Gary Archibald, Weathercaster

Thunderstorms are likely across much of the southwest, southern Plains, and southeast on Thursday. There is also the threat of showers and scattered thunderstorms over the Great Lakes region on Thursday due to a persistent upper-level low that continues to circulate unstable air.

What is going on exactly?

Well, this particular situation is a classic example of a cut-off low-pressure system…

What is a cut-off low?
A cutoff, upper-air low occurs when a counterclockwise wind circulation becomes separated from the main belt of west-to-east winds in the jet stream high above the earth's surface. With nothing to steer a cutoff low, it can simply sit and spin for days, often bringing damp, dreary weather. Such lows usually form when a strong wind speed maximum riding along the main belt of westerly jet stream winds dives into a deep upper-level trough or dip in the jet. As the wind speed maximum dives into the bottom of the trough, it elongates the trough and also helps build a large downstream ridge. The combination of the deepening trough and the strengthening downstream ridge allows the bottom portion of the trough to separate from the main belt of the westerlies. As a result, a large, circular, counterclockwise circulation forms high above the earth's surface.

Often, areas of low pressure at the earth's surface are associated with upper-level troughs and cutoff lows. A cutoff low is usually slow moving and won't exit a region until it is picked up by the westerly steering flow. Sometimes they unravel as they spin, eventually weakening and then dissipating. Either way, cutoff lows almost always bring several days of cloudy, dreary weather with periods of rain or snow.

A unique weather situation the cutoff low… Stay tuned for more blogs about unique weather patterns and dynamics as it happens across the country…right here at weatherplus.com.

GA