bluewave

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.

If go back a day and read the posts you will see that there is no mention of any silver bullet in regard to the forecast going forward. We were discussing the competing influences that we are seeing in the modeling. On one hand we had the strong +AAM rise leading to the blocking across the North Pacific. But the Pacific Jet is much stronger than when we had this pattern back in 13-14 and 14-15. So the upper low is much closer to Aleutians eroding the block from the west and displacing the ridge axis further east limiting the cold air production over North America relative to those years so far. As the influence to the +AAM begins to fade the continuing forcing near the Maritime Continent will start to have more influence leading to a moderation in the pattern by mid-December with more ridging near the East Coast.

Noisy is a term frequently used in the CPC MJO discussions when dealing with the RMM plots. So there is nothing unusual about the term. The key for us won’t necessarily be a specific RMM index value but where the actual forcing is located. The VP anomaly forecasts have been very steady in the MJO 5 for early December. Then they slowly propagate the wave closer to MJO 6 by mid-December. Forcing in that region in December usually results in a ridge developing near the East Coast. This is why the models have been showing more ridging near the East Coast in recent runs after early December. The only reason we are starting December colder than average is due to the +AAM spike boosting heights over the NPAC. Models have been indicating that this influence will gradually wane allowing more in the way of the Maritime Continent forcing influence to take over.

We had this same discussion back in November. The VP anomalies remained constant in MJO 2-3. Remember when the forecast by some models was to stay in the circle using the RMMs? But it came out above a +2.0 amplitude. The RMM amplitudes can vary quite a bit from run to run but the actual forcing as per the VP anomaly charts didn’t change much. Now as we head into December the RMMs are changing amplitude again but the location of the forcing is constant near phase 5 early in the month with a slow progression toward phase 6 by mid-December.

This is par for the course for the RMM charts which can be very noisy. The actual forcing on the VP anomaly charts is headed into the MJO 5-6 regions in December. This has been a steady forecast for weeks now. What has changed in the long range outlooks is that the +AAM influence is starting to wane allowing the more MJO 6 look for Mid-December to have more influence.