What is a cut-off low pressure system?
Recently, the Midwest saw strong winds and heavy rains as part of a low pressure storm system that refused to move eastward for a few days.
Friday, October 21, 2011 - 14:21
SKY PUNCTUATION: A low pressure system moves across the eastern U.S., looking rather like a comma. (Photo: NASA Goddard Photo and Video/Flickr)
If you live in the Midwest, you probably noticed the incredibly large weather system that just passed through this area. Heavy rain and strong wind lashed the Midwest, and it was an adventure trying to get around UIUC's campus in that weather with only two feet and a bike. I looked at the radar and saw a swirling mass of clouds that almost looked like a hurricane over the Midwest, and felt confused. I researched it, and found it was a phenomenon called a cut-off low pressure system.
Most of the time, storm systems move across the country in an eastward direction, and sometimes southeast. The strange thing I noticed about this particular storm system was that it was circling around the states of Illinois and Indiana, and not moving steadily eastward at all. This is because the system was a cut-off low pressure system. A cut-off low pressure system is a large low pressure system that is completely separate from the normal eastward weather flow, and that is why it stayed over the Midwest without passing through for three days. It is a kind of closed low pressure system, and "closed" indicates some degree of separation from the normal weather patterns. All cut-off low pressure systems are closed, but not all closed systems are cut-off.
Cut-off low pressure systems occur when there is no driving force eastward to push the clouds like they would normally go. Instead, the system sits in a sort of trough in the wind system, and usually has a counterclockwise motion. It stays over the same area for days, often bringing lots of wind and rain, or at the very least, damp and gloomy weather. Cut-off low pressure systems only leave when they are picked up by a directional wind again, and then they gradually move eastward. They can also unravel as they spin, and as they unravel, they disintegrate.
This particular system caused very large waves in Lake Michigan, and soaked the Midwest with quite a bit of rain. We needed it, since we haven't had much rain in a while, but nevertheless, it wasn't a picnic running from class to class!
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