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                   Friday, May 18, 2012 - 12:55AM CST

Chase forecast update for May 18-28

By DAN ROBINSON
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In the past 24 hours, the GFS has changed significantly in its depiction of the upper pattern beginning on Tuesday of next week. The Euro (ECMWF) model has also been trending in a similar direction. Both models now agree on some form of a dominant western US trough that will begin spreading strong (greater than 40 knots) southwest midlevel flow in a jet somewhere over the Plains (at this point, central to northern). In response, southerly winds 'switching on' the low level moisture conveyor are shown beginning across the Plains on Tuesday and continuing through the week.

It's a little early to get nitpicky about the details (like the GFS showing somewhat shallow moisture depth even into Thursday, capping issues and veered 850mb winds), but it's safe to say some Plains and/or Midwest observing is becoming more likely next week. Given the flip-flopping leading up to this, it's possible the models could be playing one of their Lucy-vs-Charlie Brown football tricks on us all. Furthermore, the Euro model is still insistent on the trough being too far north and/or west to do much for the Plains. So, I won't really start giving high observing probabilities to this system until we get a few more days of model output.

The following table plots the chance of a Plains weather expedition happening in a particular date range (Midwestern expedition days are not factored into this table):

2012 Plains Storm Expeditions - Probabilities as of May 18
May 18-212%
May 22-2550%
May 26-3130%

Midwest Chase Probabilities: The following table outlines the probabilities of a Lower Midwest chase (within 5 hours of St. Louis) happening within a particular time frame:

2012 Midwest Chases - Probabilities beginning May 18
May 18-192%
May 20-2185%
May 22-2550%

Midwestern chase forecast discussion (Update based on 5/18 00z data): A local low-key lightning expedition still looks likely on Sunday afternoon/night in and close to St. Louis as a cold front moves through. Next week, a Midwest chase or two may be in the cards thanks to the potential upper system moving into the central US - depending on whether or not I'm able to make it out west for one of the Plains setups, if they materialize.

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