Thursday, October 15, 2009

2009-2010 MWN Winter Outlook

With the winter season not too far down the road and an increased interest in long-range forecasts, I have put together some thoughts on what we may expect to see weather-wise here in the Mid-South.

There is a plethora of data, long-range forecasts, and opinions available with regard to the upcoming winter season. I studied several that I find to be helpful, informative, or just plain interesting. Most of the information contained below comes from one of these sources. These include the NOAA/Climate Prediction Center (CPC) 90-day seasonal outlooks, the CPC's El Nino Diagnostic Discussions, NOAA's 2009-2010 Winter Outlook, Memphis International Airport climate records, recent blog posts from Cloudyand, the Farmer's Almanac Winter Outlook, and the Old Farmer's Almanac Long-Range Outlook (did you know these are not the same thing?).

Current data confirms the presence of a weak El Nino event as sea-surface temperaures (SSTs)in the central and east-central equatorial Pacific Ocean regions are running approximately 0.5-1.0deg Celsius (1-2 deg F) above average (see map below). Based on trends and model forecasts, most climate scientists agree that the current El Nino will maintain itself or strengthen over the coming winter months, most likely becoming a moderate El Nino event this winter (SSTs 1.0-1.5 deg C above average).
An El Nino pattern in the winter results in a more southerly position of the Pacific jet stream, typically making landfall over California and continuing across the southern U.S. This results in an active weather pattern for the southern tier of states (wet and cool conditions) and warmer than average conditions across the northern U.S. (see map below).

For the Mid-South, El Nino effects are less pronounced than in other areas, particuarly to our south. However, typically the fall and early winter months see above average precipitation (so far so good!) with below normal precipitation during the core of the winter. Temperatures average near normal in the fall to perhaps slightly below normal during the winter.

Climate forecasts
CPC's seasonal outlooks and the recently-released NOAA Winter Outlook validate the "typical" El Nino winter (especially since they are based on those conditions occurring) - a warm/wet October-December period and a dry and slightly cooler than average January-March. The Farmer's Almanac predicts near normal temperatures and above normal precipitation. The Old Farmer's Almanac's crystal ball shows near normal precipitation and near to slightly below normal temps.

Recent Mid-South El Nino winters
Comparing to recent El Nino winters (December-March of 2003, 2005, and 2007), the Memphis area was slightly cooler (-0.9 deg) and wetter (+1.90") than average in 2003, while warmer and drier than average conditions occurred in 2005 and 2007 (2005, +3.0 deg and -3.76" and in 2007, +3.4 deg and -3.99") according to climate records for KMEM from MWN.

...what do I think? First of all, I'm NOT a long-range forecaster or a expert in climate. Given that, and with all due respect to the farmers (and old farmers), the evidence is present that El Nino has begun this past summer and decades of measurements indicate that it will not wane over the winter. There is also plenty of data to support the "typical" conditions that occur during El Nino events across the United States. I expect to see the wet conditions of late trend towards generally drier conditions as we head into the winter. However, as the winter storms move along the jet stream to our south, a few will have the potential to bring soaking rains and perhaps even the possibility of severe weather to the area. Temperatures seem to be a little more tricky, but I'm hedging on slightly above normal temps overall.

Prospects for snow
Everyone always wants to know about snow prospects for the upcoming winter as well. Given the more southerly storm track, we should have some opportunities for a round or two of precipitation getting caught up in cold air near the surface and creating the potential for snow. The timing will have to be perfect (as is almost always required in this part of the country). Precipitation coming from the southwest overrunning a cold dome of air already in place behind a preceding frontal system would be the most likely setup.

Here's some interesting data for the past 11 winters for Memphis showing yearly snowfall amounts (at the airport) and the state of El Nino/La Nina for that winter (ENSO neutral means neither El Nino or La Nina conditions). It is interesting to note that the highest totals seemed to occur during La Nina years, not El Nino. However, there have also been only 3 years in the past 11 that we did not get at least a couple of inches. [NOTE: The 30-year (1970-2000) "climate average" for snowfall for Memphis is 5.7" - which we didn't actually hit once during the past decade.]

I'm guessing we'll get at least one "couple inch" snowfall again this year, though freezing rain/sleet seem to be fairly likely at some point, once again given the southerly storm track of typical El Nino years. I am NOT predicting "the big one" - many of you received that on February 28th this year and those don't occur all that often!

1999 - Trace - La Nina
2000 - 5.5" - La Nina
2001 - 0.1" - weak La Nina
2002 - 4.5" - ENSO neutral
2003 - 2.9" - El Nino
2004 - 2.2" - ENSO neutral
2005 - Trace - weak El Nino
2006 - 3.5" - weak La Nina
2007 - 2.2" - weak El Nino
2008 - 5.4" - La Nina
2009 - 4.1" - weak La Nina

So what do you think? Educated opinions, uneducated opinions, wet fingers in the air, arthritic knee forecasts, I'll take 'em all! (And yes, I know Nino has a tilde over the second 'n'.)
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