Tuesday, February 18, 2014

Fog Wednesday AM, Storms Possible Later This Week

By Jay Grymes & Steve Caparotta
We started the day off with dense fog across much of the viewing area, prompting NWS Dense Fog Advisories to the east, south and west of the Capital City.  Advisories remained in effect over the tidal lakes and coastal waters until noon today, reflecting the extent of the fog especially near and along water bodies.
The front we described over the Bayou State last night stalled to our north and remained there through the morning, essentially fizzling out over the state during the afternoon.  With the WAFB area remaining on the south side ("warm" side) of the front, we stayed warm for this time of year, with Red Stick lows in the 60°s.  At the same time, low-level Gulf moisture kept the air "juicy" for fog development.
As Steve noted, last night's fog was mainly an "advection fog."  As the name implies, an advection fog (advection: horizontal movement) is typically formed when southerly winds move relatively warm-and-moist Gulf air across the cooler land and water bodies.  The cooler surfaces underlying the moving Gulf air cools the moist air, bringing the air temperature down until it is equal to the dew point temperature.  At that point, the air is "saturated" and microscopic water droplets begin to condense -- the onset of fog.

More Fog, Warm Again on Wednesday

Low clouds were rather persistent through the day as well and that slowed the morning warm-up.  After a morning start in the low 60°s for most WAFB neighborhoods, the ‘heavy’ clouds delayed the normal sun-driven warm-up.  Still, every WAFB community made it into the 70°s for the afternoon with a few sites making the upper 70°s.  And while the clouds were with us throughout the day, little if any rain fell over the WAFB viewing area.
Make plans for a repeat on Wednesday.
We'll begin the day with another round of dense morning fog.  In fact, the NWS has already issued an area-wide Dense Fog Advisory, which currently runs until 10AM on Wednesday.  And much like today, morning lows will run from the upper 50°s to low 60°s across the viewing area, with afternoon highs climbing into the mid to upper 70°s.  Also like today, clouds will dominate Wednesday's skies.  We can't rule out a spotty shower or two given the moist Gulf air still in place but showers will be spotty at best.  And it will likely be occasionally a bit breezy for Wednesday afternoon.
Be ready for another round of fog on Thursday morning with highs again headed into the upper 70°s.  However, changes come on Thursday as we head into the latter half of the day.  A vigorous cold front will be approaching from the west with its center of low pressure moving through the U.S. Plains.

T-Storms Possible Late Week

An updated assessment for Thursday’s approaching front by the NWS Storm Prediction Center (SPC) now puts the SW Mississippi counties and the WAFB parishes along the LA/MS state line under a "Slight Risk" for severe weather from Thursday into early Friday.  The main threat at this time appears to be damaging winds, although isolated tornadoes and large hail cannot be ruled out.  However, the real action will most likely stay farther to the north.
At the same time, the NWS Weather Prediction Center (WPC) is calling for 1.0" to 1.5" of rain across a good portion of the WAFB viewing area in response to the likelihood for active thunderstorms.  We'll be keeping a close watch on this scenario as Thursday draws nearer.
Just about all of the stormy weather should exit the viewing area before Friday’s sunrise and everything settles down and the skies should be clearing by/before Friday afternoon.  A cooler air mass moves in behind the Thursday/Friday front keeping highs in the 60°s across the viewing area for Friday and the weekend.

Extended Outlook

The weekend forecast still remains a little "cloudy" and unclear.  The latest runs of the operational American 'GFS' and the European 'ECMWF' models suggest a mainly-dry weekend ahead, whereas some of our other guidance suggests a decent rain chance for Sunday.  We should get a better bead on the weekend details in the coming days.

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