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                   Tuesday, April 24, 2012 - 12:30AM CST

Chase forecast update, April 24

By DAN ROBINSON
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In a testament to the difficulty of springtime long-range forecasting, it turns out that the GFS and Euro models appear to have blown their forecast for Thursday and beyond. Instead of a moderately amplified western trough, the GFS, Euro and now the NAM/WRF are coming together on a less than ideal chase setup for the Plains this weekend, similar to what was shown by the GFS and Euro several days ago. A shortwave is shown ejecting eastward over the Plains, providing one good expedition day on Thursday - but the overall jet pattern through the weekend remains largely unimpressive, with a few weaker intervals of southwesterly to westerly flow of up to 40 knots. While these types of patterns are capable of bringing good supercell and tornado days, they are more conditional, usually result in one one or two target storms, and storms often struggle with organization and going HP too fast. Certainly doable for those on chase vacations and local residents, but not worth the expense of a 3,000 mile trip for "outside of the Plains" expeditioners.

In another twist, a setup for northwest-flow supercells exists in the lower Midwest (Missouri, Illinois and Indiana) from tonight through Wednesday night. The big question for these appears to hinge on whether moisture return that the models show will actually occur, as currently the region is very dry in the low levels with deep moisture far into the Gulf of Mexico. It doesn't look likely that the deep moisture will make it north in time as the models indicate, nor is there any abundant local source for low-level moistening. However, if enough moisture can be realized for storms to fire, they will do so in a favorable kinematic environment with veering-with-height wind profiles: a 50 to 60 knot 500mb jet overhead, westerly 850mb winds and southwesterly surface winds (basically a southwest flow setup turned 90 degrees clockwise).

All told, later this week, deeper moisture should be back in place across the Plains and Midwest. Also, we'll have some type of upper jet in place *somwehere* close by to provide, at the very least, minimally-adequate shear for severe storms and a few tornadoes. With as bad as the models have done recently more than 3 days out, where any such setups end up is too variable to call now. For the time being, I'd expect any good chaseable, targetable setups to only become evident less than 48 hours in advance. In other words, a time to be on 'standby' for short-notice expeditions.

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 April 24
April 24-300%
May 1-1525%
May 15-3130%

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