Wednesday, December 18, 2013

Rains Arrive for the Weekend

By Jay Grymes & Steve Caparotta
As expected, it was another cold start to the day with just about everyone starting Wednesday in the 30°s.  There were even a couple of Weather Watcher sites to the north and east of metro Baton Rouge that reported lows at or near freezing just before sunrise.  And once again, as we’ve seen the past two mornings, a few early morning commuters had to deal with fog -- on the dense side in some spots.
The warming trend we’ve been chatting about this week continued today as well, with most WAFB communities getting into the upper 60°s for Wednesday afternoon highs under sunny skies.  Tonight will be noticeably warmer than the last several nights, with lows ranging from the mid 40°s north of the LA/MS state line to lower 50°s for closer to the coast.  And as we’ve seen the last few mornings, fog will greet some early morning risers.  In fact, our suspicion is that patchy dense fog could be a problem -- we’ll have to watch for the possibility of a Dense Fog Advisory for the Thursday morning start if the winds stay light.
All in all, our thinking for the coming days hasn’t changed much.  A modest upper-level ridge will continue to move our way over the next couple of days, taking afternoon highs into the 70°s. At the surface, high pressure that has been sitting over the Lower Mississippi Valley for the past couple of days will shift east and set-up southeasterly and southerly flow along the Central Gulf Coast.  That means a return of low-level Gulf moisture which will enhance cloud development on Thursday.  Expect mostly cloudy skies by Thursday afternoon and evening, but the day stays dry.
The warm-up continues into Friday and Saturday, with mid to upper 70°s expected for highs on both days.  Unfortunately, those spring-like temperatures will be accompanied by rain on both days.  In fact, rain will be the lead weather story right through the weekend.  None of the three days will be a total wash-out, but just have the umbrella by your side if you are planning on getting to those around-town errands or finalizing the holiday shopping.
An upper-level trough currently developing near the California Coast will swing through the Desert Southwest late Friday then move into the Southern Plains during the weekend.  The trough will then lift northeastward into the mid-Mississippi Valley towards the Great Lakes and Ohio Valley early next week.
With the trough to our west on Friday and Saturday, upper-level southwesterly flow overhead will work with the low-level Gulf moisture to generate scattered showers on Friday.  At the same time a frontal system will set-up over the Southern Plains by Friday.  However, that system slows and lingers to our northwest and west before heading our way Saturday. 
The timing of the weekend frontal passage is still a little fuzzy.  The latest run of the American model (the GFS) has the system moving through late Saturday and early Sunday with Sunday’s rains ended by of before the early afternoon.  The European model (the ECMWF) is a little slower, with rains extending into Sunday afternoon and possibly lingering into the early evening.  Either way, our two preferred medium-range forecast tools bring an end to the rain on Sunday and leave us “dry” through Christmas Day.
Now what about a severe weather threat with the weekend front? 
It’s still too early to make a definitive call.  Strong to severe thunderstorms can’t be ruled out, especially during the latter half of Saturday into Saturday night.  We’ll have daytime heating, a “juicy” atmosphere and the potential for support from relatively fast-moving southwesterly flow aloft.  All of these are factors enhance the opportunity for strong to severe storms, so we’ll need to remain watchful.
However, even with these key ingredients, preliminary indications keep the more active weather to our north through the weekend.  The latest NWS Weather Prediction Center projections for the WAFB viewing area show three-day rain totals running in the 1.0” to 1.5” range, implying that strong to severe storms should be isolated at best.  But as we said, we’ll keep an eye on this as the weekend gets closer.

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