These southwestern Illinois counties are under a tornado watch until 11 p.m.

The National Weather Service has issued tornado watches for counties across southern Illinois, including St. Clair, Monroe, Randolph, Washington and Perry counties.

The watches expire at 11 p.m. , according to the weather service.

Here’s how the weather service defines a tornado watch: “A Tornado Watch is issued when severe thunderstorms and tornadoes are possible in and near the watch area. It does not mean that they will occur. It only means they are possible.”

In an interview with the BND earlier Friday, National Weather Service Meteorologist Fred Glass said the biggest potential threat to the metro-east is the possibility of tornadoes, damaging winds and large hail.

Glass explained a cold front will move through the area that will clash with the unseasonably warm December temperatures — a combination that significantly increases the odds for severe weather, he said.

“We’ve got a pretty strong storm system developing,” Glass said Friday afternoon. “If you walk outside right now, it’s obviously pretty warm for December. Something has to give when that cold air wants to replace that hot air. And associated with that is a severe weather threat.”

Glass said the greatest threat for Belleville and southwestern Illinois is 7-11 p.m.

Visit the National Weather Service - St. Louis for details on the forecast and weather news.