Long Range Pattern Outlook #7

Brief Overview:

Today’s long range pattern outlook focuses on the medium range pattern going into early January, with growing indications that the pattern changes evolving during late December continue into early January with a cold pattern for the region, possibly including several snowstorms as well, one of them on December 29-30.



Pattern Outlook Archive

<< Pattern Outlook #6 – December 10, 2012
Pattern Outlook #7 – December 24, 2012
>> Pattern Outlook #8 – January 10, 2013


Expected Pattern Teleconnections:

  • NAO – neutral
  • AO – negative to neutral
  • PNA– neutral to positive
  • EPO – neutral

Medium-Long Range Outlook

Scroll down to the bottom for a brief summary

The previous pattern outlook mentioned how the first half of December ended up even warmer than expected, but a colder pattern was expected to slowly evolve with at least one snow potential for the area between 12/20 and the end of the month. So far, December has been abnormally warm, and only one snow event has been observed in Central Park with a trace of snow observed on the 22nd from isolated flurries. As expected, changes in the pattern are resulting in a gradually colder and stormier pattern for the last week of the month; three storms are expected for this coming week, with the first two producing snow for the interior parts of the area. More pronounced changes have been taking place recently in North America, with a strong block currently making its way from Alaska towards central Canada. There has been no blocking of this magnitude in this part of North America over the last year and a half. Although this block will quickly exit towards Atlantic Canada as a storm on the 26-27th affects the region, no persistent troughs are expected to redevelop in the Alaska and Gulf of Alaska region.

By the time that the third storm will reach the region, on December 29-30, more noticeable changes will develop in the US pattern. The PNA, having been firmly negative since early November, is expected to rise and turn positive for the first time since October. Ridging is expected towards the NW US and western Canada, with a persistent area of lower heights to the south, near and west of California. Meanwhile, the storm on the 26-27th is expected to move towards Newfoundland while another storm makes its way towards the region around the 29-30th. With the different pattern in place including the low near Newfoundland and rising heights in the Northwest, this storm is not expected to track far west of the area, and has the potential to be a significant nor’easter with snow for parts of the region, perhaps including the NYC area.

With this storm, the transition to a colder pattern is expected to have taken place with a large, strong trough settling over the eastern half of the US, as shown to the left from the latest GEFS 500mb height anomalies from the PSU e-Wall. These changes have been supported by the recent medium range guidance and ensembles, as well as the CFSv2, which has been consistently showing a widespread outbreak of cold into the US including the region around early January. The CFSv2 has also verified with showing a much warmer than average December with above average temperatures covering the majority of the US. The NAEFS (North American Ensemble Forecasting System), having verified with the warmth in December and in the winter of 2011-12, is showing an absence of above average temperatures in most of the US, with near to below average temperatures for the eastern half of the US. Along with the changes in the Pacific observed for the first time since October, there is a higher probability of a cold pattern developing for the region at the end of the month starting with the 29-30th storm, with average to colder than average temperatures likely through early to mid January, before the pattern potentially trends warmer again. With the cold pattern expected, the question is how much snow falls. Following the 29-30th storm, a dry New Year’s Eve is possible, and at this time the beginning of this colder pattern appears to be dry without much, if any storminess. Given the colder pattern, however, snow events are likely to affect the region during the first half of January.

Summary: The changes in the pattern that began early in the month are still ongoing, and this week marks the transition to a stormier and somewhat colder pattern for the region, with two snow events for the interior and a potential nor’easter on the 29-30th, which may produce a snowstorm for parts of the region, including the area. Behind this storm potential, a more sustained cold pattern is likely to develop for the first half of January, and while this pattern is expected to be dry at first, more snow potentials are likely before the middle of the month.

Pattern Outlook #4 Verification

Verification for Pattern Outlook #4 will be added soon.

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