Final Winter 2012-2013 Outlook

This is the only winter outlook posted this year; there was no preliminary outlook. Last year's final outlook can be found here. As with the last 2 years, an updated winter outlook will be posted in early January.

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Final Winter 2012-2013 Outlook
NYC Area Weather

For the second consecutive year, winter is already off to an early start for the NYC area. Only a year after last year's historic late October snowstorm, a significant snowstorm affected the area on November 7th this year with another minor snowstorm on the 27th, as this November ended up with the coldest monthly temperature anomalies for the area since early 2011. The early October snowstorm was as close to a winter as last year got in terms of snow, however; a mild, snowless and progressive pattern persisted through the entire season, resulting in one of the warmest and least snowiest winters on record. While the winter will be off to a somewhat mild start, unlike last year, there are already significant differences that will prove this year to be different than last year with more cold and snow, potentially even above average.

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A. Background Information


1. ENSO Overview

The ENSO, El Nino-Southern Oscillation, has two main phases: La Nina and El Nino. Sea surface temperature anomalies near the equatorial Pacific are used to determine if an El Nino, La Nina, or neither are present, with four ENSO regions between 80W and 160E longitude used to determine the ENSO state. Warm SST anomalies represent El Nino conditions, and cold SST anomalies represent La Nina conditions. Region 3.4 is often used to determine the state of the ENSO, with the other regions used to determine whether the El Nino or La Nina is west or east based. Below is a CPC graphic representing these regions:

 

Officially, an El Nino is classified when the SST anomalies in region 3.4 are at least 0.5°C warmer than average for 5 consecutive 3-month running means. The same applies for a La Nina but with SST anomalies 0.5°C colder than average. When SST anomalies fail to meet the criteria above, it is often called neutral ENSO, representing neither a La Nina nor an El Nino.

For the last two years, a La Nina has been present, with the last El Nino back in 2009-2010. El Nino conditions began to develop this summer, but have failed to progress further and instead reversed this fall when SST anomalies dropped closer to average; the Nino 1+2 region had colder than average anomalies for a brief time earlier this fall. Since then, SST anomalies have slightly rebounded, although it appears unlikely that it will be enough to reach the official criteria for an El Nino. As such, this year is likely to be classified as neutral-positive ENSO conditions, leaning towards an El Nino with positive SST anomalies but not enough to make it an official El Nino. Despite the lack of an official El Nino, the warmest SST anomalies are located further west, towards regions 3.4 and 4, meaning that west based El Nino conditions are currently in place. West based El Ninos can be more favorable for cold in the eastern US than east based Ninos, as seen with 2009-2010 which was a very snowy winter for the Mid Atlantic, but the signal is likely to be less evident this year especially with this El Nino much weaker than 2009-10's moderate El Nino. It should be noted that the recent pattern has some similarities to a La Nina-type pattern in the US, with below average precipitation in the south and above average precipitation in the north expected towards the medium range.

2. Teleconnections

Teleconnections such as the NAO, PNA and the AO also have an impact on the pattern in the US. Although it is difficult to forecast these indices past 1-2 weeks with high accuracy, they can still be useful for long range forecasting.

a. NAO

The NAO, or the North Atlantic Oscillation, is an index that can either be positive or negative, and its phase is determined by the North Atlantic pattern. The graph to the left provided by the NOAA/ESRL/PSD (http://www.esrl.noaa.gov/psd/forecasts/teleconn/) shows a typical +NAO set up, with lower than average 500mb heights near Greenland, keeping the cold air over Canada, and above average 500mb heights with warmer than average temperatures in the eastern US. A negative NAO is present when there is a ridge near Greenland, which displaces the cold air further south and is often associated with blocking, with more frequent below average temperatures in the central/eastern US. The -NAO can be west, central or east based, which have different impacts on the pattern in the US regarding cold and snow chances.

Last year, the NAO was persistently positive, with almost no ridging to be found over Greenland. Since March earlier this year, a negative NAO pattern developed, and has persisted through this fall with ridging near Greenland and high latitude blocking much more frequent than last year at this time. At this time, the NAO is likely to average out as slightly negative for the winter, with periods of positive NAO expected at times such as in the first half of December, but without a positive NAO lasting throughout the majority of the winter as with last year.


b. PNA

Another factor for this winter is the PNA, or the Pacific/North American Pattern. The graph to the left provided by the NOAA/ESRL/PSD shows a typical +PNA set up, associated with a ridge near western North America, while a negative PNA is associated with below average 500mb heights in this region. The PNA usually reflects what happens in the central and eastern US; a +PNA occasionally translates into a central/eastern US trough with colder than average temperatures, and a –PNA often translates into a central/eastern US ridge with warmer than average temperatures. A -PNA does not indicate that warmth is certain in the East, however; the warmth could be overcome by strong blocking associated with a strong -NAO and -AO pattern, as observed during December 2010.

Last year, the PNA averaged out to slightly positive, but did not result in a cold pattern for the US especially with the progressive pattern in place and the lack of significant amplification in the pattern. This fall, the PNA started out more positive, but has gradually trended towards more neutral to negative with occasional periods of a positive PNA. For this winter, the PNA is expected to average out to slightly negative to neutral, with occasional periods of a +PNA expected, especially later into December and January.


c. EPO

The EPO, or the East Pacific Oscillation, is another factor that is used for long range forecasting. The graph to the left provided by the NOAA/ESRL/PSD shows a typical +EPO set up, consisting of below average heights near Alaska and above average heights in central/SE Canada and the northern US, which results in above average temperatures across the northern US and southern Canada. A negative EPO pattern would bring the opposite conditions.

Last winter, a positive EPO pattern persisted throughout the majority of the season, with most of the cold air trapped in Alaska while a progressive pattern was observed across North America. This year, since late September the EPO has been persistently negative, although is expected to become persistently positive for the first half of December. The EPO is expected to be more on the variable side this winter, averaging out close to neutral or slightly negative.


d. PDO

The PDO, or the Pacific Decadal Oscillation, as with the other variables can be positive or negative. A positive PDO is associated with colder than average SST anomalies in the northern Pacific, with the opposite conditions for a negative PDO. The PDO has been persistently negative since 2010 and especially this fall, with a -2.21 value during September. Since 1900, 19 other Septembers had a PDO near or below -1.0; out of these 19 cases, 14 of them were followed by a -PDO winter, although a noticeable number of these years had at least some moderation in the -PDO compared to the fall values. Considering this, as well as that the PDO has been negative, strongly negative at times, since 2010, with cool SST anomalies persisting in the northern Pacific, a -PDO is expected to continue through this winter.



B. Fall - Winter Pattern Outlook


While predicting a full winter season often does not have high accuracy, one thing that is certain is that the pattern this winter will not be the same as the last one. Despite the interestingly similar beginnings, including a hurricane followed by an early season snowstorm, there are many other differences that set this year apart from last year. The simplest difference is that there is actually more frequent high latitude blocking in North America and near the north pole, without a persistent polar vortex near Alaska. Last year, the AO and NAO were almost entirely positive with a lack of any ridging near Greenland and Alaska, with ridging staying transient in every case except for a briefly stronger ridge in the western US in early February, which also broke down shortly afterwards. The pattern during this year's fall, however, set up differently, with occasional periods of blocking near the west coast as well as Greenland, which has seen occasional blocking since a pattern change in late March. The pattern for November was significantly different than last year's November, which was warmer than average; this year's November was colder and much snowier than average, and is the 6th snowiest November on record.

With the differences, it is unlikely that this winter ends up again with well above average temperatures and well below average snowfall. However, the extent of the cold and snow this year is the main question. A few days ago, a strong ridge built over the north pole with the AO becoming very negative. Initially, it had appeared that this could lead to a colder December, although the opposite is currently unfolding for the first half of the month. Despite the blocking over the north pole, however, there is none to be found near Greenland, with a persistent area of lower 500mb heights over the Gulf of Alaska expected to keep the US under the influence of a mid Pacific air mass starting on December 1st. This period of strong blocking over the pole is not expected to remain long lasting, which along with the Gulf of Alaska trough, slightly below average 500mb height anomalies near Greenland, a weaker than modeled MJO signal, and an absence of persistent western US ridging, are likely to keep the pattern warmer than average through the first half of December, perhaps beyond. While this pattern will not be as warm and snowless as last year, it will remain unfavorable for sustained cold and widespread snow events in the East, with cold air masses remaining transient and quickly moving out while the storm track stays mostly in the northern US into Canada.

Last year, the model guidance frequently showed signals for a pattern change in the medium range starting in November, yet the pattern never changed from a mild and very progressive one until late March. While changes in the pattern will likely take more time to unfold than what the models are showing, this is not expected to be another case of the pattern change that never came. Towards the middle of December, there appear to be some changes towards what could be a somewhat colder pattern for the eastern and central US, but some of the model guidance may be breaking down the Gulf of Alaska trough too quickly; along with a lack of persistent ridging near Greenland and the western US, a more gradual change over the second half of the month with somewhat colder temperatures especially in the northern half of the US into the Northeast is favored over a significant pattern change early in the month with widespread strong cold temperature departures for the East. It may still remain difficult to observe persistent ridging near the western US, with the PNA likely to average out to slightly negative this winter, although transient periods of ridging in the western US will provide more transient cold opportunities especially towards the second half of December and into January and February. While December is not expected to be significantly warmer than average like last year, the transient nature of these troughs may also limit how cold the month gets, and my forecast is for slightly above average temperatures in December; should more changes evolve in the pattern especially beyond 12/15 with more persistent ridging near the western US and ridging near Greenland, the possibility is there that December could end up slightly colder than average. At this time, there appears to be a better chance of more widespread cold departures and perhaps more snowstorms in January across the region, especially should more significant changes take place in the pattern with ridging returning to Greenland. This is expected to take time to unfold, but unlike last year, there is a higher probability of changes in the pattern that would be more favorable for more sustained cold and snow across the eastern and central US, especially with the tendency for high latitude blocking this winter compared to the last one.


C. Monthly Outlook for the Region


December:
Slightly warmer than average, slightly below average precipitation and snowfall.

NAO: neutral
PNA: slightly negative
EPO: slightly positive
AO: negative

January:
Colder than average, average precipitation and slightly above average snowfall

NAO: slightly negative
PNA: slightly negative
EPO: neutral/slightly negative
AO: negative

February:
Average temperatures, slightly below average precipitation, average snowfall

NAO: neutral
PNA: neutral/slightly positive
EPO: neutral/slightly negative
AO: neutral

Overall winter outlook:

December: +1 to +2 degrees
January: -2 to -4 degrees
February: -1 to +1 degrees
Overall: 0 to -2 degrees
Snowfall for NYC: 24-34 inches (Average: 26 inches)


D. Outlook for the United States

Overall, this winter will be more active than the last one, with more cold temperature anomalies in the US and more frequent snow events. While the winter will be slow to start, with warmer than average temperatures and below average snowfall for the first half of December, perhaps beyond, a more active pattern with more cold and snow is possible towards January and potentially February for the eastern and central US. My temperature outlook is for near to slightly below average temperatures to cover the majority of the eastern half of the US, with slightly warmer than average temperatures for the west and southwest. The best chance for colder than average temperatures would be near the Great Lakes and Ohio Valley regions. Near to slightly above average snowfall is also likely in these regions, with the best chance of above average snowfall in the Ohio Valley, Great Lakes and the Northeast.

Long range forecasts are more risky than shorter range outlooks and have a lower confidence level; unlike last year, when there was more confidence in the pattern generally staying mild and mostly snowless, there is less confidence in the pattern outlook for this winter. As such, there are areas of uncertainty with the winter outlook. This outlook assumes that the pattern will change towards late December with a colder and snowier January for the eastern and central US. Should a stronger western US ridge signal emerge with the return of blocking near Greenland, the winter may overall end up colder than currently expected, with slightly below to below average temperatures stretching across the majority of the eastern half of the US, including the Southeast. The possibility is also there that the persistent trough in the Gulf of Alaska and the recent lack of ridging near Greenland continue well into December and perhaps into January, with a lower probability of a colder pattern setting up for the eastern US. In that case, the winter could end up again warmer than average with below average snowfall. An updated winter outlook will be posted in early January, reviewing the December pattern compared to the outlook and making any necessary changes to the outlook.