By Skip Rigney

Now that the chute for cool air has opened, a parade of cold fronts is sweeping through south Mississippi. The next front is forecast to move through on Monday. Ahead of it will be a line of showers and thunderstorms that may be strong enough to inaugurate the fall severe weather season.

As I write this on Friday morning, Subtropical Storm Nestor appears to be on the verge of forming in the Gulf of Mexico. Regardless of whether it is officially named, the impact on Pearl River County will be minimal as the strongest winds and rain associated with the system stay to our south and east.

Low pressure systems are classified as either tropical, subtropical, or extratropical. The differences between the systems largely relate to their internal structure and the source of their energy. Extratropical, which literally means “outside the tropics,” describes the vast majority of low pressure systems that affect us here in the United States. Extratropical lows are usually located along or near a frontal boundary between warm and cold air. Winds spinning around extratropical lows get their energy from temperature differences along the frontal boundary.

In contrast, tropical surface lows are warm at their core. They get their energy from the moisture that evaporates from the warm ocean surface, and which then condenses into clouds, releasing heat energy into the atmosphere.

This is the only process that can support a low intensifying into a full-blown hurricane.

Subtropical lows are hybrid systems that have characteristics of both tropical and extratropical lows. Their strongest winds and heaviest rains are often displaced far from the center, a marked contrast from their tropical cousins.

This weekend should be sunny and warm thanks in part to sinking air around the periphery of Subtropical Storm Nestor.

By Sunday our winds will shift around to the south returning more humid air to the area.

The more humid air will be one of the ingredients that raises the risk that some of the storms ahead of Monday’s cold front could become severe. The other ingredient is that winds will be changing directions with height, a condition that can cause thunderstorms to start spinning and produce tornadoes. Make sure you have a way to receive weather warnings on Monday, for example via a weather app on your phone, just in case some of the storms turn severe.

Most of Mississippi has received between one and five inches of rain over the past week, ending a stretch of over six weeks of abnormally dry weather. The Mississippi Forestry Commission lifted the state-wide burn ban on Monday. Most locations in Pearl River County received between one and two inches of rain this past week.

Temperatures behind Monday’s cold front are forecast to be in the lower 50s at night climbing into the 70s in the afternoon Tuesday through Thursday. Rain chances return on Thursday as more humid air returns, drifting in from the Gulf.

Expect another shot of cool air to arrive by Friday or Saturday of the upcoming week. And, the parade keeps moving with our sixth cool front of the month likely to arrive sometime during the middle of Halloween week.