Tuesday, December 1, 2009

It's beginning to look a lot like... El Nino!

Source: NOAA/CPC

The weather pattern over the Mid-South and southeastern U.S. is starting to take on a fairly typical "El Niño" look as we approach mid-week. The pattern is marked by progressive low pressure systems that emerge from the Desert Southwest into the western Gulf of Mexico on a fast-moving subtropical jet stream that zips over the southern tier of the U.S. They then tap into copious moisture from the Gulf, bringing torrential rains and the occasional chance of severe weather to the Gulf Coast. The lows then continues east or move northeast, depending on the track of the jet stream, sometimes continuing into the major metros of the Northeast U.S.

For the Mid-South, there are many factors that determine what results these western Gulf systems have on our weather. These include the exact track of the low itself, the amount of moisture it contains and the dynamics associated with the system, as well as the degree of cold air in place over the region, particularly in the lowest few thousand feet of the atmosphere. All of these factors make these systems some of the hardest to forecast for this region of the country, especially when cold air has to be factored in.

With respect to the upcoming system, which is already bringing flooding rains to the central Gulf coastal region and the possibility of severe weather to portions of the Gulf Coast and southeast, the low will likely track through the Tennessee Valley and up the spine of the Appalachians on Wednesday. (This track is somewhat atypical of a track for the classic El Niño storm, but it's origins are fairly El Niño-esque.) The low will be strengthening and there is plenty of moisture associated with it.

Fortunately, at least this time around, there just won't be enough cold air in place during the time the precipitation is falling to have to worry too much about the chance of anything but rain. For areas to the north of the metro, there is a small chance that sufficient cold air could seep in Wednesday night as the precipitation is ending to produce a few flurries. I do not expect this in the Memphis metropolitan region however, and there should be no accumulations anywhere in the Mid-South that flurries might fall thanks to warm ground temperatures during the very early stages of winter.

After this system passes us by, the coldest air of the season will set in, with temperatures falling below freezing Friday, Saturday, and probably Sunday mornings area-wide and highs Friday maybe struggling to reach 40 degrees! Saturday morning will be the coldest morning so far this winter with lows in the mid 20s likely in the city and maybe near 20 in outlying areas. Not good news for those running the St. Jude/Memphis Marathon or Half-Marathon on Saturday or attending one of many holiday events or parades around town this weekend!

Keep an eye on the MWN Forecast for the latest details on our first winter blast!

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