Wednesday, January 16, 2013

Snow Tonight/Thursday AM?

I woke up to this morning to a few questions on social media asking about snow being mentioned in the forecast by some outlets. I was a bit surprised since we had downplayed the threat Tuesday night, but an examination of the latest computer guidance in this morning shows why that magical 'snow' word is being tossed around.

The prospects for any wintry weather are tied to a fairly potent upper-level low forecast to move across the area late tonight into Thursday morning.

NAM (computer model) forecast valid at 3 AM on Thursday. An upper-level low is noted moving across central/northern Louisiana.
The upper-level low is important for 2 reasons: 1) it generates 'lift' that can lead to the development of clouds/precipitation, and 2) these upper-lows carry pockets of cold air aloft in the atmosphere necessary for frozen precipitation.

Most guidance shows that the atmospheric profile (temperatures from near the surface to several thousand feet above ground) will be plenty cold enough late tonight to support wintry precipitation, primarily in the form of snowfall. The big question remaining is: will there be enough moisture left?

Yesterday, our computer guidance was unanimous and adamant that necessary moisture would exit the region well before the arrival of the upper-low. The big change with this morning's guidance is that much of it now hints at just enough moisture lingering to squeeze out some very light precipitation late tonight.

Our RPM model now generates some light snow near the state line into SW Mississippi near/after midnight tonight...

RPM forecast valid at 2 AM Thursday. Snow is represented by shades of blue and white. A wintry mix is represented by the pink shading.
RPM forecast of snowfall potential through Noon on Thursday. 
Here's the key...while the odds of a little wintry precip/snow may have increased a bit this morning, we still believe anything that falls should be quite light. The greatest chances for any wintry precipitation appear to be near the LA/MS state line northward into SW Mississippi. Even here, significant accumulations appear unlikely, although it's possible a little snow could briefly accumulate on trees/grass and elevated surfaces.

Here's another look at snow potential...the graph below shows that the NAM (computer model) is forecasting about 1/3 inch of snow at McComb, while the GFS only generates about 1/10 inch.

The same plot for Baton Rouge shows the models have no snow accumulating in the Capital City, although I wouldn't completely rule out some flurries or very light snow late tonight.

Bottom line: even if anything falls tonight...and it's still a big 'IF'...significant accumulations are not expected. Of course, Jay and I will have the latest on 9News at 5 & 6 this evening.

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