Long Range Pattern Outlook #5

Brief Overview:

Tonight's long range pattern outlook focuses on the end of November into early December, regarding the medium range pattern and an expected pattern change, along with verification for the 2nd pattern outlook from 10/16. The final winter outlook will be posted next Tuesday.


Click below to read the full post.

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Pattern Outlook Archive

<< Pattern Outlook #4 - November 11, 2012
Pattern Outlook #5 - November 21, 2012
>> Pattern Outlook #6 - December 10, 2012

Winter Outlook 2012-2013 - November 30, 2012

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Expected Pattern Teleconnections:

• NAO - negative
• AO - negative
• PNA- negative, becoming neutral late
• EPO - negative, becoming neutral late

Medium-Long Range Outlook

Slightly revised on 11/22

Over the last week, the pattern generally stayed the same with a zonal flow dominating the US with above average heights in the northern US into central Canada. Meanwhile, a high pressure was located on top of the Northeast, with a persistent NE flow across parts of the region for the majority of the week. Temperatures are slightly warmer through Friday ahead of a cold front on Friday night (11/24-25), which will be followed by a strong transient cold air mass.

This will be the start of a larger scale pattern change starting to take place. The polar vortex is expected to be disrupted, with above average heights extending from Alaska to Greenland as a -NAO develops. Meanwhile, a low pressure will develop in the central US and move towards the region on the 27-29th. The exact track remains uncertain, although a track is currently likely through the Northeast or Mid Atlantic regions, with rain and/or snow likely.

The more noticeable changes in the pattern take place after this event; unlike last year, when talks of a pattern change started in November but didn't verify until March, this time there are actually stronger signals supporting a pattern change within the medium to long range. A strong trough will move into the region behind this storm, with colder temperatures returning for a few days. Meanwhile, ridging is expected to persist near Greenland with a continued -NAO pattern, something that has not been seen at any point during last year's winter. High latitude blocking has been much more frequent this fall than during last year at this time, and is a trend supportive of a colder pattern for December. In addition, The MJO is likely to end up near Phase 1 by early December, which favors below average heights over the region with above average heights in Greenland and the western US. This is the opposite of last year's pattern when the MJO was stuck in phases 4-6, which favor warmth across most of the US.

At this time, a significant pattern flip to a persistent cold and snow pattern is not expected; neither is a persistently cold pattern for the start of December. Despite a negative NAO, the start of December is unlikely to feature widespread cold up through at least the first week, with temperatures at least near to slightly above average and precipitation likely near to below average. However, latest indications point that unlike last winter, there is a higher probability of actually receiving a sustained period of cold and snow this winter, with the first potential for a more sustained period of cold and potentially snow going forward into the middle of December as the pattern gradually trends colder. The winter outlook will discuss the December pattern in more details, although the latest trends are clearly pointing towards a more active December pattern than last year, with more cold and snow expected.

November Temperatures: As of the 21st, there has been no significant warmth this month other than a brief warm spell after the early month snowstorm, and the NWS stations in the immediate NYC area so far are 3 to 5 degrees below average since November 1st. With no significant warmth upcoming, this November is finally set to be the first month to feature widespread below average temperatures across the region in over 20 months, an unprecedented stretch of near-warmer than average months.

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Pattern Outlook #2 Verification

Link to Pattern Outlook #2

Pattern Outlook #2, posted on October 16, expected a brief surge of warmth around 10/21-23, but with a strong ridge failing to build into the region as temperatures remain close to average with a storm track through the north central US. The potential was mentioned for a colder pattern at the end of October into early November, although a persistent cold pattern was not expected to set up.

Above, I posted the GEFS initialized 500mb height anomalies across North America from 10/23 at 18z, the same time frame from the posted GEFS image in the pattern outlook. As modeled, this shows a strong trough in the western US along with significant positive height anomalies from Greenland through northern Canada and into Alaska. There was minor ridging in the central and eastern US but without significant positive height anomalies. Following this time frame, ridging was stronger than expected going into the end of October, although this came as a result of the highly anomalous pattern that set up, allowing hurricane Sandy to strike the region as a historic hurricane with a highly unusual track. This pattern outlook was written 13 days before Sandy; while the potential was there for a tropical cyclone to form in the Caribbean, it was too far out at that time to expect a tropical cyclone to move up the coast and strike the Northeast; the first model to catch onto Sandy having direct impact in the region wasn't until 10/20. Although hurricane Sandy was not forecast, the colder pattern did verify from the end of October through the first week of November, and exceeded expectations with its intensity and duration while resulting in a significantly colder than average start to the month across the region.