temperature of the deep ocean currents that upwell during AMO phases have probably changed little and are very cold for tropical waters - and probably colder than the coldest upwellings in the Pacific during ENSO phases.  I will be placing more AMO papers into the Research Portal during the next week or so and then I'll review several of them.  Some of the research on AMO impacts has been partly done. We now have much more sophisticated observational and measuring equipment and we should learn a lot more about the AMO during the next few years as we move further into the cold phase. 

 

For me, although I like extreme weather like many on this forum, I'm fascinated by all the causes and how all the drivers and different factors interact.  This is going to be a really unusual hurricane season and will also be a big learning curve for all of us.  Beryl was an extraordinary feature proving that very small hurricanes can be just as interesting as major ones. David :) 

  • Like 3

Share this post


Link to post
Share on other sites

Yakov’s post is verification that the actual NUMBER of storms means absolutely nothing in the end, it is storm HITS that count. Look at Andrew in Aug 1992 and the destruction he caused, that was a well below normal season. 

  • Like 2

Share this post


Link to post
Share on other sites
8 hours ago, Andrew Maddis said:

Yakov’s post is verification that the actual NUMBER of storms means absolutely nothing in the end, it is storm HITS that count. Look at Andrew in Aug 1992 and the destruction he caused, that was a well below normal season. 

Ala Sandy as well. An 1821 or 1815 "gale"track would be a damaging storm/devastating  to the east coast from OBX up to RI/NE.

Share this post


Link to post
Share on other sites

THE RETURN OF BERYL!

 

I've already said in several recent posts how extremely unusual "Beryl" has been and that several papers may be written on her life cycle. Well she's back and I think that she deserves a whole book in the hurricane archives!  Here's the latest from NOAA:

 

5 Day Outlook:

b1.PNG

 

NOAA advisory:

"Tropical Weather Outlook NWS National Hurricane Center Miami FL 200 PM EDT Sat Jul 14 2018 For the North Atlantic...Caribbean Sea and the Gulf of Mexico: The National Hurricane Center is issuing advisories on recently re-developed Subtropical Storm Beryl, located about 290 miles north of Bermuda. Tropical cyclone formation is not expected for the next five days. && Public Advisories on Subtropical Storm Beryl are issued under WMO header WTNT32 KNHC and under AWIPS header MIATCPAT2. Forecast/Advisories on Subtropical Storm Beryl are issued under WMO header WTNT22 KNHC and under AWIPS header MIATCMAT2. $$ Forecaster Beven".

 

Satellite Image of Beryl Steadily Weakening Again During Today:

goes16_ir_02L.gif

 

Let's just go through Beryl's history (please refer to earlier posts for full details): 

  • She formed as a disturbance coming out of Africa and was carried across the Atlantic on an African Easterly Wave
  • Conditions were initially not particularly conducive with very marginal SSTs and dry air intrusion from the north inhibiting development
  • Beryl managed to develop into a tiny tropical depression and strengthened into a tiny tropical storm some 500 to 1,000 miles east-south-east of the Lesser Antilles.
  • She strengthened rapidly into a category 1 hurricane but only maintained this intensity for a few hours.
  • She then weakened rapidly into a tropical storm and as she approached the south-eastern Caribbean islands she weakened further into a tropical depression.
  • She then dissipated "almost" completely into an area of disorganised showers - due to non conducive upper conditions with adverse wind shear and dry air intrusion.
  • It is almost unheard of for hurricanes at that latitude, on an west-north-westerly track to weaken so rapidly "prior" to reaching landfall.
  • Despite higher SSTs towards the Caribbean upper conditions remained marginal for a few days but NOAA suggested that conditions would improve as the remnants moved north
  • This patch of disorganised showers continued to track northwards past the Bahamas and west of Bermuda encountering higher SSTs and improving upper conditions.
  • Beryl reformed as a "subtropical storm" earlier today but is not expected to reach tropical cyclone strengthen again and is already weakening into a subtropical depression (within 24 hours) as she tracks north-east
  • Beryl was one of the smallest tropical storms/hurricanes "by area" on record
  • Although hurricanes can weaken considerably and then re-intensify as surface and upper conditions vary, especially making landfall before moving out to sea again, it is extremely unusual for almost complete dissipation over a few days and then to reform again. In fact, it's more a case of a "new" system forming out of the remnants, although NOAA seem to prefer to retain the same name.

 

What Next?

As I suggested in several earlier posts, this is likely to be a very unusual hurricane season.  We have below average SSTs over the tropical eastern Atlantic but rising SSTs in the Caribbean and above or well above average SSTs in the western tropical Atlantic, the Gulf of Mexico and particularly off the eastern and south-eastern US seaboard.  The vast majority of major hurricanes (category 3, 4 and 5) develop on disturbances moving out of Africa and are transported on African Easterly Waves.  With SSTs being mostly below the critical point, how many of these disturbances this season will manage to develop into mature hurricanes? Although Beryl was a highly unusual feature might this be a sign of these "difficult" surface conditions?

 

Of course we need to look at the upper conditions which are at least as important.  There are some small changes going on in terms of upper winds and moisture.  I posted 2 GFS forecast charts last week which I now show as animations using the "gif" option:

gfs_mslp_pcpn_atl_fh6-120 (1).gif

As these are forecast charts, I have taken the period up to T+120 for 5 days as this is in the more reliable period and coincides with the NOAA 5 day hurricane advisories.  The main belt of high pressure in the central Atlantic changes very little.  There are several  areas of low pressure forming in west Africa and disturbances moving westwards but weakening as they move into the tropical Atlantic. We can track "Beryl" moving steadily north-north-east to Newfoundland but unlike Chris she weakens and becomes a minor feature rather than a mid-Atlantic depression.. 

gfs_midRH_atl_fh6-120 (1).gif 

There is still a considerable amount of dry air pushing around the eastern and southern flanks of the Atlantic high pressure. This appears to mix with the disturbances moving out of Africa and probably makes the upper conditions non conducive to any storm developments on African easterly waves for the forecast period.  Dry and moist air alternate coming out of the eastern US and running south-eastwards on several troughs and that area east of Florida would seem to be rather more conducive to storm develop should one move into that region (not currently predicted to) and with those well above average SSTs this area may well remain fairly "storm friendly" for a few weeks. It will be interesting to see when the next African disturbance makes it across the Atlantic and if it manages to develop into something more significant than Beryl. I still feel that very few will manage it this season for the reasons I stated in my last few posts. Perhaps a very slow moving system can develop around the Caribbean when upper conditions are much more conducive than now and then take advantage of the higher SSTs there and further north.  

 

East Pacific:

There is an area of storms but NOAA feel only a low chance of cyclone develop and it's well out at sea and moving westwards. I won't show the chart but here's the latest advisory:

 

"Tropical Weather Outlook NWS National Hurricane Center Miami FL 1100 AM PDT Sat Jul 14 2018 For the eastern North Pacific...east of 140 degrees west longitude: 1. Showers and thunderstorms are persisting in association with a trough of low pressure located a little over 1500 miles west-southwest of the Baja California Peninsula. Little development of this system is expected today or tonight, but environmental conditions could become more conducive for development in a day or two while the system continues moving westward at around 10 mph. * Formation chance through 48 hours...low...10 percent. * Formation chance through 5 days...low...30 percent. Forecaster Zelinsky".

 

UPDATE: NOAA have just increased the chances of this east Pacific disturbance developing into a tropical depression as 60% in 5 days:

 

"Tropical Weather Outlook NWS National Hurricane Center Miami FL 1100 PM PDT Sat Jul 14 2018 For the eastern North Pacific...east of 140 degrees west longitude: 1. A low pressure system has developed more than 1500 miles east-southeast of the the Hawaiian Islands, and associated thunderstorm activity has increased and become a little better organized during the past several hours. Environmental conditions are expected to gradually become more conducive for development, and a tropical depression could form during the next couple of days before the system moves westward into the Central Pacific basin. * Formation chance through 48 hours...medium...40 percent. * Formation chance through 5 days...medium...60 percent. Forecaster Stewart"

 

I'll be back when there are some developments to report.  David :) 

 

  • Like 2

Share this post


Link to post
Share on other sites

MJO - LIKELY INFLUENCE ON HURRICANE SEASON THROUGH JULY 2018?

 

The current phase of the MJO is the latest teleconnection likely to be currently influencing this hurricane season. Research presented by Phil Klotzbach at the AMS 33rd Conference on “Hurricanes and Tropical Meteorology” held in Florida in April 2018 shows a strong correlation between the different phases of the MJO and hurricane activity:

 

The Madden-Julian oscillation (MJO) has been documented in a variety of prior studies to significantly impact Atlantic hurricane activity through modulations in atmospheric conditions including vertical wind shear, mid-level moisture, vertical motion and surface pressure.

 

We find that US hurricane landfalls are more likely in MJO phases 1-4 (MJO enhanced convection is centered over Africa, the Indian Ocean, or the western portion of the Maritime Continent) and is less likely when the MJO is in phases 5-8 (MJO enhanced convection is centered over the eastern portion of the Maritime Continent, the western Pacific or the Western Hemisphere).

 

A video of the conference presentation can be found in the Teleconnections Research Portal here. The following two charts from the presentation clearly show the impact on activity. (There are many other charts shown in this excellent presentation including maps of where storms form, subsequent landfall and damage caused by phase).         

 

Hurricanes by MJO Phase Klotzbach 2015.jpgHurricane_Cyclone Landfall by MJO Phase Klotzbach 2015.jpg

 

The latest Wheeler-Hendon phase plot forecasts from ECMF for 17th to 31st July shows MJO activity in the ‘lower activity’ Phases 5 to 6 over the coming 2 weeks.

 

MJO Phase Plot 15July to 31July ECMF.gif

 

And Michael Ventrice has just tweeted:

 

“After a swift start to the 2018 Atlantic Hurricane Season, things have quieted down. It's not coincidental that this lull in activity is timed with the passage of the convectively suppressed phase of the Madden Julian Oscillation across the Atlantic Basin.”

 

Ventrice tweet hurricane 17July2018.jpg

 

https://twitter.com/MJVentrice/status/1019258434397659139

  • Like 3

Share this post


Link to post
Share on other sites

WHY HAS THE ATLANTIC HURRICANE SEASON GONE SO QUIET?

 

In this post, I shall bring together some of the influential factors that I and others have already covered, elaborate on some of these, look at the current set up and review a fascinating highly relevant "hurricane" presentation and short video which I placed into the Teleconnections Research Portal yesterday.  As I said before, this is and is likely to continue to be, a particularly "unusual" hurricane season with many conflicting signals which will probably make using analogue years far less useful than in a more straight forward season.

 

The Season So far:   

It is still very early in the Atlantic hurricane season and it would be easy to point to an early start with three named storms before mid-July which briefly became category 1 hurricanes but I feel that this would be very misleading as I explain during this post.  Let's put things into context and have a look at the climatological norm for the number of named storms and hurricanes by season, by month and their origins and tracks.  The vast NOAA site is bursting with current and archived information which would almost take a lifetime to fully explore!  The "Tropical Cyclone Climatology" section is fascinating and can be found on this link:   https://www.nhc.noaa.gov/climo/   This covers data on Atlantic and east Pacific storms for the last 40 to 70 years - I'll focus on the Atlantic in this post. 

 

ah1.PNG

This table (up to 2009 - I'll look at the most recent years below this section) tells us that the average number of named storms for the season is 11, with 6 hurricanes and 2 major hurricanes.  The average date for the first named storm is 9th July, first hurricane is 10th August and first major hurricane is 4th September.  We know that there are huge variations from season to season with quieter and much more active seasons (or period of years). 

 

ah2.PNG

The next section breaks the season down into 10 day periods from 1st May to 30th November with maps showing all the hurricanes from 1851 to 2015 recorded during each period, their origin and track.  I just show one as an example, covering the next 10 days.  Going through the charts, you'll see that just 2 hurricanes formed during the first period of 1st to 10th May. Up until early July, you'll see that most Atlantic hurricanes have shorter tracks and tend to form around the Caribbean and the Gulf of Mexico (and these very rarely achieve major hurricane status - over 83% of those form on African easterly waves, especially in the second half of the season).

 

ah3.PNG     ah4.PNG

These maps show the prevailing hurricane tracks from June to November for each month.  The coloured areas relate to when the disturbance, tropical depression or tropical storm has reached at least category 1 hurricane strength and nicely illustrates the point I made above (more on this later on).

 

ah5.PNG

This table clearly shows a trend with a rather higher number of named storms in the last 60 years compared to previous 60 years but this still varies considerably from year to year. Interestingly, there is very little change in the number of category 1 and 2 hurricanes but there are more major hurricanes in the latter period. The reasons for these changes are the subject of much debate and some is highly political and disputed.  It may partly be due to climate change and global warming but other factors such as the AMO and AMOC mentioned by several members on this thread in earlier posts (and I plan to do a more focused post on these in due course) produce "naturally occurring" cyclical changes too.

 

For all the US members - the vast majority on this forum, this NOAA link also provides details and maps of "hurricane return periods", "CONUS hurricane strikes" and CONUS hurricane strike density" at cities and counties along the US coastline for similar time periods.

 

Just to complete the series, I now focus on the last three seasons. Another excellent source of data is provided by the UK Met Office.  This link provides separate data for all (global) hurricanes and cyclones for each year from 1988 to 2017:    https://www.metoffice.gov.uk/weather/tropicalcyclone/observations

 

2015 Atlantic Hurricane Season

ah6.PNG

2015 was an average in number but weaker in intensity as well as a long season with the first named storm as early as 8th May and the last one on 9th November.  Only Bill made US landfall in the Gulf of Mexico on a very short track having formed there. Erika hit the central Caribbean islands.

 

2016 Atlantic Hurricane Season

ah7.PNG

2016 was a fairly active hurricane season which saw hurricane Alex in January - an extremely rare event (just a couple in 150 years) and it made landfall in Greenland as a sub-tropical depression by then!  Many storms originating on African easterly waves  were quite well developed at an early stage of their crossing.  SSTs off Africa were much higher then compared to this season (but important other factors apply too - see later). 

 

2017 Atlantic Hurricane Season

ah8.PNG

As many will recall, 2017 was a very active hurricane season with 18 named storms, starting off will Arlene in April.  As usual, the majority of storms originated on African easterly waves with those long tracks producing the major hurricanes.

 

Returning to this season and the 3 early storms which reached category 1 hurricane strength very briefly,  two formed east/north-east of the Caribbean in that large pool of well above average SSTs between the Florida coast and Bermuda and stretching northwards, east of the Carolinas.  Beryl formed on an African easterly wave and was a very early example of one of these.  I've already posted in detail on the exceptionally unusual features of Beryl. 

 

Factors Likely to Influence the Remainder of the 2018 Hurricane Season:

1. Sea Surface Temperatures (SSTs)

SSTs are only one ingredient that influence storm formation, development, longevity and track. These have been covered extensively on here by a number of posters - so just a brief update now.  

ah9.PNG

The trend continues with those well below average SSTs in the east and above or well above average SSTs in the west.  The Caribbean which was still marginal several weeks ago is now almost entirely above the critical 27c level. The Gulf of Mexico is closer to 30c and that very warm pool of 28c+ continues off the south-eastern and eastern US coast. As the charts above on the previous seasons demonstrate, hardly any tropical storms and hurricanes that do develop in that area around or within 500 miles of Bermuda (such as hurricane Chris 2 weeks ago) make US landfall (there are just a few exceptions since 1850).  What is needed are much longer track storms such as those that develop on African easterly waves (which account for over half of all hurricanes and 83% of all major hurricanes - according to the very latest updated research). Some storms develop in the Caribbean and the Gulf of Mexico and these can make landfall in the US.  With the low SSTs in the east, disturbances that develop on the easterly waves are likely to struggle this season to make it into mature hurricanes. We have already seen an early exception with Beryl but she was a tiny system that intensified rapidly and only reached hurricane strength for a few hours prior to reaching more friendly SSTs but upper conditions were not conducive to further development and she dissipated into an area of disorganised showers in the eastern Caribbean.  That area of showers then moved northwards across those much higher SSTs and 5 days later Beryl re-formed (or a new tropical depression did out of Beryl's remnants) for several days but again without making landfall.  What this proves is that we might only need disturbances to make it across the Atlantic on the easterly waves and then they "might" develop after reaching much more favourable development conditions (depending on other factors - see below). 

 

2. El Nino Southern Oscillation (ENSO):

I have provided a number of updates on ENSO during the last few months on the Teleconnections thread and also on here. Malcolm (@Blessed Weather) posted a short ENSO update on the Teleconnections thread yesterday (at the bottom of page 3). The trend from La Nina to ENSO neutral conditions continues (-0.5c to +0.5c SST anomalies). We are moving towards weak El Nino conditions which NOAA give a 65% chance of developing during the fall and a 70% chance of continuing during winter 2018/19.  Malcolm referred to a Philip Klotzbach tweet in his post which suggested that El Nino type behaviour was already starting to exert itself.  This produces less active Atlantic hurricane seasons and more active east Pacific seasons - the opposite influence to La Nina conditions.  I agree with Phil and this is "one" of the important general background influences. 

 

Just one chart from the most recent NOAA weekly report:

ah10.PNG

If anything, the values are marginally lower than or very similar to 2 weeks ago. Overall, SSTs are slightly positive (except in the extreme east).  The changes are rarely in a straight line and I'm pretty sure that this is a temporary blip. The reason for my confidence is that sub surface temperature anomalies in the top 300 metres are much more positive (+0.5c to +1c) and are continuing to upwell.  The full NOAA report can be found here:   http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf

 

3. The Madden-Julian Oscillation (MJO), the Atlantic Multi-decadal Oscillation (AMO) and the Atlantic Meridional Overturning Circulation (AMOC)

I have been placing many more "hurricane related" papers and short conference video presentations into the Research Portal during the last few weeks. There were several on the strong influence of the MJO on the Atlantic hurricane season and Malcolm posted on this thread on Wednesday (just above this one) with his comments and a link to one of these excellent papers. This is a shorter term influence associated with the different phases of the MJO and how amplified it is. Indeed in the Michael Ventrice tweet that Malcolm refers to he suggests that this is a key reason why we have this current lull in activity. Backed up by the paper, this is undoubtedly "one" of the reasons but I feel that there are other important factors in play too.

 

Malcolm and I in recent posts (and several other members in earlier posts on this thread) have already commented on the AMO and the AMOC - so no more this time but I will review a couple of excellent very recent papers on this topic in due course. Changes in the AMOC (also referred to as the Gulf Stream which part of it) are actually triggered by deep currents in the southern oceans around Antarctica - if no one else does, I will produce a full post on this on the Teleconnections thread before long.

 

4. Wind Shear, Dry Air Intrusion, Saharan Dust, Mid-tropospheric Humidity and More on African Easterly Waves

The upper and mid level conditions are even more influential than the surface conditions and also how they all interact.  The NOAA "Tropical Weather Outlook Texts" always refer to whether the upper conditions are conducive or not to tropical storm develop and reference various factors.

 

Vertical wind shear with upper and/or mid level winds blowing in an opposite or different direction to surface winds can have significant impacts on storms and disturbances. A well developed system can literally hit the buffers when it encounters adverse wind conditions. Opposing forces can rip a storm apart and cause it to dissipate rapidly. On the other hand, a degree of favourable wind shear can spark off storm intensification. It's a highly complex subject and I'll be placing more papers on this in the portal shortly (and review several on here in due course).

 

Dry air intrusion occurs when air sourced outside of a storm system is sucked into its circulation. Most storms can sustain a degree of drier air but when they encounter major regions of much drier air (at various levels) this usually effects convective activity, reduces precipitation and weakens the storm.  As African easterly waves are generated with disturbances moving out of equatorial Africa, they often mix with drier air moving out westwards from sub-tropical Africa further north. The "Saharan Air layer" is often a significant contributor to this dry air intrusion.  Furthermore, Saharan sandstorms can whip up vast quantities of dust into the lower and middle tropospheric layers. Under the right synoptic conditions this dust can be carried for hundreds or even thousands miles away from their source region. Sometimes this dust can cross the Atlantic. It can be sucked into the circulation of storms and disturbances with dramatic impacts. A limited amount of dust can sometimes assist storm convective development by providing the nuclei for raindrops to form on.  Larger amounts of dust can be very destructive and dry out the moisture layers but with the fastest growing African easterly waves, the dust can increase the growth rate substantially with powerful barotropic and baroclinic energy conversions.  It's far more complicated than this and the interactions of different levels of dust in adjacent layers have been the focus for many papers, studies and model simulations increasingly over the last 20 to 30 years.  I've placed many papers in the portal covering all these influences and impacts (with more to follow) and I will review a few of these in due course.  This involves a lot of reading and work and I'd be delighted if a few other forum members get involved with their comments and reviews - there is so much to cover and get your teeth into. :) 

 

With or without dry air and/or dust intrusion tropical storm development is very dependent on the amount of moisture within its circulation, especially towards the storm centre and within a radius of 200 km and are particularly sensitive to "mid-troposhperic humidity". I just viewed an excellent conference presentation video entitled A satellite analysis of mid-tropopsheric humidity and its impact on tropical cyclone genesis and intensification - Presentation (click on the title for a link to the Research Portal entry and a direct link to the video).  Here's the abstract:

 

 "We use microwave satellite observations of mid-tropospheric humidity (MTH) to examine the relationship between environmental humidity and tropical cyclone (TC) activity in the North Atlantic basin over the period 2002�2013. Our analysis reveals spatially coherent changes in MTH over the tropical North Atlantic that are highly correlated with variations in accumulated cyclone energy (ACE). Fewer storms are generated in dry years and those that do form tend to originate outside of the driest regions. In contrast, genesis locations in humid years are more evenly distributed across the Atlantic main development region (MDR) with a substantially higher percentage of storms developing into hurricanes. By compositing MTH measurements as a function of future storm intensity change, we show that Atlantic Easterly Waves (AEWs) with higher MTH environments are more likely to develop into tropical depressions and that this signal is evident at least 24 hours prior to genesis. Similarly, TCs of all categories are more likely to intensify (weaken) over the subsequent 24 hour period when the mid-troposphere is more (less) humid. We also demonstrate that this signal is primarily evident near the storm center rather than in the larger scale environment, emphasizing the importance of microwave measurements for its detection and quantification. The MTH signal is also shown to be temporally and spatially distinct from microwave-based retrievals of total precipitable water (TPW) which underscores the distinction between low-level and mid-level water vapor. The potential applications of these measurements for real-time forecasting and model evaluation will be highlighted."

 

Although a small part of this presentation includes mathematical equations (which I always find hard to follow) most of it is a fairly simple explanation of all the slides and charts used. The researchers use satellite imagery to measure the amount of water vapour and the relative humidity within and adjacent to the storm circulation.  They tested the accuracy with very satisfactory results. They queried whether more storms are generated due to the prevailing high humidity or whether higher humidity is generated by the storms.  They resolved this "chicken and egg dilemma" and found it was the former.  Here is a small selection of the charts shown with my brief comments below each one.

ah11.PNG

They produce this type of chart from satellite imagery. The blue colours represent high humidity, thru green to yellow, orange and red representing much lower humidity. 

 

ah12.PNG

They compared the dry and inactive season of 2013 (top chart) to the very humid and active season of 2005 (middle chart).  Note that the blue colours represent dry air with orange, brown and red representing increasingly moist air.  That vast blue area in the Atlantic from 10N northwards is very similar to the conditions that we are seeing right now (more after this review). Lower humidity produces both less intense storms as well as fewer disturbances that can develop. 

 

ah13.PNG

This charts shows the amount of humidity for 6, 12, 18 and 24 hours "prior" to storm intensification in the cases of waves through to hurricanes. You can see what a useful tool this is for forecasters - especially to gauge rapid intensification (or weakening as in the next chart).

 

ah14.PNG

A similar chart but this time "prior" to a wave or storm weakening.

 

The researchers have been refining their methods and computations and plan a major paper very soon. They do seem to have found a strong correlation between relative humidity and storm development, in terms of numbers, intensification (moister air) and weakening (drier air).  In several recent posts on here, I have been showing the mid-tropospheric moisture charts for the 400mb to 700mb level (the researchers use 400mb to 800mb) so let's have a look at the current chart. 

 

ah15.PNG

Firstly, please do not be confused by the colours which are in reverse to those shown in the researcher's tables above. Here, the blue colours represent the moister air and brown colours the drier air.  The drier air has, if anything, intensified/expanded over the tropical Atlantic in recent days. There is some moist air over equatorial Africa (as there should be in mid-summer) but there is a lot of dry air over Africa north of 10N and also from 30W westwards. Until or unless this changes significantly, this is another very strong reason for the lull in Atlantic storm activity. Of course, many of the factors I have covered above, interplay with each other.  The ENSO state (weak El Nino starting to develop) and the below average SSTs over the eastern tropical Atlantic seem likely that they will have a negative impact on Atlantic hurricane formation all season.  The MJO may enter a more positive phase within a few weeks and the expanse of dry air may diminish.  Meanwhile, SSTs in the western tropical Atlantic are likely to rise slightly further and this is the most positive of all the factors but may not be enough on its own.

 

The east Pacific, on the other hand, is seeing an active hurricane season with numerous disturbances and storms developing.  There are currently 3 disturbances in the 5 day frame, one of which seems likely to develop into another named storm.  2 more from earlier in the week have now moved into the central Pacific.  I shall have a closer look at the east Pacific in my next post, especially if the lull in Atlantic developments continues as seems likely at least for the remainder of July.

 

  • Like 3

Share this post


Link to post
Share on other sites

Excellent tweet from Ben Noll:

 

"The late-July SST anomaly pattern in the tropical Atlantic during 2018 is the complete opposite of 2005, which was the most active Atlantic hurricane season on record. The difference is striking."

 

July 2005:

SST Anom 26July2005.jpg

July 2018:

SST Anom 26July2018.jpg

 

https://twitter.com/BenNollWeather/status/1023188777198530560

  • Like 3

Share this post


Link to post
Share on other sites

A SHORT UPDATE + A LITTLE MORE ON SEA SURFACE TEMPERATURES AND A REVIEW OF A RELEVANT PAPER

 

In my last post I looked into some of the reasons why the 2018 Atlantic hurricane season has gone so quiet. Malcolm's (@Blessed Weather) post above helps explain one of the reasons - the negative sea surface temperature (SST) anomalies in the tropical eastern Atlantic. Let's look at the latest chart.

h29a.PNG

This is little changed over the last few days with sub 26c SSTs over most of the east and 28c + limited to the Caribbean, the Gulf of Mexico (with some 30c +) and that area centred around Bermuda extending east and north-eastwards from Florida and the Carolinas.

h29b.PNG

The 7 day anomaly chart will be updated by Tuesday morning but last week's chart shows the SSTs continuing to fall in the east and levelling off in most other "key" development areas.

 

There has been a debate raging for a number of years as to what is the critical level for SSTs for enhancing or inhibiting cyclogenesis. There are a number of papers in the Research Portal on this subject.  I found a fascinating one recently (published in 2014) which I placed into the portal yesterday.  This is entitled:  Revisiting the 26.5°C Sea Surface Temperature Threshold for Tropical Cyclone Development  (just click on the title for a link to the portal where you'll find the abstract plus links to the full paper and a supplement). I show the abstract here:

 

"A high sea surface temperature is generally accepted to be one of the necessary ingredients for tropical cyclone development, indicative of the potential for surface heat and moisture fluxes capable of fueling a self-sustaining circulation. Although the minimum 26.5°C threshold for tropical cyclogenesis has become a mainstay in research and education, the fact that a nonnegligible fraction of storm formation events (about 5%) occur over cooler waters casts some doubt on the robustness of this estimate. Tropical cyclogenesis over subthreshold sea surface temperatures is associated with low tropopause heights, indicative of the presence of a cold trough aloft. To focus on this type of development environment, the applicability of the 26.5°C threshold is investigated for tropical transitions from baroclinic precursor disturbances in all basins between 1989 and 2013. Although the threshold performs well in the majority of cases without appreciable environmental baroclinicity, the potential for development is underestimated by up to 27% for systems undergoing tropical transition. An alternative criterion of a maximum 22.5°C difference between the tropopause-level and 850-hPa equivalent potential temperatures (defined as the coupling index) is proposed for this class of development. When combined with the standard 26.5°C sea surface temperature threshold for precursor-free environments, error rates are reduced to 3%–6% for all development types. The addition of this physically relevant representation of the deep-tropospheric state to the ingredients-based conceptual model for tropical cyclogenesis improves the representation of the important tropical transition-based subset of development events."

 

This paper studies all worldwide tropical cyclones (1,757) from 1989 to 2013 and shows that in the vast majority of cases the critical SST threshold is indeed around the 26.5c to 27c mark but that there are some exceptions. Several other studies have shown that SSTs of 25.5c or lower can still be conducive to development but further research found that if the disturbance passes over sub 26.5c SSTs for 2 or more days then this will suppress development. It is possible for a disturbance on an African easterly wave (AEW) to pass over SSTs below or well below the threshold as "Beryl" did last month in its early stages. Then, as the SSTs become more conducive, rapid development is possible later on. With the length of the track over favourable SSTs greatly reduced, the chances of a major hurricane developing are also reduced (remember recent studies show that 83% of major hurricanes develop on long tracks originating on AEWs). Please refer to my two earlier posts on hurricane Beryl for further details on that highly unusual tropical storm. 

 

The study finds that up to 27% of forecasts underestimate development when the threshold is met.  It shows that using another threshold of 22.5c for 850-hPa temperatures in combination with the SST threshold minimises the error rate. 

h29c.PNG

This table shows the error rates for using only SSTs (26.5c threshold), only the 850s (22.5c threshold) and then using the combined thresholds divided into non-TT(tropical transition), weak TT and strong TT events. The main errors occur with underestimations of a strong TT and shows how using the combined threshold greatly reduces this error rate. There is much more in this fascinating paper which is an "easy read" and well presented. I recommend it to all those interested, reading the full paper and the supplement.

 

Of course, as stated by myself and some other posters on this thread, there are many other factors involved.  I will not repeat them all again now but let's have a look at the latest upper level humidity chart which is such an important factor.

h29d.PNG  

The pool of dry air from the north has expanded even further and almost all the tropical Atlantic from around 10N to 40N is covered in very dry upper air. There are some slight signs that a larger patch of moist air is developing over a wider part off western Africa from around 5N to 25N.  While we should expect that up to at least 10N, there is additional moisture showing further north. We need to keep this monitored as renewed convective activity and disturbances may start developing again in that region. Whether there will be sufficient moisture en route across the tropical Atlantic is still far from certain. A certain amount of dry air (such as in the Saharan Air Layer) can mix in with the moist air layers and can be conducive to develop (and depending upon many other factors). Further west there is a channel of deeper moisture stretching from the Caribbean and northwards adjacent to the eastern US seaboarrd.  That does coincide with the above average SSTs there - so storm initiation may become more likely there.

 

Finally, let's finish with the MJO outlook. I placed a number of papers in the portal on the MJO/hurricane teleconnection and Malcolm (@Blessed Weather) posted on this and reviewed one of these papers on here quite recently. Remember that we are looking for the MJO in phases 1 to 4 with enhanced MJO convection over Africa, the Indian Ocean or the western maritime continent for a greater chance of US landfall hurricanes and not in phases 5-8 when development is far less likely.  We have spent the whole of July in the non conducive phases 5 and 6.

h29e.PNG  h29f.PNG

Looking at all the models, there is a lot of uncertainty as we move into the first half of August. This is highlighted by the big 2. NCEP/GEFS shows the MJO moving from phase 6 thru the circle of death (inactive) and re-emerging in phase 1 in 2 weeks time. Meanwhile, ECM holds the MJO in phases 6 and 7. So, another one to watch carefully. 

 

I might be able to report slightly better news next week but I feel that we may need to be patient to see if this season can come to life again.  David :)  

  • Like 2

Share this post


Link to post
Share on other sites
On 7/28/2018 at 11:22 AM, Blessed Weather said:

Excellent tweet from Ben Noll:

 

"The late-July SST anomaly pattern in the tropical Atlantic during 2018 is the complete opposite of 2005, which was the most active Atlantic hurricane season on record. The difference is striking."

 

July 2005:

SST Anom 26July2005.jpg

July 2018:

SST Anom 26July2018.jpg

 

https://twitter.com/BenNollWeather/status/1023188777198530560

Wow. 

Share this post


Link to post
Share on other sites

Hurricane Outlook, A Closer Look At The Very Active East Pacific Season Plus More Paper Reviews - Part 1: North Atlantic

 

In my last few posts on here, I have been stressing how unusual the 2018 hurricane season has been and examining the conflicting background signals. This time, I'll review the North Atlantic outlook in part 1 of this post before moving onto the East Pacific in part 2.  I will examine "cross basin storms" - ones which transition from the Atlantic to the Pacific and, very rarely, from the Pacific to the Atlantic. I'll review several fascinating papers on this particular topic.

 

North Atlantic:

Although we had some early season tropical storm activity with three category 1 hurricanes, the North Atlantic hurricane season has been really quiet for the last few weeks. I have been mentioning the well below average SSTs in the eastern tropical Atlantic which have been steadily falling and this was highlighted in Malcolm's (@Blessed Weather) tweet post yesterday.  This, plus the progress towards a weak El Nino, the MJO in an unfavourable phase and the vast expanse of dry air (see earlier posts on here for full details) are all inhibiting storm formation and development. There is still no sign of any significant storm development on an African easterly wave which is the most favourable formation zone for long track storms and 83% of all major hurricanes.  So, what do have here?

hur 4.8a.PNG

hur 4.8b.PNG

A non tropical depression which is moving southwestwards closer to that area of well above SSTs as shown below.

h29a.PNG

 This storm may well be a "slow burner" and we need to keep an eye on it. On its current track, in 5 days it will have reached the critical 26.5c SSTs. If it carries on moving southwest or westwards it will find even more conducive SSTs (but there are other key factors - see later). If you refer back to my (and several other) earlier posts on this thread which show storm tracks, a storm forming in the mid Atlantic and moving south-westwards is almost unheard of (not to be confused with tropical storms or hurricanes which can often end up there towards the end of their life cycle).  Might we be on the verge of history being made?  It would fit into this highly unusual set up! 

hur 4.8e.PNG

The weekly anomalies are updated each Monday/Tuesday.  Those even lower SSTs off Africa in the key 10N to 20N region show up clearly.  Most other areas have levelled off.  The well above average SSTs off Florida have fallen back very slightly but further north and east, they have increased further and perhaps that explains why that disturbance "may" develop during the next week or so. Much will depend on available moisture and other upper conditions, so let's look at the latest mid level humidity chart.

hur 4.8h.PNG

That Atlantic depression shows up clearly at around 38N and its normal northeasterly track is being cut off by the Azores HP which has shifted towards the UK and further HP to its northwest and west. Although there is much dry air on its western, northern and eastern flanks, there is a decent patch of much moister air to its south. Right now there is further dry air well ahead to the south-west which may need to change within the next week or so to allow additional storm development.  So, while a really fascinating feature, conditions may be just too marginal going forward.  Elsewhere, although there is still much dry area across the tropical North Atlantic, it has eased slightly over the key 10N to 20N region and there are renewed signs of disturbances forming over Africa with greater moisture over the easterly wave track just west of Africa.  Still marginal but something of an improvement on the last month. 

 

Part of the reason for all the unusual conditions throughout the mid latitudes with vast areas of exceptional heat has been the weak and meandering jet stream, often taking a particularly northerly route.  Here's the current north hemisphere chart with the North Atlantic view.

hur 4.8i.PNG

That "reverse" breakaway spiral to the south (in the mid Atlantic) has the circulation there moving from northeast to southwest and marks the northern edge of this depression.

 

Overall, still little sign of any major hurricane development over the next few weeks but perhaps some minor ones in more unusual positions. I'll be back later today with part 2 and the very different season in the tropical eastern Pacific. Can any of those tropical storms impact on the US?   David :) 

 

 

  • Like 2

Share this post


Link to post
Share on other sites

Hurricane Outlook, A Closer Look At The Very Active East Pacific Season Plus More Paper Reviews - Part 2: East Pacific

 

Following on from part 1, where I reviewed the North Atlantic hurricane season, I now turn to the East Pacific which in contrast, has seen a very active season so far.  I shall look at current conditions and compare this season to a normal East Pacific season and look at typical tracks. Then I consider the cross basin storms and the more unusual tracks to see what chances there are that these might impact on the US in any way.  There are several excellent papers covering this particular topic and I shall review these.

 

The current outlook is pretty typical of the season so far but becoming even more active with more frequent and intense storms.

hur 4.8l.PNG

We have 4 systems showing up. Hector has become a category 3 "major" hurricane (see below). Two more disturbances are likely to become named storms during the next 5 days and another one seems set to continue this trend later next week. The satellite imagery shows a long band of convective activity.

hur 4.8j.PNG

Note that this chart has not updated from the previous forecast (6 hours ago) and all the systems are now shown to be more intense and/or with a greater chance of development!

hur 4.8l.PNG

Hurricane Hector is expected to maintain hurricane strength well into next week. 

hur 4.8k.PNG

It is not expected to present any land hazards but it may get pretty close to Hawaii as it tracks just to the south of the islands - so one to watch.

hur 4.8k.PNG

The train of systems shows that each is currently on a similar track and follow a pattern seen for much of the early part of the 2018 season.  The tracks have generally been on a west-north-west trajectory.  Storm development has started slightly further east with the more recent systems, closer to the west Mexican coast. More on this later. 

hur 4.8f.PNG

Unlike the topical Atlantic, there is an almost uninterrupted expanse of STTs above the critical 26.5c mark from 5N to 15N and 28c or higher SSTs closer to the Central American, Mexican and Southern Californian coasts.

hur 4.8g.PNG

The weekly anomalies are either neutral or slightly positive with the greatest positive values off Southern California.

hur 4.8m.PNG

Again, in stark contrast to the tropical Atlantic, there is an abundance of moisture from at least 5N to 15N and just enough dry air to the north (not too much) to mix into the systems.  A similar "easterly wave" train can be established in the tropical eastern Pacific to that in the tropical Atlantic.  In fact, the motion of African easterly waves are usually (but not always) traced all the way from west Africa, over the Atlantic, then across central America and well out into the eastern Pacific.  This season it has been much more interrupted and the easterly waves have been inconsistent and even absent at times in the Atlantic but much more prominent in the eastern Pacific. I've placed many papers and presentations into the Research Portal that deal with various aspects of easterly waves and at some time, later this season, I will review a few more of them on here.

 

So just how active is the 2018 East Pacific hurricane season?  Several weeks ago I posted some charts on historical North Atlantic tropical storm and hurricane activity and tracks. Let's have a closer look at some similar charts and tables for the East Pacific.

hur 4.8q.PNG

This table compares all global tropical storms and hurricanes for each basin from 1981. Comparing the top two, we see that the average annual number of named tropical storms is 12.1 in the Atlantic and 16.6 in the East Pacific and for hurricanes (category 1 upwards) it is 6.4 and 8.9 respectively.  Both basins have seen a maximum of 28 tropical storms in their most active season and a maximum number of hurricanes of 15 and 16 respectively.  

hur 4.8p.PNG

Unlike the Atlantic, there has not been a trend to greater storm activity during the last 30 years (compare to the Atlantic chart in my 21st July post).  There is a slight increase (outside year to year fluctuations) in the number of storms that developed into major hurricanes and 1992 managed 10 and the exceptional season of 2015 managed 11 (more on this in a minute). The East Pacific season is usually slightly longer than the Atlantic season with more storms in May and November. So, we are not yet half way thru the season and in 2018 we already have 8 named storms which may well reach 11 within a week or so which emphasises that this is indeed an active season.  The recent activity has been steadily building.

 

Some readers may wonder why I am focusing so much on the East Pacific basin when the typical track of tropical storms and hurricanes there is west-northwesterly and away from the US and central America. Whilst this is true, there are some exceptions which dispel the myth that they never affect the US .

hur 4.8r.PNG

This map shows the tracks of all the East Pacific named storms from 1980 to 2005.  More recent season by season charts are available from NOAA. Here's a UK Met Office link to named storm tracks for each season from 1992 to 2017:   https://www.metoffice.gov.uk/weather/tropicalcyclone/tracks/composites/nep  (click on the year for each map).  Some of these tracks take a swerve to the north-east. The usual westerly trajectory is due to the easterly trades winds.  When these are weak, tracks tend to be more erratic. Later in the season, especially after the September equinox, the ITCZ has drifted back to the equator on its migration southwards.  The mid-latitude jet stream gradually takes up a more southerly route.  The trades on the northern edge of the sub-tropics generally start to weaken.  Meanwhile, SSTs are still close to their summer peaks.  This is when a few tracks of both Atlantic and Pacific storms can be more likely to take more northerly or north-easterly routes. The SSTs off the western US coast rapidly reduce further north (as can be seen in the Reynolds SST chart further up) and there is a cold ocean current there too.

 

Several storms each season do usually make landfall into Mexico and Southern California.  The best example of this was the record breaking Hurricane Patricia in 2015. This was a very active season in the East Pacific and the storm track maps are divided into two.

hur 4.8s.PNG

hur 4.8t.PNG

Patricia was a late season storm, that formed very close to central America and after moving west-northwestwards then swerved to the north and north-east making landfall in Mexico as a record strength category 5 hurricane and second only in strength to Typhoon Tip in 1979.  Both storms should be assigned to a new category of 6!  This Wikipedia link provides all the main facts about this famous storm:  https://en.wikipedia.org/wiki/Hurricane_Patricia  On October 23rd 2015, Patricia reached  maximum intensity with sustained winds reaching an incredible 215 mph (345 km/h). She broke many records including those for rapid intensification and even more rapid weakening as she made landfall, which reduced the amount of widespread damage that had been anticipated.

 

There are several papers and video presentations in the Research Portal which consider the reasons for and factors behinds this exceptional storm. This one is a brilliant 16 minute video:  The Extreme Intensification and Predictability of 2015's Hurricane Patricia - Presentation  (click on the title for a link to the presentation summary in the portal where there's a direct link to the video). Here is another excellent video presentation, also from the 2018 Hurricane Conference:   An Investigation of the Formation of Hurricane Patricia (2015) - Presentation (click as above).

 

Now, not even Patricia had any impact on the US, so let's consider those storms that have done so. Here's a list of "cross over" storms from 1842 to 2016.

hur 4.8n.PNG

Of the 18 tropical storms or hurricanes, 12 crossed from the Atlantic to the Pacific and 6 from the Pacific to the Atlantic - so quite a rare event but not "never"! The last one of these being Hermine in 2010 which formed out of the remnants of an ex tropical depression from the East Pacific. The disturbance crossed Guatemala and then hit the warm waters in the Gulf of Mexico and it regenerated into a tropical storm which then made US landfall causing more widespread damage, flooding and loss of life. Full details here:   https://en.wikipedia.org/wiki/Tropical_Storm_Hermine_(2010).

 

Apart from the storms in the table above, there are quite a few other examples of storms from tropical disturbances crossing central America as Wikipedia show in this table:  

hur 4.8o.PNG  

EDIT: This table did not enlarge - so here's the Wikipedia link to both tables:   https://en.wikipedia.org/wiki/List_of_Atlantic–Pacific_crossover_hurricanes

To get a good idea of this type of regeneration, just refer to my posts earlier on this thread about the extraordinary Hurricane Beryl.  After developing as a tiny tropical storm on an African easterly wave, she briefly reached category 1 strength before hitting adverse upper conditions and rapidly dissipating into a small area of disorganised showers. 5 days later the remnants moved across more favourable SSTs and Beryl briefly reformed into a tropical storm east of Florida.  So, the remnants of any storm can provide some of the ingredients for further storm development if upper and surface conditions become more conducive again.

 

Now let's get down to the nitty gritty of this post. While La Nina conditions favour more active Atlantic hurricane season as in 2017, El Nino conditions favour far less active conditions as I and several others have already pointed out on this thread. With ENSO neutral conditions on the warmer side of average and with predictions of a weak El Nino developing during the fall and into winter 2018/19 this is already impacting on the 2018 season (this and other factors referred to previously). The reverse applies to the East Pacific basin with El Nino favouring a more active season as is underway now.  I shall review another paper that I recently placed in the Research Portal.  Just click on the title for a link to the portal entry and a direct link to the full paper):   Intrabasin Variability of East Pacific Tropical Cyclones During ENSO Regulated by Central American Gap Winds

 

Here's the abstract:

Hurricane Patricia in 2015 was the strongest Pacific hurricane to make landfall in Mexico. Although Patricia fortuitously spared major cities, it reminded us of the threat tropical cyclones (TCs) pose in the eastern North Pacific (ENP) and the importance of improving our understanding and prediction of ENP TCs. Patricia’s intensity and the active 2015 ENP hurricane season have been partially attributed to the strong El Niño in 2015, however there is still a lack of fundamental understanding of the relationship between El Niño-Southern Oscillation (ENSO) and ENP TCs. Here, we demonstrate that ENSO drives intrabasin variability of ENP TCs, with enhanced (reduced) TC frequency in the western portion of the ENP during El Niño (La Niña), but reduced (enhanced) TC frequency in the eastern nearshore area, where landfalling TCs preferentially form. This intrabasin difference is primarily driven by the Central American Gap Winds (CAGW), which intensify (weaken) during El Niño (La Niña), producing low-level anticyclonic (cyclonic) relative vorticity anomalies and thus an unfavorable (favorable) environment for TC genesis. These findings shed new light on the dynamics linking ENP TC activity to ENSO, and highlight the importance of improving CAGW representation in models to make skillful seasonal forecasts of ENP TCs.

 

This 2017 paper takes us from Hurricane Patricia onto a study of the storm variability in the Atlantic and especially the East Pacific basins during different phases of the ENSO. it shows that the "Central American Gap Winds" (the winds that blow across that region and drive storms or disturbances across the peninsular) intensify during El Nino conditions and weaken during La Nina conditions.  

hur 4.8u.PNG

These charts show the difference in tropical cyclone genesis averaged over 13 El Nino and 11 La Nina events between 1970 and 2015 (in a and b above) and relative to their climatological norms for the 46 year period (in c and d below).  It confirms that statistically, El Nino increases tropical cyclone development in the western development region (in the East Pacific) and reduces it in the eastern development region, particularly in the Caribbean and Gulf of Mexico and the main activity generally switches westwards. The reverse applies during La Nina conditions.

 

The main part of the study looks at the Central American Gap Winds (CAGW) and also takes account of SSTs, upper moisture and wind shear.  it shows how the mountainous region of central America has an influence on low level vorticity changes and strongly impacts on reduced cyclogenesis in the Atlantic basin. (particularly in the Gulf of Mexico). This part of the study also confirms the relevance of those upper level humidity charts that I've been posting.  The developing weak El Nino conditions favouring a greater expanse of drier air over the tropical Atlantic and greater moisture over the tropical East Pacific. The authors of the paper suggest that the study is incomplete:

 

"Discussion: 

This study provides a new perspective on the impact of ENSO on ENP TC activity. It suggests that the previous view of increased (decreased) basin-wide ENP TC activity during El Niño (La Niña) caused by changes in vertical wind shear, SST, and mid-tropospheric relative humidity is incomplete. The ENP TC response to ENSO is characterized by intrabasin variability, with enhanced TC activity confined to only the WDR and reduced TC activity in the EDR during El Niño, and the opposite during La Niña. Furthermore, this study identifies a topographically locked feature – CAGW– as a primary player in regulating TC activity in the EDR through low-level relative vorticity changes. As all TCs that make landfall onto the Pacific coast of Central America and Mexico form in the EDR, the results presented here have important implications for improving seasonal forecast skill of ENP TCs and their impacts. It points to the necessity of properly resolving and representing the CAGW and its response to ENSO in TC forecast models.

We note that the overall reduction of TC activity over the EDR in response to El Niño does not exclude occurrences of individual extreme hurricanes in the region during El Niño. Indeed, Hurricane Patricia in 2015 and Hurricane Pauline in 1997, both among the strongest hurricanes to make landfall over Mexico, occurred during strong El Niño years. Individual hurricane genesis can be initiated by short-lived synoptic events that may not contribute significantly to the mean environment for TC genesis. Hurricane Patricia, for example, appeared to be initiated by a synoptic-scale CAGW strengthening event near TT that provided an injection of cyclonic relative vorticity on the cyclonic shear side and spurred the formation of the initial tropical depression7. These types of synoptic CAGW events are especially effective in triggering tropical cyclogenesis when they interact with the monsoon trough37. Understanding synoptic CAGW events as a mechanism of tropical cyclogenesis is an active research area, and more studies are clearly needed to elucidate the underlying dynamical processes and the distinction between their roles in synoptic versus seasonal and longer timescales."

 

The authors point out that despite the general influence of the ENSO state on overall conditions, that this does not rule out completely the formation and development of major individual storms.  I have read in other studies that even in very inactive hurricane seasons, there can still be one or two major events.  The extremely unusual combination of teleconnective patterns, particularly with those low SSTs in the eastern tropical Atlantic and those well above average SSTs further north towards the eastern US coast may still have an influence on the season. Unfortunately, for those seeking US landfall hurricanes during this season, there may be very few opportunities but if a storm does manage to develop, we may see some rapid development over those higher SSTs, if /when the dry air expanse is at least temporarily reduced. That almost unheard of mid Atlantic disturbance I mentioned in part 1 of this post is another feature of this very odd hurricane season.  

 

David :) 

  • Like 3

Share this post


Link to post
Share on other sites

Good stuff David.

 

For the uninitiated...this hurricane season has been weak as F for the Atlantic.

  • Like 2

Share this post


Link to post
Share on other sites

Thanks Geoff, whilst I like to see extreme conditions as much as the majority on here, I also get a lot of enjoyment out of trying to understand the causes of them or the lack of them. As I've demonstrated in a number of posts on here, despite the lack of activity in the North Atlantic (which is likely to continue for much of August if not all season) there have been a number of very unusual events. I've highlighted all of these. Meanwhile, the East Pacific is going into overdrive and may well end up close to its all time most active season.  There are storms starting to queue up.  When this happens, the disturbances at the eastern end of the train in the formation zone begin to bunch up closer and closer to the Mexican coast and start to take different tracks.  We may see one of those rare "cross over" storm events that I described in my last post.  One day, perhaps this season (which offers a decent opportunity with the weak El Nino slowly developing, the high SSTs and plenty of moisture in the formation zone.) one of these storms will cross into the Gulf of Mexico and rejuvenate there, possibly into a major hurricane which may make US landfall.  Those higher SSTs (in the G of M) may be put to good use after all.  The latest 7 day anomaly charts are just out this afternoon:

6.8h1.PNG

The anomaly trend continues in the North Atlantic with extensive negative values from Africa now spreading further west thru to as far as the Florida coast (note that these are 7 days changes and please refer to the charts I posted in my 2 posts above for current/recent actual temps.). Meanwhile the warm pool further north continues to warm up further.  More neutral conditions elsewhere with temps levelling off but still slightly positive in the G of M.   

 

6.8h2.PNG

The formation zone in the East Pacific off the Mexican and South Californian coast continues to see rising SSTs which were already well above average.

 

So my message is look west (or south-west) and watch the East Pacific updates.  Some of you who need an extreme event fix could take a holiday in Hawaii and watch the eruptions and larva flows and time it to coincide with a hurricane there - they're getting closer and are generally easier to predict over a week out compared to their Atlantic cousins.  David :) 

 

  • Like 2

Share this post


Link to post
Share on other sites

TROPICAL STORM DEBBY

 

Well that mid Atlantic depression did managed to take on tropical characteristics and is now a named storm.  Debby had moved south-westwards but the upper flow has changed and she will move away north-eastwards and move back over cooler waters. So a brief and extremely rare event typical of this season.

debby1.PNG

 

debby2.PNG

 

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
Issued by the NWS Weather Prediction Center College Park MD
200 PM EDT Tue Aug 7 2018

For the North Atlantic...Caribbean Sea and the Gulf of Mexico:

The National Hurricane Center is issuing advisories on newly formed
Subtropical Storm Debby, located a little more than 1000 miles west
of the Azores.

Tropical storm formation is not expected over the next five days.

&&
Public Advisories on Debby are issued under WMO header WTNT34 KNHC
and under AWIPS header MIATCPAT4.  Forecast/Advisories on Debby are
issued under WMO header WTNT24 and under AWIPS header MIATCMAT4.

$$
Forecaster Roth/Avila
NNNN

 

David :) 

 

  • Like 2

Share this post


Link to post
Share on other sites

I've not signed up and watched it myself, but if anyone fancies taking a look, the latest Bryan Norcross podcast is "Forecasting the rest of the hurricane season" with highly respected Research Scientist Phil Klotzbach from the Department of Atmospheric Science at Colorado State University:

https://www.local10.com/weather/hurricane/bryan-norcross-podcast-forecasting-the-rest-of-the-hurricane-season-with-phil-klotzbach

 

 

  • Like 2

Share this post


Link to post
Share on other sites

HURRICANE ALERT - US LANDFALL IMMINENT

 

Now that I've got your attention, I had better quickly say that I'm referring to Hurricane Hector in the Central Pacific that passed just south of Hawaii and is now heading for Johnston Island - an atoll in the mid-Pacific - which is a US territory!

 

800px-Midway.jpg

 

So some of you storm chasers like Geoff (@33andrain) had better book your flights now - you may need to charter a private plane but here's the good news...

800px-Johnston_Atoll_satellite_map.jpg

.....there's a large airstrip - in fact not much room for anything else 😁

 

two_cpac_2d0.png

 

EP102018_3day_cone_no_line_37A.png

Hector is still a major category 3 hurricane.

BULLETIN
Hurricane Hector Intermediate Advisory Number 37A
NWS Central Pacific Hurricane Center Honolulu HI   EP102018
800 AM HST Thu Aug 09 2018
 
...MAJOR HURRICANE HECTOR CONTINUES TO MOVE DUE WEST FAR SOUTH
OF KAUAI...
...TROPICAL STORM CONDITIONS ARE POSSIBLE ON JOHNSTON ISLAND
LATER THIS WEEK...
 
 
SUMMARY OF 800 AM HST...1800 UTC...INFORMATION
----------------------------------------------
LOCATION...16.7N 160.8W
ABOUT 365 MI...585 KM SSW OF HONOLULU HAWAII
ABOUT 580 MI...935 KM E OF JOHNSTON ISLAND
MAXIMUM SUSTAINED WINDS...120 MPH...195 KM/H
PRESENT MOVEMENT...W OR 275 DEGREES AT 16 MPH...26 KM/H
MINIMUM CENTRAL PRESSURE...957 MB...28.26 INCHES
 

I'm sorry that this post was rather tongue in cheek but I thought Geoff needed cheering up a little! I'm away on a business trip and that's my excuse.  David :) 

 

 

 

 

 

 

 

 

  • Like 3

Share this post


Link to post
Share on other sites

Sea Surface Temperatures  in the Atlantic can't support hurricanes. Although we made a slight recovery, water temperatures are only 22-27 degrees celsius (72-81 degrees fahrenheit) in the tropical Atlantic. Still though, we could have a short window in which a tropical cyclone could form in that region.

 

08092018 Sea Surface Anomaly.png

  • Like 3

Share this post


Link to post
Share on other sites
2 hours ago, Blessed Weather said:

I've not signed up and watched it myself, but if anyone fancies taking a look, the latest Bryan Norcross podcast is "Forecasting the rest of the hurricane season" with highly respected Research Scientist Phil Klotzbach from the Department of Atmospheric Science at Colorado State University:

https://www.local10.com/weather/hurricane/bryan-norcross-podcast-forecasting-the-rest-of-the-hurricane-season-with-phil-klotzbach

 

 

Hi Malcolm,  thank you for posting this. I just listened to it and you do not need to sign up - in fact my Malwarebytes software blocked the external link (so do not press the green button).  Just click on the Podcast pic and it'll play.  It lasts about 40 minutes overall and mostly features the interview between Bryan and Phil with a little bit more at the end.  Well worth listening too.  It's actually very much along the lines of what we (and several other posters) have been saying during the last few weeks.  I'll summarise it below:

 

  • The developing El Nino (not quite there yet) is dominating
  • The higher East Pacific SSTs which are causing a very active season there are inhibiting Atlantic development
  • A low or very low number of Atlantic tropical storms, hurricanes and major hurricanes are expected during the remainder of this season
  • A few may occur and there "could" still be a major hurricane making US landfall - perhaps during the peak month of September
  • The high SSTs north of the usual formation zone may cause some more mid Atlantic storms just like Debby right now
  • As the El Nino develops further, the East Pacific season may become even more active (I suggested a record season is quite possible)
  • No talk of cross over storms that I posted on last weekend (but one to watch)

Several questions were answered towards the end including one about the very active 2005 Atlantic season (almost the opposite to 2018) - they said that the most unusual feature then was that hardly any storms developed on long track African Easterly Waves (AEWs), including major hurricanes.  This was completely contrary to expectations as well as all the recent research which shows that over half of all storms form on AEWs and 83% of all major hurricanes do.  I'll need to do a post on that season to examine the prevailing conditions and the reasons - there are already a couple of papers I've read on it and I'll ensure that they go into the Research Portal.

 

So, overall it's not good news for all those US members wanting US landfall major hurricanes, we could still see one or two big ones. For those, like me, who want to understand the causes, this highly unusual hurricane season is providing loads of meteorological interest.

 

David :) 

 

 

  • Like 2

Share this post


Link to post
Share on other sites

WHAT'S THIS? AN UNEXPECTED DISTURBANCE POPPING UP IN THE TROPICAL ATLANTIC

 

ah1.png"Tropical Weather Outlook NWS National Hurricane Center Miami FL 800 AM EDT Fri Aug 10 2018 For the North Atlantic...Caribbean Sea and the Gulf of Mexico: 1. An area of disturbed weather is located about midway between Africa and the Lesser Antilles. Environmental conditions are expected to become conducive for some gradual development while the system moves slowly west over the next few days. By the middle of next week, stronger upper-level winds could limit the chance for further development. Formation chance through 48 hours...low...10 percent.  Formation chance through 5 days...low...20 percent. Forecaster Zelinsky".

 

Well, against the odds and contrary to the previous NHC bulletin which stated "no developments expected within the next 5 days" we have a disturbance which is on a similar track to "Beryl".  Formation conditions are even less conducive on African Easterly Wave tracks but there is a fairly small chance that this disturbance might development as it moves west over progressively higher SSTs. Upper conditions are highly marginal and this system is likely to fail to develop into a tropical storm. 

 

I'm still away on a business trip and I am not able to pull together the upper humidity charts which are highly important. We know that the ENSO state - positive and moving towards a weak El Nino is warming those East Pacific SSTs and inhibiting Atlantic storm development. If you refer back to a couple of recent posts (further up this page) from Malcolm

(@Blessed Weather) who reviewed an excellent MJO/hurricane paper and I referred to that in my long post on hurricane influences, we stated that the MJO can also influence Atlantic cyclone development. Remember that we are looking for the MJO in phases 1 to 4 with enhanced MJO convection over Africa, the Indian Ocean or the western maritime continent for a greater chance of US landfall hurricanes and not in phases 5 to 8 when development is far less likely.  We spent the whole of July in the non conducive phases 5 and 6. The MJO signal then weakened in the "circle of death".  Conflicting forecasts "had" (about a week ago) suggested either a re-emergence at low amplitude in phase 2 or 3 (GEFS) or phase 5 or 6 (ECM).  Here's the GEFS/NCEP chart: 

mjo1.gif

 

GEFS  (I cannot obtain/post the other charts at this time) currently expects the MJO to re-emerge in phases 2/3 and at moderate amplitude which (if it happens) might be slightly more conducive for slightly increased activity but other factors still favour the weak Atlantic hurricane season to continue. Fingers crossed that the new disturbance manages to defy the odds. If any convective remnants can reach those much higher SSTs further north-west and north then it might "do a Beryl" - dissipate into a band of disorganised showers and than spring to life again. At least it's something else to watch out for following Debby earlier in the week.  David :) 

 

  • Like 3

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×