March 11, 2010 - Cut It Out!

By: Mallory Brooke
By: Mallory Brooke

The beautiful weather of Monday and Tuesday is a thing of the past. A cutoff low over the Midwest has continually brought clouds and periods of rain up the eastern seaboard. What makes a low pressure "cutoff"? Read today's blog!

While the rain hasn't affected us too much Wednesday and Thursday, the cloud cover has certainly increased. Low pressure situated in the Midwest has become a cutoff low. The official definition from the National Weather Service is "A closed upper-level low which has become completely displaced (cut off) from basic westerly current, and moves independently of that current."

So how does this happen? It all comes back to the jetstream. A strong wind speed maximum rides along the main belt of the westerly jet and dives into a trough in the upper levels of the atmosphere (or a dip in the jetstream). When the area of high wind moves into the bottom of the trough, it eventually elongates the trough and the bottom portion of the trough separates from the main belt of the jetstream. When the trough separates, a large counterclockwise circulation forms in the upper levels of the atmosphere.

There's your cutoff low. It can sit and spin in the same place for days on end. The cutoff low won't move much until the westerly flow picks it up again. On some occasions, it will simply weaken and dissipate on its own. Other times, the cut off low will actually retrograde, and move back towards the west.

The continuous clouds and (eventually) rain showers occurs because upper level disturbances continually move around the low pressure. That's why there are so many days of rain and clouds in the forecast!

We'll start to see some clearing on Tuesday as strong high pressure develops to the west.



On a side note: I had two wonderful groups of second graders from William Perry Elementary School come in for a tour of the WHSV Studio Thursday. One of the teachers asked if the March 18th snowstorm was really happening. Where do these rumors start?! Computer models have our temperatures barely dipping below 50 degrees in the next 10 days, let alone bringing a monstrous snowstorm. I don't think I've ever heard more weather rumors than I have this year.

Now, that doesn't mean it won't get cold again before we finish out March, and we've had measurable snow in April several times. But for now, enjoy the sunshine and 50's!

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