Fall preview didn’t last long

Published 7:00 am Tuesday, September 1, 2015

Wasn’t last week’s weather a refreshing change from our normal summer sauna?

An unusually strong cool front for August passed through early last Tuesday morning. Behind it was the driest air mass to move into south Mississippi since early summer.

It was still sunny and and warm in the afternoons with highs near 90, but it didn’t feel as hot because the humidity was so much lower.

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Overnight and early morning was when you could really tell a difference. Temperatures cooled down down into the upper 70s after sunset. By sunrise Wednesday and Thursday mornings, lows dipped into the high 50s, temperatures we haven’t seen here since May.

The humidity of this continental air mass was much less than the mugginess we have grown accustomed to. Dew point temperatures were in the high 50s and low 60s compared to the 70-80 degree dew points we typically get in summer from air coming off the Gulf of Mexico.

Water vapor absorbs some of the heat energy radiated by the ground and keeps more of that heat trapped in the air near the surface. When there are clear skies and less water vapor, more heat can radiate away from the ground and the lowest levels of the atmosphere. That’s what allowed us to cool off each night from Wednesday through Saturday of last week.

A southward dip in the jet stream carved out a deep low pressure trough at altitudes of three to seven miles along the eastern seaboard states of the U.S. On the western edge of that trough, upper winds over the northern Gulf Coast were from the northwest, and that kept the cool front south of us.

Hopefully, you didn’t unpack your sweaters. The trough moved on and so did the dry air.

Over the weekend more humidity, warmer nights, and higher daytime heat indices began their return. This week the weather will be typical for summer: highs in the lower 90s, lows in the lower 70s, with a slight chance of getting rain from one of the widely scattered afternoon showers and thunderstorms that will be popping up in the Gulf South.

Don’t expect any big changes for at least the first half of September.

While we were enjoying a break from summer, two tropical cyclones, Danny and Erika, trekked near the Greater Antilles islands of Puerto Rico, Hispaniola, and Cuba. Both were sheared apart by strong west and southwest upper level winds. This is not surprising given that El Nino is going on in the tropical Pacific. One of El Nino’s side effects is more wind shear in the tropical Atlantic, making it tough for hurricanes to form or to hold together for long.

Hurricane Fred formed over the weekend in the easternmost tropical Atlantic. The storm is 3500 miles away from the Gulf, which means that even if it doesn’t recurve out into the Atlantic, and even if it survives the enhanced shear of this El Nino year, it will take at least until early next week for the storm to make it all the way to the Gulf.

In other words, for now, ignore Fred. And, keep the air conditioner running.

By Skip Rigney