Climatological factors in the forecast
As many of you will remember, last year's winter pattern was driven by two factors, a strong La Nina and a strongly negative North Atlantic Oscillation (NAO-) (both link to Wikipedia). Combined, they produced a hellacious winter in the Northern Plains and eastern U.S. While the NAO is somewhat difficult to predict more than a couple of weeks in advance, scientists are fairly confident that we will experience a second straight La Nina winter after briefly emerging from La Nina conditions earlier in 2011. (La Nina is a cooling of the waters of the equatorial Pacific.) The consensus seems to be that it won't be as strong of a La Nina as last year, however. The graphic below shows the general or typical conditions expected for North American winters during a La Nina pattern. The jet stream pattern under average La Nina conditions blows across the southern U.S. from origins in the central Pacific (off the Mexican coast) as shown below. However, at times, the polar jet stream will take large dips into the eastern U.S bringing a blast of cold air.
|A typical La Nina pattern and resultant effects on North American winter|
Based primarily on a weak to moderate La Nina episode this winter, NOAA, Accuweather, and others have put together their winter outlooks for 2011-2012. In general, the effects are similar to last year only slightly tempered due to the weaker La Nina expected.
NOAA, the parent organization of the National Weather Service, predicts well above normal temperatures for the southern U.S. from the Desert Southwest to the Lower Mississippi River Valley, while cooler than normal conditions will be found on the west coast and colder than normal across the Northern Plains into the western Great Lakes. Precipitation-wise, the northern half of the country is expected to be wet, especially across the Pacific Northwest and Northern Rockies, as well as the Ohio Valley. Drier than normal conditions are forecast across the southern tier of the U.S., particularly the Southern Plains (where an exceptional drought is ongoing) and the Gulf Coast into the southeast Atlantic coast. Graphics below depict the NOAA forecast for December - February.
|NOAA Temperature Outlook for Winter 2011-2012. Click for larger image.|
|NOAA Precipitation Outlook for Winter 2011-2012. Click for larger image.|
|Accuweather Winter Outlook for 2011-2012. Click for larger image.|
|Farmer's Almanac Winter Outlook for 2011-2012. Click for larger image.|
I'm the first to admit I am not a climatologist, nor a very good research scientist. I'm better at the next couple of hours than the next couple of months. So in long-range outlooks, I typically take the path of least resistance and that is what I will do this time. Last winter, La Nina produced several snow storms and late winter severe weather in February and March. Temperatures were below average in December (in particular) and January and below normal precipitation throughout the winter, despite above average snowfall.
Given we are likely in for a tempered La Nina winter and the forecasts described above, I expect that precipitation and temperatures will be near normal over the course of the entire winter. However, I also believe that we will see some extremes at times, with a chance of late winter severe weather once again and at least a couple of decent chances at snow or ice. We'll see how it all works out! One thing you can be assured of - just because it's warm one day doesn't mean it will the next! In fact, it almost guarantees it won't!
Here's to a few snow storms and no ice this winter!
Erik Proseus, Meteorologist
Cirrus Weather Solutions/MemphisWeather.net
Visit MemphisWeather.net on the web, m.memphisweather.net on your mobile phone, and download our iPhone or Android apps. Nowcasting services provided on Facebook and Twitter.