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                   Friday, October 23, 2009 - 4:12PM

Severe weather possibility next week

By DAN ROBINSON
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25 Years of Storm Observing
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We may have a system to watch next week. Models have been hinting at an impressive upper level trough taking shape and sweeping across the country by the end of next week. Here is the GFS model's current version:


GFS 500mb pattern for next Friday

Notice how the winds in the trough start curving back west slightly after they round the southern edge - this is called a 'negative tilt' configuration, which can result in strong upper divergence (an enhancing ingredient for severe storms) where the upper winds 'fan out' to the north of the trough base.

Models have been fairly consistent in this pattern happening in some form next week. The main question is how fast it sweeps across the country. A slow-moving trough will allow longer period of moisture return from the Gulf of Mexico, while a faster trough will plow through before any decent low-level moisture can move in place northward ahead of it. More moisture and higher dewpoints mean better severe weather chances, and with an upper configuration like this, a decent moisture return could mean a major outbreak of severe weather.

Of course, a system of this magnitude this time of year means we'll probably be dealing with lots of cloud cover, a big limiting factor on severe potential. If the sky clears out ahead of this trough, then watch out. A good shot of cold air should come with the back side of this trough, which could bring a quick round of snow to a large part of the country. In other words, a big trough means 'pay attention to what's ahead'. It's still way too early to look at specifics, but if the models are correct about the large-scale pattern, we'll have an event to look at next week.

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