Ask Tom why: How long can a cut-off low weather system keep going?

September 29, 2011

Dear Tom,

The cut-off low has lost its charm, but it is fascinating. Given no outside interference, how long could it realistically keep going?

—Jim Baum, Mount Prospect

Dear Jim,

Several readers have raised similar questions about the weather system responsible for the cloudy, cool, showery weather of recent days. Variously known as a cut-off low or closed upper low in meteorological circles, it's a storm system in the upper atmosphere (generally above 15,000 feet) around which winds circulate in a closed counterclockwise loop. The system is usually several hundred miles in diameter and consists of a pool of cold air. Ground-level weather is unsettled. A closed upper low lies south of the usual westerly wind flow and moves slowly or not at all, sometimes persisting six to eight days before its cold pool warms and gradually dissipates. Preferred U.S. locations for cut-off lows are the Great Lakes region and the Southwest.