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December 2020 General Discussions & Observations Thread


bluewave
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29 minutes ago, MJO812 said:

Vortex split on the Euro according to Webb

 

 

That doesn’t look like a true vortex split, the model shows something, then backs off, shows something then backs off. I hope this doesn’t turn into yet another winter of chasing phantom “vortex splits” and SSWEs from now until March....

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5 minutes ago, snowman19 said:

That doesn’t look like a true vortex split, the model shows something, then backs off, shows something then backs off. I hope this doesn’t turn into yet another winter of chasing phantom “vortex splits” and SSWEs from now until March....

  it  certainly is a  a full split 

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Milder air is now poised to move into the region. The remainder of the week will feature above normal to perhaps some much above normal temperatures. Somewhat cooler air could return to the region after next weekend.

A sustained warmer pattern could develop toward the winter solstice. The closing 10 days of December could wind up generally warmer than normal. The EPS weeklies and new subseasonal guidance are in strong agreement that the last week of December will likely be warmer than normal. The latest CFSv2 weekly guidance favors much warmer than normal readings.

Historic experience following exceptionally warm November cases suggests that a warmer than normal December remains the base case. Almost 90% of cases with a November mean temperature of 51.5° or above in Central Park (2020 saw the temperature average a record 52.9° in November) went on to record a warmer than normal December and just over three-quarters of such cases saw December register a monthly mean temperature of 40.0° or above.

The ENSO Region 1+2 anomaly was -0.5°C and the Region 3.4 anomaly was -1.1°C for the week centered around December 2. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.77°C and the ENSO Region 3.4 anomaly has averaged -1.35°C. La Niña conditions will likely prevail at least through the winter.

The SOI was +5.29.

Today, the preliminary Arctic Oscillation (AO) figure was -0.913.

On December 8 the MJO was in Phase 5 at an amplitude of 0.901 (RMM). The December 7-adjusted amplitude was 0.678.

Based on the latest guidance, no significant stratospheric warming event is likely through the first half of December.

Since 1950, there have been five cases where a La Niña developed during June-July-August or afterward following an El Niño winter. 4/5 (80%) of those cases saw a predominant EPO+/AO+ winter pattern. The most recent such case was 2016-17. 10/11 (91%) of the La Niña winters that followed an El Niño winter featured a predominantly positive EPO. A predominant EPO+/AO+ pattern is very likely for winter 2020-21. It is likely that the Middle Atlantic and southern New England areas will see a warmer than normal winter with below normal snowfall.

Based on sensitivity analysis applied to the latest guidance, there is an implied 73% probability that New York City will have a warmer than normal December. December will likely finish with a mean temperature near 40.0°.

 

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