Thursday, July 11, 2013

Less Rainfall on Friday

By Jay Grymes & Steve Caparotta

Most of you made it into the 90°s this afternoon before the showers and t-storms arrived.  The majority of this afternoon’s rains over the WAFB viewing area were the product of a persistent outflow boundary moving towards us all day long from the northeast.  The outflow was produced by a cluster of strong storms over Alabama and eastern Mississippi earlier in the day.

What is an “outflow boundary?”  Stated simply, an outflow boundary -- also known as a gust front -- is the leading edge of gusty, cooler air produced by thunderstorm downdrafts.  Cooler air from the thunderstorm sinks to the surface then spreads out ahead of the thunderstorm, effectively creating a small-scale “cold front” with a wind shift and a noticeable drop in temperature and dew point (much like a “traditional” cold front).  Outflow winds can easily gust into the 30-40 mph range and occasionally rise well above the 40s for brief periods.

These gust fronts can also serve as a lifting mechanism for the warm and humid air that it advances through.  As a result, the outflow boundary/gust front can generate new thunderstorm development.  If you watch us regularly, you may have noticed how we point out occasions where different outflow boundaries collide: intersecting outflow boundaries can produce explosive development of thunderstorms.

Our forecast for Friday calls for a drier day ahead.  We’ll start of Friday with a warm and humid morning -- like the past few mornings, expect sunrise temps in the mid 70°s for metro Baton Rouge.  But Friday afternoon’s forecast only calls for a 20% -- to maybe 30% -- rain chance under otherwise partly cloudy skies.  But then, “drier” usually means hotter: look for Friday highs up around 93° for metro BR, with some WAFB neighborhoods reaching the mid 90°s.

We still expect that cool front we’ve advertised this week to make it into south Louisiana over the next day or so before fizzling out along the coast.  The “drier” air (less humid, lower dew point temps) should allow for lows on Saturday and Sunday to slip back to something closer to 70° or so, but we’ll still see afternoon highs in the low 90°s.  Saturday should be a mainly dry day weatherwise, with rain chances at less than 20%.  By Sunday, however, the air will already have started to moisten up thanks to the return flow off the Gulf: we’ll set Sunday afternoon and early evening rain chances at about 30% to maybe 40% closer to the coast.

By next week, we’re back to typical summer-season weather: low to mid 70°s at sunrise, daily highs in the low 90°s, and scattered mainly-afternoon rains.

And in the tropics ... although Chantal lost her surface circulation yesterday, ‘her’ remnants have shown some hints of mid-level spin today.  As a result, the National Hurricane Center is posting this area of disturbed weather with a 30% chance of returning to depression or tropical storm intensity over the next two days.  Were it to re-develop, the latest computer guidance indicates little chance for the system to head into the Gulf.  However, that assumes that any new center of circulation will emerge between Cuba and the Bahamas.  Should the center develop elsewhere, we’ll have to re-assess the Gulf threat.  But for now, let’s just relax -- this doesn’t look like it will be a storm for us to worry about regardless of what happens.

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