Showers and some storms on Thursday

Published 7:00 am Tuesday, March 28, 2017

Thursday will be our active weather day this week. Forecasters expect showers and thunderstorms across Pearl River County for several hours during the day Thursday and possibly into Thursday evening.

The rain will impact all of Louisiana, Mississippi, and Alabama at various times between Wednesday night and Friday.

The cause of the rain will be a large low-pressure system in the atmosphere several miles above ground level. Computer weather models predict that on Wednesday afternoon the low will be centered over the Texas Panhandle.

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The low will move northeastward and by Thursday afternoon will be over Kansas or Missouri.

Like all low-pressure systems in the Northern Hemisphere, the winds will be circulating counterclockwise around the center of this low. That means that Wednesday and Thursday the winds above us here in south Mississippi will start off from the southwest and then slowly shift to the west as the upper low’s center moves due north of us in Missouri.

At the same time, our surface winds will be from the south. The change in direction from southerly at the surface to westerly several miles above the surface creates the kind of shear that can cause thunderstorms to become severe and even produce tornadoes.

Another ingredient that could contribute to the potential for severe storms is that there will be plenty of warm, moist air at the surface. As cool air aloft associated with the upper low comes closer this could result in the atmosphere becoming quite unstable on Thursday.

Associated with the upper low-pressure, a weak surface cool front will pass us on the back side of the rain Thursday night.

By Friday and Saturday the upper low will have continued moving off to our northeast. Because of the counterclockwise circulation around the center of the low, our winds several miles above the ground will shift to a dry northwesterly direction.

The result should be sunny spring days on Friday and Saturday. The forecast high temperatures on Friday and Saturday show how weak the cool front will be as highs both days will be close to 80 degrees. Low temperatures will only drop to around 60.

Meanwhile, low pressure troughs and high pressure ridges several miles above ground level will continue to move quickly from west to east. Meteorologists often call this a progressive pattern when weather systems are progressing rapidly eastward.

The computer models show that on Friday in the upper atmosphere our next weather maker will already have slipped down the west coast from Washington state to Arizona and will be making the turn eastward toward us.

The models predict that the counterclockwise wind circulation of this low-pressure system several miles above ground level will cover most of the southwestern United States.

This system will probably be even stronger than the one that affects us on Thursday.

It is an open question as when the system will bring showers into our area.

The European computer model indicates it could be as early as Saturday night, while the long-range American model delays the rain until Monday.