Wednesday, September 4, 2013

Plenty of Heat, A Few Storms Each Day

By Jay Grymes & Steve Caparotta

We mentioned it yesterday and the local outlook looks just about the same through the weekend and possibly right through the next seven days.  Plan on mainly-dry morning starts with sun-up temperatures in the low 70°s around metro Baton Rouge.  The afternoons are likely to remain on the “warm” side of normal into next week, with highs in the low 90°s for most but flirting with the mid 90°s for some locations.
For now, we are calling for mainly-afternoon and early evening showers and occasional storms through the weekend.  Rain probabilities (in other words, areal coverage) each day should be on the order of 20% to 30% or so into early next week.
We’ve got somewhat competing influences driving our day-to-day rain chances through the week and weekend.  At the surface, warm-and-humid Gulf air will be ready to fuel afternoon showers and storms with daytime heating.  Add in a meandering, quasi-stationary front across the Gulf States which will linger over the region for another day or so before dissipating.  These two factors normally would encourage afternoon showers and storms.  At the same time, upper-level ridging centered to our northwest will maintain a northerly upper-air flow over the lower Mississippi Valley.  The ridge -- associated with dry, sinking air -- will act to counter some of the instability promoted by the Gulf air mass and frontal lift. 
However, should a few storms be able to overcome the ridge’s effects (since we will be located on the eastern periphery of the upper-air dome), they could become quite strong.  This is exactly what we experienced on Tuesday afternoon and evening -- isolated pockets of very active thunderstorms, producing frequent lightning and even some hail, moving towards the south and southwest along the eastern edge of the upper-level flow.
As we often remind you, when t-storms are moving from north-to-south, they tend to have cooler-than-normal air in the higher levels.  This cooler air aloft encourages more lightning and hail once a thunderstorm “matures.”   Fortunately, most of Tuesday’s bigger storms were limited in coverage, but if you were under one of them, you certainly knew it!
So far this afternoon (through 4PM), storms have not gained the same strength as what we saw on Tuesday.  But we’ll need to remain alert for a potential repeat over the next few days.
We’re still watching two areas in the tropics: a poorly organized tropical wave near the Yucatan Peninsula and recently upgraded Tropical Depression #7, located just south of Puerto Rico. 
Neither system poses a threat to the central Gulf Coast.
In fact, as of right now, it looks like the Yucatan disturbance will struggle to reach depression strength before it runs out of open-water real estate.  By contrast, the National Hurricane Center (NHC) calls for T.D. #7 to strengthen and become T.S. Gabrielle overnight as the system moves northwest.  The forecast cone for Thursday extends from the eastern Dominican Republic to the western tip of Puerto Rico.  Based on the current model consensus, a moderately strong Gabrielle will move into the western Atlantic but take on a more northerly course, likely passing to the east of the Bahamas and headed towards Bermuda.  At least for the time being, the NHC does not expect T.D. #7 to reach hurricane intensity.

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