Final Winter 2010-2011 Outlook

Final Winter 2010-2011 Outlook
NYC Weather

Meteorological winter is only days away, and the pattern we are seeing now is already becoming a winter-like pattern, with several strong arctic air masses dropping into the Midwest, which will reach the area in a modified version after Thanksgiving. Winter 2009-2010 was a historic one for the Mid Atlantic, with over 80 inches of snow in the Washington DC area, the snowiest month on record in the NYC area, as well as a late February blizzard that brought NYC one of its biggest snowfalls on record.

Last winter, most signs pointed out to a cold and snowy winter in the Mid Atlantic, and there were even indications that the winter could be a historic one before the winter started. While this year’s winter outlook was a little more difficult to make, as there are more uncertainties than there were last year, signs are pointing towards a much less extreme winter in the northern Mid Atlantic, with snowfall ranging from slightly below to above average, with the cold and snow focusing further north and west, which I will discuss in more details below.


Part A: Factors For The Winter Outlook


La Niña:

So far in 2010, we've gone from a strong El Niño in the start of the year to a borderline strong La Niña in the fall. Such a rapid transition is not something that was common in the last 60 years, though it has happened before. Last winter's strong El Niño was different from most strong El Niños, which are typically east based and associated with a mild and dry northern US and a cold and wet south. Last winter had a west based El Niño, which combined with the right amount of cold and an active southern branch, led to historic snowfall in the Mid Atlantic.

This year's La Niña is also different than most strong La Niñas. While a typical strong La Niña would be west based, with a +NAO and -PNA pattern leading to a storm track through the Great Lakes/Midwest with a warm east and a cold west, such as the winter of 2007-08, this La Niña started out east based and is still east based, even though it may transition to more of a central/slightly west based La Niña later on in the winter.In addition, the NAO has generally been negative, when it would typically be positive with a strong La Niña. On average, an east based La Niña has less warmth and more cold in the east, with very cold temperature departures in the north central US.

The La Niña developed as an east based one in the summer and fluctuated between east and central based in the fall, however recently the La Niña became much more east based as the western ENSO regions saw weakening of the cold departures, which gives a high confidence level on this winter staring out with an east based La Niña. In my preliminary winter outlook, I expected the La Niña to be more central based by now, though this has failed to happen so far. Despite this, some La Niñas that started out as more east based became west based by the end of the winter, and while I do not think that this La Niña becomes a completely west based La Niña, I continue to expect gradual shifting to a central based, potentially slightly west based La Niña by the end of the winter. It is also possible that the La Niña simply stays east based through the whole winter, which could lead to slightly colder/snowier conditions in the East than I am expecting.

For the intensity of the La Niña, it has reached a peak earlier this month, and while La Niñas typically peak in the middle of the winter, it is possible that the La Niña may already have peaked. From the start, the models were showing this La Niña peaking unusually early, it’s now November and it appears that the La Niña either peaked in intensity or may peak in intensity within the next month, with weakening expected later in the winter.

NAO:

The North Atlantic Oscillation (NAO) is an important factor in addition to the La Niña. When the NAO is negative, it typically results in a colder East coast, and when it is positive, warmer temperatures generally take place in the East. On average, the NAO tends to be positive during La Niña winters. Last winter, we saw a strong -NAO which contributed to the unusually favorable set up of cold and snow in the Mid Atlantic. This year, we saw a consistently negative NAO, strong at times, which was one of the longest lasting -NAO periods in the last few decades. The La Niña is currently moderate, and we have been seeing a -PNA so far, which is also frequently associated with stronger La Niñas, however the NAO continues to be negative, with a strong -NAO expected for the end of November into early December. While a -NAO does not always mean that it will be cold in the East, as we are now seeing with a -NAO and a -PNA a southeast ridge is expected for early this week, but it won't be as strong as it could've been as the -NAO is preventing the SE ridge from becoming even stronger, which will be something that may repeat itself at times this winter.

For this forecast, I am expecting a generally negative NAO to continue through December into potentially early-mid January with an occasional period of +NAO possible, with a more variable NAO from January into parts of February where +NAO periods could become more frequent, but with –NAO periods also expected.

PNA:

The Pacific/North American Pattern (PNA) is also an important factor in the general pattern across the US. When the PNA is positive, that means there is a ridge in the western US resulting in above average temperatures, and can sometimes mean a trough in the eastern US resulting in below average temperatures. When the PNA is negative, a trough is likely for the western US with below average temperatures, which can mean a ridge in the eastern US bringing above average temperatures. So far this month, we have been seeing a –PNA in place, which led to cold conditions in the west and north central US. Despite the NAO being negative, the –PNA and –AO which have produced and will likely continue to produce strong arctic air blasts into the north central and NW US have occasionally led to a ridge in the eastern United States. This ridge would typically be a strong one with a +NAO, however since we are seeing a –NAO, this ridge is weak compared to what it could have been.

At this time, the models are expecting a trend towards a neutral PNA by early December, leading to an increased potential of cold and snow in the Mid Atlantic/Northeast. For this forecast, I am expecting a variable PNA through most of December into early January, which can range from negative to neutral with occasional positive periods, with an occasional +PNA possible in the second half of the winter but with a –PNA more common.


Other Factors:


It has been observed that sometimes, colder than average Octobers result in colder than average winters for Central Park, NY and Philadelphia, PA. 2009’s October was slightly colder than average and resulted in an average winter. 2007’s October had record warmth and the following winter was unusually warm. October 2010 brought below average temperatures, and the following winter brought near-slightly below average temperatures to the Mid Atlantic. This October ended up with slightly above average temperatures, which could support a slightly warmer than average winter.

The SE ridge is something that is usually seen with La Niñas, especially strong La Niñas. The SE ridge is a high pressure off the East Coast, which tends to bring above average temperatures to the SE US, or if it’s strong enough, to all of the eastern US. We have already seen the SE ridge appear from time to time this month due to a –PNA keeping the cold further west, however it was unable to become strong due to a persistent –NAO. While I think that the NAO will be slightly positive at times, I am expecting a generally neutral to occasionally slightly negative NAO, meaning that I’m expecting the SE ridge to generally be weak when it does appear, though there could be a few periods of time where the SE ridge becomes strong, combined with a +NAO, especially in parts of January and February. There will also likely be some times where we see a –NAO/–AO and a strong cold spell can briefly push into the Mid Atlantic and/or parts of the SE, but such strong cold spells likely won’t be too common, with most of the intense cold spells focused in the north central US.


Part B: Forecast For The Mid Atlantic


As previously mentioned, a –NAO, -AO and neutral PNA pattern is expected to start December, which may lead to a favorable time frame for cold and snow in the Northeast and potentially the Mid Atlantic for the first week of December. The pattern in December will likely be a variable one with periods of cold and snow, however the pattern will mainly depend on the PNA, as when the PNA is negative, the cold will be focused further west, with milder conditions for the East and an inland storm track, and when the PNA is more neutral, there should be more cold/snow periods affecting the region, especially the northern parts of the Mid Atlantic. The storm tracks could range from a few coastal lows to mainly inland tracks, Alberta clippers, and a storm track through the Great Lakes or Northeast with a Miller B low, or redevelopment near or off the Mid Atlantic/NE coast. For December, I am expecting slightly below average temperatures with occasional periods of slightly above average temperatures, with near to slightly below average snowfall for the southern Mid Atlantic and slightly above average snowfall for the northern Mid Atlantic.

December patterns:
Variable PNA, negative to slightly positive
Negative NAO


January could be a more variable month, though it appears that especially by the second half of January, we could see a transition to a pattern a little more similar to that of a typical La Niña, with a more negative PNA, and the AO/NAO still slightly negative but with more periods of slightly +NAO possible. The SE Ridge should appear more frequently in this month, and could also be briefly strong at times. As a result, I am expecting slightly above average temperatures further north and above average temperatures further south, with slightly below to below average snowfall.

January patterns:
Negative to occasionally neutral PNA
Slightly negative – slightly positive NAO


February is not expected to be a cold and snowy month at this time, though there should be differences from January. I am expecting the pattern to become more active in February, and based on analogs from most La Niñas, above average precipitation could be a good possibility from the central/northern Mid Atlantic into the Northeast and further west. The SE ridge could be weaker in February than in January, with colder temperatures returning to the NE US, which combined with the colder temperatures could lead to increased snow in the Northeast and snow/mix events in the central/northern Mid Atlantic. While the main storm track should be inland, there should also be snowfall from Alberta clippers, overrunning events, and Miller B storms that are inland first then redevelop off the coast of the Mid Atlantic. At this time, I am expecting slightly above average temperatures and snowfall slightly below average in the south and near average in the north, though the forecast departures for February have less confidence than those of the earlier months.

February Patterns:
Neutral-slightly negative NAO
Negative-neutral PNA


Winter Summary:


Overall, this winter should be less extreme than last winter, but near to slightly above average snowfall is possible in the northern Mid Atlantic, with near-slightly below average snowfall in the central Mid Atlantic and slightly below to below average snowfall in the southern Mid Atlantic. The average storm track should be inland, in the Northeast and Great Lakes, though a few storms may be coastals or move into the Midwest, with other tracks including Alberta clippers, overrunning events, and Miller Bs that are inland storms which redevelop off the Mid Atlantic coast. There should be more mixing with sleet and freezing rain than last year.


Part C: Forecast For The Rest Of The US


Western US: As the storm track moves through the NW US, expect above average precipitation in the Pacific Northwest, with a wetter winter than last year. With colder than average temperatures also likely, expect above average snowfall.

California will see a much drier winter compared to last year, and while some rainstorms are still possible, most of the storms as well as the storm track will be focused to the north, with near average temperatures and below average precipitation likely.

Midwest: (Minnesota, N/S Dakota, Montana): Expect below to well average temperatures this winter, as strong cold air masses dropping from Canada will focus on this region. Precipitation is expected to be near to slightly above average, with snowfall also slightly above average.

Great Lakes: As the storm track will focus on the Great Lakes and Northeast, this will likely be an active region this winter. Above average precipitation is expected for most of the Great Lakes, and as the storm track is expected to be close to the region with colder air to the west, below average temperatures are also likely, but not as cold as those in the Midwest. While there could be a few times where the SE ridge could be strong enough that the storm track is near the Great Lakes/Midwest and some places see rain/mix, there will likely be a lot of snow in this region, with above average snow likely, especially in the second half of the winter.

Southeast: December could be more of an average month, but as the SE ridge develops, warmer than normal temperatures and drier than average conditions are likely to develop and persist for a good part of the winter. There will be a few times when strong yet brief cold spells could make it down to the SE along with some snow, but this will likely not be too frequent. Above average temperatures and below average snowfall is expected.


Experimental forecast snow totals by city:


New York City
December: -1 to -3 degrees
January: +1 to +3 degrees
February: 0 to +2 degrees
Overall: -1 to +2 degrees
Snowfall: 20-32 inches (Average: 26 inches)

Philadelphia, PA
December: -1 to -3 degrees
January: +1 to +3 degrees
February: 0 to +2 degrees
Overall: 0 to +2 degrees
Snowfall: 15-25 inches (Average: 19 inches)

Washington DC
December: -1 to -3 degrees
January: +2 to +4 degrees
February: 0 to +2 degrees
Overall: +1 to +3 degrees
Snowfall: 9-16 inches (Average: 16 inches)


Part D: US Winter Maps









**Note:** As this is a long range outlook, there are some uncertainties on the exact patterns for each month. If necessary, a separate monthly outlook will be posted for each month as it gets closer.


Edit On November 22:

While I did not mention it in the winter outlook when it was posted, the pattern is expected to turn more favorable for more cold and snow in the Northeast by the second half of February, which may reach the northern Mid Atlantic. Depending on the La Nina and the pattern, it is possible that March could end up being a colder and potentially snowy month for the Northeast and even parts of the Mid Atlantic.

As said in the note above though, there are uncertainties on the exact patterns as this is a general long range outlook, and a separate monthly outlook will be issued as March gets closer.