Thursday, October 21, 2010

MWN 2010-2011 Winter Outlook

2009-2010 Winter Outlook
It was about this time last year that I issued my MWN Winter Outlook for 2009-2010, in the midst of a mild El Nino winter. At that time, I expected the winter to be drier than average and temperatures slightly above normal. I went one for two. According to climate statistics from Memphis International Airport for December 2009 through March 2010, temperatures and precipitation were below average in each of those months. (In fact, February temps averaged a whopping 7.5 degrees below normal.) In addition, I expected at least one "couple inch" snow event. That is exactly what happened, when a 5"+ snowfall occurred on February 8, 2010 (much more fell not too far north of the airport), in addition to an inch-and-a-half event in January.

Global patterns
For the upcoming winter, the global setup is opposite of last winter. A fairly strong La Nina event is in progress, which means cooler than average sea surface temperatures (SST) in the eastern equatorial Pacific. The bottom image below shows the Pacific SST anomalies as of early-October - note the below normal SSTs in the eastern Pacific.

What does a wintertime La Nina typically mean for the eastern U.S.? The map below shows what to expect in general terms, courtesy of NOAA. The dark blue lines are the average positions of the jet streams. From this map, the Mid-South could expect above normal temperatures, wet conditions, and a potentially active pattern due to the jet stream being positioned overhead. This combination could mean the potential for strong to severe weather at times.

Indeed, NOAA's Climate Prediction Center long-range forecasts for this winter indicate above normal temperatures and precipitation for the December-February time period - or a warm and wet winter. The last La Nina winter was 2007-2008, which you may recall produced the Super Tuesday tornado outbreak on February 5, 2008. Earlier today, NOAA issued their official winter outlook which nearly mirrors the conditions that would be expected under a La Nina, as it will be the most dominant feature impacting North American weather this winter. The images below show the temperature and precipitation outlooks from NOAA (click for larger images).

MWN's Outlook
Given the La Nina conditions in place and the confidence that this scenario will continue throughout the winter months, I believe we could see an end to the drought conditions by mid-winter (if not before) as above normal precipitation will be likely. In addition, the possibility of a mid-winter severe weather event would appear to be slightly higher than normal given the average jet stream position over the region. This jet position would favor storm systems of Pacific origin moving across the region fairly often, resulting in above normal precipitation. Increased clouds and precipitation would result in above normal low temperatures, while the position of the Arctic jet stream well to the north would mean above normal temperatures as well. Thus, temperatures will average above normal.
Farmer's Almanacs side-note: Both the Farmer's Almanac and Old Farmer's Almanac are forecasting cold conditions and average to below average precipitation. With all due respect to both, I disagree.

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