Cut-off low, meteorologists’ woe. We are living that old adage this week.

A week ago it looked like the start of this week would be sunny and 80, but a cut-off low has thrown a wrench into that forecast. Instead, we are looking at mostly cloudy, cool and breezy weather through Wednesday, if not Thursday. Afternoon highs are going to run about 10-15 degrees below average for time of year.

So what's a cut off low? The National Weather Service defines a cut-off low as "a closed upper level low pressure system that has been cut off from the basic westerly current and moves independently of that current.” It’s kind of like a car without a driver. It's moving, you're just not sure where or how fast, if it moves at all.

The upper low is easy to pick out on satellite images. The center of the upper level low is located over western Illinois.

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Upper lows often form when a blocking weather pattern develops. That's what's happening this week. We've got a blocking weather pattern called an "Omega Block" just look at the jet stream map. These are winds at about 25,000 feet or about half way up into the atmosphere. See how the weather pattern resembles the Greek letter Omega.

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What's causing this block? There are a lot of ingredients but Tropical Storm Arthur off the east coast is playing a role. Arthur and its counter-clockwise wind flow is blocking the upper low from moving toward the east coast.

At the same time, a large area of high pressure formed west of the upper low. This combination pinched the low off of the westerly winds aloft, leaving the low meandering around the Mississippi River through Thursday.

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As less air flows into the low-pressure area and more air flows away from the low-pressure area, the system will dissipate by the end of the week. That will open up the westerly flow aloft and should lead to a more typical may weather pattern as we head into the Memorial Day weekend. That is, unless another cut-off low forms. Then I will be writing another blog about the cut-off low, meteorologists’ woe.

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