Preliminary Winter 2010-2011 Outlook

Preliminary Winter 2010-2011 Outlook
NYC Weather

With October coming to an end and only a little more than one month left until meteorological winter begins, I decided to work on my preliminary winter outlook. I consider it to be preliminary because there is still uncertainty on the month by month pattern and the smaller details such as the NAO and AO, details that I discuss in more details with my final outlook which will be posted around early-mid November. In this outlook, I will discuss the current scenario we are seeing with the La Niña, the patterns that are expected to make up this winter, and a general outlook for the Mid Atlantic and the rest of the United States.


Part 1: Factors for my winter forecast


La Niña:

Last winter, we were dealing with a strong El Niño. During the spring, the El Niño rapidly collapsed, with a La Niña starting to form. The models, however, did not handle this rapid drop well, most of them expecting the La Niña to stay in the weak territory, however the La Niña intensified more than expected and is unofficially a strong La Niña. In order to officially become a strong La Niña, it would need to have departures greater than -1.5o C for 3 months. I expect the La Niña to peak in intensity within the next month, then slightly weaken and remain steady in the moderate territory.

When the La Niña developed in the summer, it was generally east based, meaning that the coldest departures were found in ENSO regions 1+2 and 3, or the eastern regions, with smaller negative departures in the western ENSO regions, 3.4 and 4. Since then, however, the La Niña became less east based and more basin wide, occasionally fluctuating to slightly east based. At this time, I am expecting ENSO regions 1+2 to remain steady with regions 3.4 and 4 slightly cooling down, leading to a basin wide, occasionally slightly west based La Niña for the winter. In general, a west based La Niña means widespread warmer than normal temperatures in the East and below normal temperatures in the west with a stronger SE ridge, and an east based La Niña has colder departures further east, into the Northeast and northern Mid Atlantic with a weaker SE ridge.


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. Currently, we are in a negative NAO which lasted through most of the year, making it one of the longest lasting –NAO in the last few decades. The negative NAO has been weakening, though it is still negative as of now. While the NAO is more predictable in the shorter range, and I will discuss the NAO/AO/PNA in more details with my final outlook, I currently expect a slightly negative NAO for December, with a neutral to occasionally positive NAO for January and February.


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. As with the NAO, it is more difficult to predict the PNA in the longer range, though at this time, I expect a slightly positive PNA for December, with a neutral to negative PNA for January and February.

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 is expected to end up with slightly above average temperatures, which could support a slightly warmer than average winter.

The SE ridge, typically associated with a +NAO pattern, 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. This winter, I expect a neutral to occasionally positive NAO to develop for January and February, which is when I think the SE Ridge will become more frequent, but especially with the La Niña being central based, I do not think it will extend as far inland as it did in some La Niña years, but there could be some times where it briefly extends further inland. 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 2: Forecast For The Mid Atlantic (Including NYC Area) And Northeast


This winter will be the complete opposite of last winter. Unlike last winter, when suppression took place with frequent snowstorms for the Mid Atlantic with below normal temperatures, and drier and warmer than normal conditions for New England, the storm track this year will be further north, though there will be a few times where we could see coastal storms, but nothing like those of last year.

With a negative NAO/AO expected, winter will likely start with near/below average temperatures, with opportunities for snow as well as mixed precipitation in the Mid Atlantic. By January, however, I am thinking that the NAO/AO could become more neutral, occasionally becoming positive, and with a SE ridge developing, should bring drier and warmer than normal conditions for the southern Mid Atlantic, slightly above normal temperatures and average precipitation for the northern Mid Atlantic, and near to potentially slightly above average temperatures in most of the Northeast. The storm track will frequently run through the Northeast, occasionally into the Great Lakes, leading to heavy rain events for the northern Mid Atlantic and southern Northeast, with wintry precipitation in the central/northern Northeast.

The ridge could persist into parts of February, however it is likely to then start weakening, which would lead to colder temperatures returning to the Northeast and potentially the northern Mid Atlantic. With above average precipitation likely, heavy snowfall is possible in the Northeast, with snow chances returning to the northern Mid Atlantic. There could also be occasional brief yet strong cold spells that drop into the Mid Atlantic.

Summary: Overall, winter is likely to bring above average temperatures for most of the Mid Atlantic and potentially parts of SNE, with slightly below average temperatures for northern New England and the western Northeast. Precipitation will be below average in the southern Mid Atlantic, near normal in the central and parts of the northern Mid Atlantic, and above average from the northern Mid Atlantic into the Northeast. Most of the snow in the central/northern Mid Atlantic will come from overrunning events which will also bring mixed precipitation, weak Alberta Clippers that bring light to moderate snowfall amounts, and potentially a few coastal storms.


Part 2A: Forecast For Parts Of The United States


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 average temperatures this winter, as strong cold air masses dropping from Canada will focus on this region. Precipitation is expected to be near average, with snowfall also near, potentially slightly above average.

Great Lakes: As the storm track will occasionally move through the Great Lakes, or in the 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 and some places see rain/mix, there will likely be a lot of snow in this region, with above average snow likely.

Southeast: December will likely start out as 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, but this will likely not be too frequent.


Uncertainty:

This is only a winter forecast, and with it still being my preliminary outlook (my final outlook will be posted in early-mid November), there is still some uncertainty with parts of the forecast. Some uncertainties include the southern/western ends of the above average snow area, the extent of below/above average temperatures in the west, and how far north the above average temperatures extend in the Mid Atlantic.



Experimental forecast snow totals by city:

New York City: Average: 26. Forecast: 19-31
Philadelphia: Average: 19. Forecast: 15-24
Washington DC: Average: 16. Forecast: 9-16
Richmond: Average: 13. Forecast: 3-7


Part 3: Forecast Maps For The United States