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January 2021 General Discussions & Observations Thread

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20 minutes ago, Allsnow said:

Euro ensemble mean snowfall for next week is 10 inches between 3 storms 

Very active pattern

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A weak disturbance brought a round of light snow and snow showers across the region today. Snowfall amounts included:

Allentown: Trace
Bridgeport: Trace
Islip: 0.2"
New York City: Trace
Newark: 0.3"
Philadelphia: Trace

The remainder of the week will be fair and seasonably cold. A much colder air mass will arrive for the weekend. Parts of the region could struggle to reach freezing on Saturday and Sunday.

As Arctic blocking persists, next week could feature one or perhaps two more opportunities for snowfall, with the greatest risk focused on the January 25-28 timeframe. However, a widespread significant snowfall (6" or above) still appears relatively unlikely. A light or moderate snowfall is more likely.

AO-/PNA- patterns typically do not produce significant or major snowstorms for the Middle Atlantic or southern New England regions in late January or early February. During the January 20-February 7, 1950-2020 period, there have been 5 storms that brought 4" or more snow to at least two of the following cities--Boston, New York City, or Philadelphia--during an AO-/PNA- pattern. None of those storms brought 6" or more snow to two or more of those cities. Therefore, until the potential events are within 72 hours or less and the guidance suggests otherwise, the base case is against a widespread significant or major snowfall.

Afterward, the evolution of the AO will determine whether potential for measurable snow events will continue into February.

The ENSO Region 1+2 anomaly was -0.3°C and the Region 3.4 anomaly was -1.2°C for the week centered around January 13. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.73°C and the ENSO Region 3.4 anomaly has averaged -1.08°C. La Niña conditions will likely prevail at least through meteorological winter.

The SOI was +4.86 today.

Today, the preliminary Arctic Oscillation (AO) figure was -2.124.

On January 19 the MJO was in Phase 5 at an amplitude of 0.425 (RMM). The January 18-adjusted amplitude was 0.638.

Following a significant stratospheric warming event, the stratosphere is now cooling. The cooling will likely accelerate during the second half of January. As is typical for vortex-splitting events, the major piece of the polar vortex migrated to Eurasia. The end result has been an absence of severe cold in much of North America.

The significant December 16-17 snowstorm during what has been a blocky December suggests that seasonal snowfall prospects have increased especially from north of Philadelphia into southern New England. At New York City, there is a high probability based on historic cases that an additional 20" or more snow will accumulate after December. Were blocking to disappear, snowfall prospects would diminish.

Based on sensitivity analysis applied to the latest guidance, there is an implied 90% probability that New York City will have a warmer than normal January. January will likely finish with a mean temperature near 35.0°.

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