Why do storm systems stall out?

July 30, 2008

Erica's Answer:

For the most part, our weather relies on winds in the upper atmosphere to steer weather patterns around the world. So, if a storm system is in an area with weak upper-level winds, it will stop moving!

This happens sometimes in the summer, when a frontal boundary will hang around in the same general location because it is disconnected from the jet stream winds that would otherwise get this front moving.

Another common scenario, especially in the winter, is the "Omega" block. If you are familiar with the Greek alphabet, then you know what a capital Omega looks like- a large arc pointing upward in the center, with 2 smaller arcs pointing downward on either side of the center arc. This pattern forms in the winter jet stream on occasion, and stalls out weather patterns across the United States.

Here in the Northeast, we usually get stuck in one of those side arcs, which meteorologists refer to as a trough. If a low pressure system gets "caught" in one of those troughs, then it has a very difficult time getting back out.

Again, a big change in the weather, such as a strong area of high pressure, needs to come along to change a wintertime Omega block pattern, just as with a summertime stalled front.

- Erica

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