Tuesday, February 5, 2013

More Clouds, Rains Ahead This Week

By Jay Grymes & Steve Caparotta

Tuesday morning’s fog was more dense than we anticipated for some neighborhoods – make sure that you are ready for more fog each of the next several mornings.  In fact, we’re anticipating a Dense Fog Advisory for most or all of the viewing area by Wednesday’s wake-up.  The low-level air is moist, the ground is soggy, and winds will be light overnight: three ingredients that readily promote dense fog when temps are this mild.  Allow yourself a little extra time for that morning drive!
As Steve showed you on Monday, we expect an active sub-tropical jet (the “southern branch” of the jet stream) to remain intact for the next several days, carrying mid-level storm systems west-to-east across the southern United States while providing fast-moving upper-level winds that enhance vertical lifting (more lift, more clouds & potentially more rain).   The combination will make for “on-and-off” rains over the rest of this week and right through the Mardi Gras extended weekend.
The presence of a persistent sub-tropical jet (STJ) overhead during winter also signals mild winter weather in most instances: note that temps throughout the entire 7-day forecast will stay at or above early February norms! 
Our next east-bound disturbance is already taking shape near the Big Bend of Texas and will be a rainmaker for us by Wednesday night.  The only question right now regarding this storm system is how early on Wednesday it might arrive?  Two of our long-time reliable models -- the American (GFS) and the European (ECMWF) -- bring rains into the viewing area by the mid-evening.  On the other hand, our in-house RPM model keeps most of the rain to our west until after midnight.  Which is right?
That’s what makes forecasting storms flowing with the sub-tropical jet trickier than you might expect:  how fast will the storm complex move? 
We’re putting a 20% to 30% rain chance into the Wednesday afternoon forecast, with rain chances rising through the evening.  Just about everybody gets wet with this system, and based upon what we’re seeing now, local rain totals of 1” or more will be common.  That system clears to our east during the latter half of the day on Thursday. 

We’re still keeping fingers crossed for Friday’s Southdowns and Saturday’s Spanish Town Parades!  Although we can’t rule out a spotty shower on either day, both days still look to be “mainly dry” and mild: wake-up temps in the 50°s (with fog) and afternoons in the 70°s.  As of this afternoon, we’ll call it a sun/cloud mix for Friday with partly cloudy skies expected for at least the better part of the day on Saturday.
In the extended outlook, timing becomes even more difficult  -- therefore, confidence is low.  From what we see right now, a low pressure center over the Plains on Sunday will track towards the Great Lakes, dragging a cold front through the region on Monday.  The result: isolated to scattered rains return to the forecast for Sunday with rain likely on Monday (Lundi Gras).   The front may be slow to clear the Louisiana coast, and that could mean scattered rains for at least the first half of our Mardi Gras day (Tuesday) forecast, with rain chances expected to be a little higher in the Crescent City.

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