Heat backs off later this week

Published 7:00 am Tuesday, August 11, 2015

A couple of weeks ago, I mentioned that statistics compiled by NOAA’s National Centers for Environmental Information show that the hottest day in Pearl River County most frequently occurs during the last week of July or the first week of August.

This summer our hottest temperatures have come toward the end of that period, continuing into this second week of August. Our highs the last several days across most of southern Mississippi and Louisiana have soared into the upper 90s with some locations exceeding 100 degrees. Factor in the effect of the high humidity, which is what the Heat Index does, and it felt like it was 105 to 115 degrees each afternoon in the shade.

We have been in a hot air mass centered in Texas and extending over much of the south-central U.S. This air mass is associated with a dome of higher pressure in the middle and upper parts of the atmosphere. Sinking, drying air within this dome has, for the most part, prevented our typical afternoon showers from forming.

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But, this past Saturday afternoon and evening, a small disturbance rotating southward through central Mississippi along the eastern flank of the heat ridge provided enough upward lift to allow the very unstable hot, moist air in the lowest layers of the atmosphere to bubble upward almost ten miles high into the atmosphere forming severe thunderstorms. Those storms swept southward through Pearl River County Saturday evening bringing high winds and one-half to one inch of rain. It was the heaviest widespread rain in the county since the last day of July.

Later during this workweek the hottest air will begin to slide westward. The pressures in the atmosphere three to eight miles above us will fall as we come more under the influence of a trough extending southward from Canada along the U.S. eastern seaboard states into the Southeast. This upper level trough will push a surface “cool” front down from the north and past us tomorrow. At this time of year here on the Gulf Coast, by the time “cool” fronts reach us they are mostly just boundaries with drier surface air behind them.

The drier air behind this front will wipe out any chance of rain on Thursday and Friday. The highs on those days will be in the middle 90s, but humidities will be lower, so the heat won’t be quite as oppressive on Thursday and Friday as it has been.

Over the weekend the frontal boundary in the Gulf will dissipate allowing higher humidities to return to our area. But a low pressure trough at all levels of the atmosphere will remain in the northern Gulf.

All of this points to it being less hot this weekend and early next week — highs only in the lower 90s — and a better chance for showers and thunderstorms as the low pressure trough enhances upward motion in the atmosphere.

There is a slight chance that lower pressures in the Gulf late in the weekend or early next week could get organized enough to cause some tropical concerns. Not likely, but a reason to check the weather, especially if you have any offshore boating plans during that period.

By Skip Rigney