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December 24-25, 2020 -- High-Impact Weather Event


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  • 33andrain changed the title to December 24-25, 2020 -- High-Impact Weather Event
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Good Thursday morning everyone,  Just a quick cut and paste edit.   HIGH IMPACT wind damage and flood potential event by this time Christmas morning. Strongest winds in NJ/ne PA/se NYS rough

Here are some NAM forecast soundings for this upcoming event, compared to some soundings for April 13th of this year, when parts of the NJ shore gusted over 70 and 50+ gusts occurred all the way to Sc

12/24-25:  I think the wind damage potential is the largest threat from this rapidly deepening 980s surface low moving northward up the front into NYS predawn Christmas. SVR's possible (in my mind lik

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From OKX 7am update:

 

Despite the timing differences among the models, moderate to heavy
rainfall will occur Thursday night, with a growing consensus that it
occurs just before midnight west of the Hudson River and after
midnight from the Hudson River corridor on east. The location of the
the heaviest rain is still uncertain. However, given PWAT pushing
1.50 inches, a widespread 1 to 2 inch rainfall is likely. Another
factor to consider is the liquid equivalent( currently 0.50-2.00
inches) in the remaining snow pack. Some of this will melt before
the heavy rain reaches the area Thursday night, especially during
the day Thursday, but the ground will be saturated and the heavy
rainfall could lead to some flooding. To reiterate, there is still
some uncertainty, but the highest potential for flooding would exist
across the interior, where there is higher snow water equivalent.
Additionally, some of the faster responding streams in New Jersey
may also exhibit sharp rises and come close to bankfull during
this time frame.

An additional concern is with strong to potentially damaging winds
Thursday night into early Friday morning. The GFS and NAM forecast
soundings show 65 to 75 kt winds at 950 mb by Thursday night. The
00z NAM is actually a bit stronger just above 950 mb with winds 80-
85 kt at 925 kt. The core of the low level jet is most likely to
pass over Long Island and southeast Connecticut. Forecast soundings
indicate an inversion around or just below these high winds and this
is typically a difficult forecast on how much will mix down to the
surface. It does appear that wind advisory level winds seems
plausible over at least Long Island and southeast Connecticut, but
some of the higher winds could occur near the NYC metro and Hudson
River Corridor as well. The strongest winds should occur late
Thursday night or early Friday morning, along and ahead of the cold
front. Will also have to watch for the potential of winds reaching
high wind warning criteria across Long Island and southeast
Connecticut.
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KBOX has a similarly troubling wind disco:

 

Christmas Eve and Christmas Day... Did someone say a White Christmas? Unfortunately, if they did they weren`t in southern New England. If you`re looking for a refill of the white stuff you`ll have to go much further east. That`s thanks in large part to the far inland track of the parent low that will be the origin of our wet and windy holiday. Before we get there we`ve got to get the high out of here which sticks around Wednesday night into the first half of Thursday helping to keep things dry, while pulling increasing cloudcover and warm air northward into the region. Taking a look at the synoptic setup it`s a very amplified pattern with building ridges over the Atlantic and intermountain west and a midwest trough in between stretching clear down to the Gulf of Mexico. Given the high amplitude nature of the trough the trend in both model guidance and the latest forecast update has been to slightly slow the onset of rain Thursday night and the ending of precip on Friday (as is often the case in these situations). The latest GFS seems to have caught on to the slower solution of other international guidance, while the NAM falls on the other end of the extreme. For now the forecast has stuck with a compromise. At the surface the parent low lifts out of the Great Lakes while a secondary low is generated along the frontal boundary over the northeast US. This system as it lifts into Canada has all the necessary ingredients for an impactful storm. As for moisture, there is plenty. We could call this an atmospheric river event, with an anomalous tap of moisture evident all the way from the tropics into New England (PWATs near 1.5"). Placement beneath the right rear quadrant of a 170 kt 300 mb jet together with some impressive jet dynamics in the low and mid levels will provide ample lift for a widespread rain event. Anywhere from 1 to 2 inches of rain is expected from this system, with locally higher amounts possible. This amount of rain on top of the current snowpack is leading us to become increasingly concerned about the threat of localized street flooding and even some river flooding. Compounding that threat is the extremely warm and moist airmass present Thursday into Friday, when temperatures climb into the mid to upper 50s and dewpoints in the 50s as well! Typically strong winds plus high dewpoints equals a rapidly receeding snowpack. The other hazard from this system will be strong, potentially damaging winds. The previously mentioned good low level jet dynamics were referring to an 80 to 90 kt low level jet that develops at the 925-850 mb level. Besides providing intense lift for precip, these winds may mix down and cause damage at the surface. While model soundings currently indicate a low level inversion keeping the bulk of the winds aloft, even a portion of winds of this magnitude could cause scattered power outages. Any downpours overnight will help to mix down those winds as well. The unusually warm airmass persists overnight and those dewpoints won`t allow for much cooling off, so lows should be in the upper 40s. Friday the high temperature will occur early in the morning before the cold front swings through later in the day causing temperatures to plunge and cutting off the heavy rain. There is potential for some novelty snow on the back end as the cold air wraps in and the precip exits, but don`t expect much out of it. Saturday through Monday... Much colder post frontal airmass in place for the weekend as high pressure moves in bringing dry weather. Temperatures won`t get out of the 30s. The next storm system brings another chance of showers to start next week.

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9 hours ago, AspiringMet said:

Let’s keep in mind those winds won’t actually verify. These models have a tendency to significantly overdo wind, especially when an inversion comes into play

I live in suffolk county. Wind events usually verify. Different story west of here

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8 minutes ago, Psv said:

I live in suffolk county. Wind events usually verify. Different story west of here


LI usually verifies on the wind.  Further E one gets the better chances of seeing higher winds from S direction in CWA thanks to the ocean.

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2 minutes ago, Sundog said:

 

Not usually to the extent the models like to portray in their output. 


Models are usually overdone it seems with the max gust potential maps.  Slice 15-20 mph off IMO.

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Per Mt. Holly - 

 

A southerly LLJ will increase to 70-80 kt at 925 mb or so.  
  Based on latest BUFKIT profiles, it seems there should be a  
  strong enough inversion that would keep the strongest winds  
  aloft, but at this time, cannot rule out 35-40 kt gusts along  
  the coast, and a Wind Advisory may be needed. With heavy rain  
  showers, also cannot rule out some of those winds convectively  
  mixing down to the surface. 
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2 minutes ago, RAllen964 said:

 

They were dead on w/ Isaias. 


That’s a tropical system with no inversion, different animal than the LLJ ahead of a front and getting those max winds to mix down to surface.

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3 minutes ago, uofmiami said:


That’s a tropical system with no inversion, different animal than the LLJ ahead of a front and getting those max winds to mix down to surface.

 

Correct, but up to now, I have not seen a true inversion on soundings. I am glad the guys over at Mt. Holly are seeing something in the BUFKIT. 

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6 minutes ago, RAllen964 said:

Per Mt. Holly - 

 



A southerly LLJ will increase to 70-80 kt at 925 mb or so.  
  Based on latest BUFKIT profiles, it seems there should be a  
  strong enough inversion that would keep the strongest winds  
  aloft, but at this time, cannot rule out 35-40 kt gusts along  
  the coast, and a Wind Advisory may be needed. With heavy rain  
  showers, also cannot rule out some of those winds convectively  
  mixing down to the surface. 

 

10 hours ago, Jake302 said:

The reason I haven't gotten excited over those wind maps is because they typically don't verify that high in my experience. Not even close sometimes. I've been through this before, getting excited over a euro wind gust map showing hurricane force gusts very close to the event while in reality we only got 50-55 MPH gusts tops

 

I'm not sure about the lack of inversion. Typically there is one with these kinds of events

 

Edit: Can someone move this to the right thread? 

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Starting to doubt the inversion materializes, or it may be weak. Temps above freezing/clear skies should work to melt the pack over the next few days. Without that, you lose your primary mechanism that drives it. 

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