bluewave

The +AAM influence will start to fade since it was just a brief pattern driver which emerged for a few weeks from late November into early December. Just like the models originally underestimated the event from back in mid-November when it wasn’t showing up. Now we can see they have been underestimating the return of the milder La Niña background pattern from last week. This also matches the December seasonal progression that I posted on yesterday in the NYC Metro forum. The WPAC warm pool continues to be a dominant force as we have seen since the 15-16 winter. So it’s difficult for us to get more than a week or two of cold at a time before the warmth returns. But it will be interesting to see if there will be some more +AAM intervals to at least push back against the milder La Niña pattern at times after December.

Yes, the physical evidence is that the 2nd half of December has been warming at a faster rate than the 1st half of December since 1981. So as a function of this warming NYC and other locations have made it to 55° or warmer every year between the 17th and 25th since 2011. This late month warm up has been very reliable even in years like 2017 that dropped into the single digits right after Christmas. It was also a feature in 2020 following the record 40”+ snowstorm in BGM mid-December before the record flood cutter and widespread 50s and 60s on Christmas. Many times the models haven’t forecast this warm up during the first week of December but have shifted warmer in later runs as we approached the 15th. So when models are showing a relaxation of the colder pattern during the 2nd week of December, chances are the following period up to the holidays will see at least 1 day reaching 55° or warmer at spots like NYC and EWR. NYC maximum temperature December 17th to 25th since 2011 Time Series Summary for NY CITY CENTRAL PARK, NY Click column heading to sort ascending, click again to sort descending. 2023-12-25 62 0 2022-12-25 58 0 2021-12-25 60 0 2020-12-25 61 0 2019-12-25 57 0 2018-12-25 61 0 2017-12-25 55 0 2016-12-25 58 0 2015-12-25 72 0 2014-12-25 62 0 2013-12-25 71 0 2012-12-25 56 0 2011-12-25 62 0 12-01 to 12-16 average temperature trend line NYC….1981…39.5°…..2023…41.3…..+1.8 12-17 to 12-31 average temperature trend line NYC….1981….34.7°….2023…39.6°….+4.9°

The heat island doesn’t have anything to do with it since it’s air mass related and not UHI related. The heat island effect has been constant in NYC for a long time. The outlying areas haven’t any any similarly cold Decembers since 2010 either.

Very unlikely that we see a December this year anywhere near 32° in NYC given the record warmth across North America and the globe this fall.

Yeah, we don’t get Decembers that cold anymore regardless of La Niña, neutral, or El Niño.

While I am not a fan of using the CFS model, its forecast 500mb pattern at that time looks more like -AAM.

The November warming with the record EPAC WWB back in December is fading as the Niña-like EWBs are returning. You can see the +AAM influence beginning to weaken after early December. The 500 mb pattern is starting resemble something more Niña-like. New run around the 10th showing weakening +AAM influence and taking on some Niña-like elements like higher heights near the East Coast and a more +WPO Older run stronger +AAM with deeper Eastern Trough and more -WPO Nino 3.4 shifting back to cold neutral from warm neutral

My guess is that the first week to 10 days of December may have the coldest departures we see this month. The extended EPS is already backing off the cold it was showing for the 9th through 16th. This has been typical December local climatology with the usual warm up as we approach the solstice and Christmas. New run closer to average December 9th through 16th Old run was colder Longer term December temperature trends cooler to start and warming up as we approach the solstice and holidays

The main difference was that 2010 stayed cold the whole month with no moderation.

December 2007 had frontloaded cold which moderated mid to late month with low 60s as we approached Christmas.

First time that December 1st dropped into the 20s in NYC since 2007. Data for December 1 - NY CITY CENTRAL PARK, NY Click column heading to sort ascending, click again to sort descending. 2023-12-01 51 43 0.41 0.0 0 2022-12-01 43 35 0.00 0.0 0 2021-12-01 49 40 T 0.0 0 2020-12-01 62 37 T 0.0 0 2019-12-01 39 30 0.62 T 0 2018-12-01 46 36 0.05 0.0 0 2017-12-01 52 42 0.00 0.0 0 2016-12-01 54 42 0.07 0.0 0 2015-12-01 51 44 0.33 0.0 0 2014-12-01 65 42 0.09 0.0 0 2013-12-01 49 36 0.00 0.0 0 2012-12-01 41 36 0.00 0.0 0 2011-12-01 50 37 0.00 0.0 0 2010-12-01 60 40 1.20 0.0 0 2009-12-01 47 35 0.00 0.0 0 2008-12-01 55 43 0.11 0.0 0 2007-12-01 38 25 0.00 0.0 0

The reason the modeling has been jumping around so much last few weeks is that this was the most volatile November for 500mb height changes since 1985 due to all the competing influences.

Looks like a colder pattern for the first week of December with a trough in the East. But the fast Pacific flow and storm track through the Lakes is a continuation from recent winters. Moderation in temperatures week 2 as the Southeast Ridge returns.

Top 5 driest and warmest fall on record across the region.

That winter was one for the ages. The early signs of the extreme cold began to become evident near the end of August. The last time NYC saw the December 1-10 average dip below freezing was 2002. Time Series Summary for NY CITY CENTRAL PARK, NY Click column heading to sort ascending, click again to sort descending. 1 1926-12-10 26.2 0 - 1886-12-10 26.2 0 3 1876-12-10 26.4 0 4 1989-12-10 28.5 0 - 1871-12-10 28.5 0 5 1880-12-10 28.7 0 6 1882-12-10 29.1 0 7 1910-12-10 29.2 0 8 1976-12-10 29.3 0 9 1890-12-10 30.0 0 - 1869-12-10 30.0 0 10 1904-12-10 30.1 0 11 2002-12-10 30.6 0 - 1875-12-10 30.6 0 12 1958-12-10 30.9 0 13 1917-12-10 31.0 0 14 1906-12-10 31.3 0 15 1964-12-10 31.8 0

Yeah, I grew during that much colder era. I can remember when NYC dipped into the single digits on 12-3-76. With the higher energy costs these days we are lucky we don’t get that kind of extended cold anymore.

More like the start of December is a 10 day relaxation period from one of our warmest falls on record. It will feel much colder due to how warm it has been. But the cold departures will be modest compared to colder starts to December in the past.

You can see overlapping influences. The lingering +AAM still has a +PNA ridge near West Coast. But the higher 500mb heights in East than previous runs is the MJO 5-6, +SOI, more Niña-like influence. If later model runs develop more of a -PNA then it will signify that the +AAM influence has decreased even more.

You can see the more Niña-like influence beginning to show up in the 500mb pattern as we approach mid-December with higher heights in the East relative to previous runs. New run Old run

While the amplitude of this latest RMM extension may vary, the slow motion through the Maritime Continent moderating phases aligns well with the VP anomalies. So we get the ridge returning to the East in Mid December. With a little luck the wave can progress into phase 8 in January and not reload again in 5-7.

The magnitude and duration of the more +AAM influence will be key. If the +AAM signal can linger into mid to late December, then we’ll have a colder than average December in our area. But should the +AAM influence wane by mid month, then there will be enough Maritime Continent forcing influence so that the early month cold departures are largely erased.

It takes time for the models to catch up with the correct amplitude. So I just look at the longitude along the equator where the strongest convection and VP anomalies are showing up. But I agree that the number of the phase through a week to 10 days is fairly close on both the RMM and VP anomalies. It’s after about 10 days that the models can really loose the RMM signal correct phase in the noise and the VP anomalies are a bit more reliable for both phase and magnitude. After 15 days models often loose or dampen the convection too much so that even the VP anomalies struggle. But I know enough about model MJO biases 2-3 weeks out that I can correct the models manually and get a general idea where the MJO is going when it’s active enough.

Big cutters going north of the record warm lakes with that amped up northern stream will do it.

Spots like NYC, ISP, and BDR were able to narrowly avoid their driest fall with the beneficial rains. Time Series Summary for NY CITY CENTRAL PARK, NY - Sep through Nov Click column heading to sort ascending, click again to sort descending. 1 1908 4.00 0 2 1909 4.67 0 3 1881 4.93 0 4 2024 4.94 2 5 1931 4.97 0 Time Series Summary for ISLIP-LI MACARTHUR AP, NY - Sep through Nov Click column heading to sort ascending, click again to sort descending. 1 1965 3.31 0 2 2024 3.70 2 3 2001 4.02 0 4 2013 4.86 0 5 1973 5.38 0 Time Series Summary for IGOR I SIKORSKY MEMORIAL AIRPORT, CT - Sep through Nov Click column heading to sort ascending, click again to sort descending. 1 1964 3.68 0 2 2024 3.74 2 3 1965 4.16 0 4 2001 4.20 0 5 2013 5.55 0

You can’t go by the forecast amplitude that those RMM charts show. They often have the wrong amplitude beyond a few days out. It could be a function of the algos each individual model uses. The location and strength of the VP anomalies are usually a more reliable guide. Plus we have had RMM indices not far from the circle have an impact similar to amplitudes of a much higher range. When the MJO becomes the sole driver of the pattern at times, we can even get a well defined 500mb response when the RMM is near or in the circle with the VP anomalies more representative of the pattern. So if the +AAM response begins to wane by mid to late December, even a little tropical forcing in the warmer phases can go a long way when it becomes more of a pattern driver. Our best bet for mid to late December is that the long range models are underestimating the +AAM response. And that the other tropical forcing like the MJO remains in background. Also remember that background tropical forcing in the warmer regions can have some influence on the storm tracks. Even when other features are stronger pattern drivers. We see this at times when a storm takes a cutter track and the cold comes in after the storm with Niña-like background forcing and too dominant of a northern stream.