What happened to mild, fair weather last weekend?

Published 7:00 am Tuesday, October 6, 2015

I kept thinking about the headline of my column published last week, “Typical mild, dry air returns by Thursday,” as I shivered in the cool north wind Saturday riding my bike under grey, overcast skies along the Longleaf Trace between Hattiesburg and Prentiss.

I knew that the problem wasn’t that I had driven sixty miles north. The weather was the same at home in Picayune.

The day was dry, but it certainly wasn’t typically mild. Temperatures never got out of the 60s on Saturday, and stayed in the low 70s Friday and Sunday. “Typical” for the first few days of October in south Mississippi is low to mid 80s!

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And, what about my claim in that same column that once a cool front came through on Thursday, the northwesterly winds behind the front would bring “mostly fair skies for Thursday, Friday, and Saturday.” Instead Friday, Saturday, and much of Sunday were relentlessly cloudy.

What happened?

Last Wednesday and Thursday things were going pretty much as predicted by the models earlier in the week. A surface cool front and an associated trough of low pressure in the middle and upper altitudes of the atmosphere swept into our region flushing out the warm, humid air that had been over us earlier in the week.

High pressure at the surface began to build in as expected, putting an end to rain chances.

But, then the trough of low pressure three to seven miles high in the atmosphere did something that the numerical weather models from earlier in the week had not predicted. It transformed into a feature called an upper level “cutoff low.”

Unlike upper level ridges of high pressure or even the upper level troughs of low pressure from which they form, the winds around a cutoff low make a complete, closed circulation around the center of the low. The name “cutoff” refers to the fact that these systems become isolated from the main west-to-east upper level flow. They then tend to move quite slowly, often hanging around the same area for days.

The center of our cutoff low spent most of the weekend wandering around Alabama, Georgia, and north Florida. It was a large system, covering most of the southeastern United States.

This same cutoff low was one of three strong weather systems that combined to cause the epic rains in the Carolinas over the weekend. Much of eastern South Carolina received 10-15 inches of rain. Over 20 inches fell in some areas!

By Skip Rigney