Tuesday, December 17, 2013

Not Quite As Cold Wednesday AM

By Jay Grymes & Steve Caparotta
Yes -- Jay was caught by surprise with this morning’s light freeze.  Metro Airport temps dipped to 32° before 5AM and stayed there until well past 7AM.  And the light freeze extended over much of the viewing area, with some 32°s reported as far south as the coastal parishes.
While such a light, brief freeze is little more than an inconvenience for most of us, this morning’s low serves as a great example of how even here in the 21st Century, we have a long way to go to in the world of weather forecasting.  If you were watching last night, you saw that we were calling for a Tuesday morning low in the low to mid 30°s for metro Baton Rouge, but staying above freezing across the Capital City region.  Dew point temperatures -- a first rough guess at how low the temperature can drop -- were running in the mid 30°s between 7-10PM.  We did acknowledge the possibility of a light freeze near and north of the LA/MS state line, but didn’t think the air would get that cold for most WAFB communities.
In fact, our forecast for today’s low was still lower than those posted by the computer guidance models as of yesterday afternoon: our favorite computer forecasts were calling for lows in the 36°-38° range for Metro Airport (AP).  Instead, the low-level air continued to “dry out” (indicated by falling dew points) through the night and that allowed Metro AP’s air temperature to slide to 33° by 3AM and to below freezing well before sunrise. 
Sometimes Mother Nature has her own ideas about how to do things -- even our best forecast models can’t always figure ‘her’ out.
So after another cold start to the day, the clear skies, sunshine and the approaching upper-level ridge of high pressure resulted in a nice warm-up as afternoon readings climbed into the 60°s across the entire viewing area.  We expect mainly clear skies to remain over the central Gulf Coast through Wednesday.  The BR metro area can expect one more morning wake-up in the 30°s for Wednesday’s sunrise -- but this time the BR metro area stays above freezing.  Even our northern viewers up along and north of the LA/MS state line should miss out on a freeze for Wednesday’s wake-up.  Highs on Wednesday afternoon should reach the mid to upper 60°s across the viewing area.
Clouds will return on Thursday while the warming trend continues; just about everyone gets into the 70°s for Thursday afternoon.  And our 7-day forecast keeps afternoon highs in the 70°s right through the weekend.
Thursday’s clouds will be a function of a steady increase in atmospheric moisture through the day but Thursday stays dry, at least through the afternoon and evening.  By late Thursday evening and Friday morning, the first in a series of mid/upper-level disturbances arrives. 
Yesterday we posted scattered rains in the forecast for Friday, but now we’re upping that to “rain likely” by Friday evening.
Yesterday we also mentioned the development of an upper-level trough along the West Coast during the week.  That forecast remains on track, with the trough pushing eastward towards the end of the work week.  It will be centered over the Desert Southwest by Friday, helping to energize a disturbance that will be impacting us, then moves over the Central and Southern Plains through the weekend before lifting towards the northeast by Monday and into Tuesday.
So for Saturday, we’ll have the upper trough to our west, which means low-level moisture flowing inland off the Gulf will be working hand-in-hand with a steady southwesterly flow at the upper levels and occasional upper-level disturbances riding along the upper-level track -- those are the ingredients that should make for a fairly “wet” Saturday.  By Sunday, the trough will help swing our next cold front through the lower Mississippi Valley.  The timing of the front is still a bit in question, but Sunday looks “wetter” than Saturday as the front moves through.
We’ll tweak this weekend outlook in the coming days, but for now we’ll go with a Sunday forecast that has the front draped over the southern parishes by the latter half of the day with the cold front out over the Gulf by Monday morning.  We’ll keep a watch out for the potential for active or severe thunderstorms on Sunday as the front moves through days draw closer -- it’s still a little too early at this stage to be making a call.  Just remember that our winter fronts can produce severe weather at times.
Some of us might still get a light shower or two early Monday, but the skies should be clearing by Monday afternoon if not sooner.  And temperatures behind the front should show a significant drop-off after the run of days in the 70°s -- expect highs to drop into the 50°s for the early part of the week with morning lows in the 30°s by Christmas Eve and Christmas Day.
Papa Noel should be pleased: our extra-long-range outlook is indicating a couple of cool but fair-and-dry days for December 24th and 25th!

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