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Observations & Discussion: Autumn 2021


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Some are saying this Fall could be coolish/below average across the East U.S. Particularly Nov-Dec. Though me thinks 1999 and 2008 are good analogs to start Autumn respecting the 500mb pattern evolution. Those two years were also chosen for this hurricane season. 

No doubt September will start above average temp wise(with and uptick in tropical activity) due to a CCKW and MJO  wave over the Western Hemisphere.  But as we get towards the second half of the month the MJO wave should move on over to the Nina sectors. Eastern Hemispheric/La Nina MJO forcing during the Fall typically favors PNA ridges and repeated dips in the Jetstream across the East. Particularly phase 5(as long as destructive interference doesn't throw it off)! 

I remember HM once commented on Twitter that in order for Fall to be persistently below average across the East a proper mix of Nino and Nina forcing needs to be present. Hopefully La Nina will not come in too strong.

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Howdy Everyone - been a while since I posted some weather pictures, but I couldn't help but share this wonderful moment.  Here is a cloud inversion in the Delaware Water Gap National Recreation Area f

8:42 a.m. in Vernon, 41*      

It begins. 🤣

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1 hour ago, Snowman11 said:

Of course NYC is the warmest 

 

Looks like a radiational cooling night, of course the city will be baking relative to its surroundings. It's one of the worst qualities of the city weather wise. 

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31 minutes ago, Event Horizon said:

The 18Z GFS has a massive cold front on our doorstep around September 2nd. 

 

It's less intense on the 00Z GFS. More realistic. However, the H5 has many troughs from the 2nd onward in the Northern US. 

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5 hours ago, Event Horizon said:

 

It's less intense on the 00Z GFS. More realistic. However, the H5 has many troughs from the 2nd onward in the Northern US. 

 

The 06Z GFS continues that theme. It seems that a taste of Autumn is nearing medium range. 

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8 hours ago, Event Horizon said:

 

The 06Z GFS continues that theme. It seems that a taste of Autumn is nearing medium range. 

The 12z follows up the 2nd cold front with a much stronger, sweeping front on the 9th-10th. That’s the 15 day range, but if we get one front on the second, it’s not unlikely to have a follow up that is stronger. Does happen once we get into September. Either way, an end to summer is clearly in the crosshairs. Just gotta ride out the rest of the heat this week, and honestly I think we’re largely done with extended periods of excessive humidity and heat unless we get a tropical system that draws those air masses up behind them like Henri just did. Even so, climatology will win the battle as September moves on.

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From what I see, today’s 12z GFS has a strong cold front from the 1st-3rdish and a VERY strong, long lasting push of cool dry air around the 9th or so. Heat haters only need to hang on so much longer. I think these fronts are legit as the first one seems to be moving up in time consistently and the second seems to be gaining consistency as well.

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37 minutes ago, Archerdude44 said:

From what I see, today’s 12z GFS has a strong cold front from the 1st-3rdish and a VERY strong, long lasting push of cool dry air around the 9th or so. Heat haters only need to hang on so much longer. I think these fronts are legit as the first one seems to be moving up in time consistently and the second seems to be gaining consistency as well.

 

Some used to call that the 'summer backbreaker'  but if the GFS is right, it will live up to the name - usual long range GFS caveats apply - at least in the sense of being overdone at range. To emphasize the point changes are coming, here's Newburgh, with a 39 for a low. 

 

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3 hours ago, Archerdude44 said:

From what I see, today’s 12z GFS has a strong cold front from the 1st-3rdish and a VERY strong, long lasting push of cool dry air around the 9th or so. Heat haters only need to hang on so much longer. I think these fronts are legit as the first one seems to be moving up in time consistently and the second seems to be gaining consistency as well.

 

same deal on the 18z GFS. summer is over by september 7th

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5 hours ago, fireguy286 said:

 

Some used to call that the 'summer backbreaker'  but if the GFS is right, it will live up to the name - usual long range GFS caveats apply - at least in the sense of being overdone at range. To emphasize the point changes are coming, here's Newburgh, with a 39 for a low. 

 

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Yes, GFS does like to overdo cold pushes in the LR. But areas around here did in fact drop to 30 degrees last year in mid September. I had frost before September 20th. Can’t remember the exact date, but it isn’t completely out of the realm of possibilities. Though, I think a more realistic approach is better for now. Still does seem to be our “summer backbreaker” anyhow as you stated.

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18z GFS was pretty much anyone's dream after this long stretch of heat and humidity, but compared to other models it is really quite bullish on the cold. Still a nice cold shot is expected to start September and will be our first step down into fall!

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A positive AMM and GWO(Nino phases) this time of year(i.e. late Summer/Fall) typically favors dips in the jet stream over the East. Couple that with the MJO eventually going into Nina phases(which also favors cool weather this time of year) and we're in business! 

 

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Through 8 pm, Ida's remnants have already produced September 1 daily record rainfall totals in a number of locations including:

 

Harrisburg: 6.52" (old record: 3.60", 1952)
Hartford: 1.38" (old record: 0.91", 1952)
Mount Pocono: 4.72" (old record: 4.54", 1952)
New York City-JFK: 0.68" (old record: 0.63", 2002)
Philadelphia: 2.00" (old record: 1.80", 1952)
Poughkeepsie: 2.76" (old record: 2.50", 1952)
Scranton: 4.86" (old record: 2.59", 2003)
York, PA: 6.51" (old record: 2.01", 2003)

 

Unofficially, Newark picked up 1.36" rain in the past hour. The last time Newark picked up 1.00" or more in an hour was June 26, 2020. This was also Newark's largest hourly rainfall since May 28, 2013 when 1.84" was measured.

 

Ida will bring heavy rain and possible tornadoes to the region overnight into early tomorrow. Much of the region will see a general 3"-6" of rain with local amounts at or above 8". Flash flooding is likely on account of the saturated ground from excessive August rainfall.

 

Based on 1971-2020 data, there is an implied 97% probability that New York City will pick up 50" or more precipitation this year. There is also an implied 40% probability that the City could pick up 60" or more precipitation. Since 1869, New York City has had 34 years with 50" or more precipitation (65% of which have occurred after 1970) and 8 years with 60" or more precipitation (all of which have occurred after 1970). Records go back to 1869.

 

Despite a cooler than normal first 7-10 days, September will likely feature near normal to somewhat warmer than normal temperatures in much of the region.

 

Fall 2021 will likely be wetter to much wetter than normal in the northern Middle Atlantic region. Since 1869, there have been 9 August cases where New York City picked up 20.00" or more rainfall during the summer. Two thirds of those cases (and 4/5 of those with summer mean temperatures of 73.0° or above) had 17.00" or more fall precipitation in New York City. 2011 is probably the closest match in terms of precipitation and a nearly identical summer mean temperature. Mean fall precipitation for those 9 cases was 14.86". The median was 17.35". The 1991-2020 normal value is 12.27".

 

The ENSO Region 1+2 anomaly was 0.0°C and the Region 3.4 anomaly was -0.3°C for the week centered around August 25. For the past six weeks, the ENSO Region 1+2 anomaly has averaged +0.35°C and the ENSO Region 3.4 anomaly has averaged -0.47°C. Neutral ENSO conditions will likely prevail into at least mid-September. Afterward, La Niña conditions could begin to develop.

 

The SOI was +9.03 today.

 

The preliminary Arctic Oscillation (AO) figure was -1.846 today.

 

On August 30 the MJO was in Phase 2 at an amplitude of 1.481 (RMM). The August 29-adjusted amplitude was 1.331 (RMM).

 

 

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In the wake of Ida's intense rainfall and devastating flooding, bright sunshine returned. The remainder of the week and weekend will feature abundant sunshine and cooler than normal readings.

 

Despite a cooler than normal first 7-10 days, September will likely feature near normal to somewhat warmer than normal temperatures in much of the region.

 

Fall 2021 will likely be wetter to much wetter than normal in the northern Middle Atlantic region. Since 1869, there have been 9 August cases where New York City picked up 20.00" or more rainfall during the summer. Two thirds of those cases (and 4/5 of those with summer mean temperatures of 73.0° or above) had 17.00" or more fall precipitation in New York City. 2011 is probably the closest match in terms of precipitation and a nearly identical summer mean temperature. Mean fall precipitation for those 9 cases was 14.86". The median was 17.35". The 1991-2020 normal value is 12.27".

 

The ENSO Region 1+2 anomaly was 0.0°C and the Region 3.4 anomaly was -0.3°C for the week centered around August 25. For the past six weeks, the ENSO Region 1+2 anomaly has averaged +0.35°C and the ENSO Region 3.4 anomaly has averaged -0.47°C. Neutral ENSO conditions will likely prevail into at least mid-September. Afterward, La Niña conditions could begin to develop.

 

The SOI was +2.74 today.

 

The preliminary Arctic Oscillation (AO) figure was -1.499 today.

 

On August 31 the MJO was in Phase 2 at an amplitude of 1.182 (RMM). The August 30-adjusted amplitude was 1.481 (RMM).

 

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