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                   Tuesday, March 15, 2011 - 8:31AM CDT

Chase forecast update for March 15, 2011

By DAN ROBINSON
Editor/Photographer
25 Years of Storm Observing
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Taking a look at the models out to 7 days, it appears at least one wave of severe weather will sweep across the Plains and Midwest region starting as early as Saturday through Monday. Of course, details (and even the general pattern) are impossible to nail down this far out, but we can see some potential from the indicated large-scale pattern. Everything I'm about to say is subject to change (likely to in fact) as the event draws closer, so don't take this as a concrete forecast! A strong western US upper trough is shown moving into the central US over the weekend, spreading 40 to 60 knot southwest flow at the (500mb) midlevels, strengthening to over 80 knots by Monday.

Plains: The best tornado potential with this system lies, as usual, over the western Great Plains - this model run would place targets along the dryline near the northern Texas/New Mexico border into southwest Kansas on Saturday and central/southwestern Kansas on Sunday. These areas are shown with very good directional shear in place from the surface all the way up through the mid and upper levels. Surface and 850mb winds are shown backed nicely in these areas, with a surface low developing and moving in the general vicinity of central Kansas by Sunday afternoon. Moisture, as is common in March, will be the limiting factor - with dewpoints only in the 50s on Saturday. This of course will not prevent the development of storms and supercells, and with the degree of shear in place, tornadoes are a decent possibility, on Sunday in particular near the surface low in Kansas. A strong but breakable capping inversion between 850m and 700mb should prevent a big squall line so typical with March systems, however this feature may cause storms to initiate late in the evening or after dark.

Midwest: The intensifying +80kt jet streak shown in the image above would increase shear over the central US on Monday. As usual, without the Rockies to channel low-level flow into a more southerly direction, 850mb winds veer sharply under the midlevel jet streak over the Midwest. This decreases the deep-layer directional shear component, however 850mb winds are shown increasing to over 50 knots, introducing strong speed shear. The surface low unfortunately is shown lifting far to the north by Monday afternoon, which will decrease surface wind backing and low-level convergence/directional shear in the lower Midwestern region (including St. Louis). The GFS model shows strong convection developing by midday Monday in western Missouri, and given the shear profiles and degree of upper forcing, this should be in the form of a strong squall line moving across Missouri and the St. Louis metro overnight into Tuesday morning.

So, in a nutshell, this appears like more of an expeditionable setup for the Plains than the Midwest as far as tornadoes and supercells are concerned. The degree of moisture return and directional shear over the entire central US this weekend would promote a possible supercell or two over the Midwest if we can see a strong warm frontal system develop in conjunction with the Kansas surface low. However, the model doesn't indicate that occuring at this time - but will be an outlier possibility to keep an eye on. My plans, as stated before, will be to forego the Plains and stay here in the St. Louis area for the low-probability setups on Sunday/Monday with lightning photography on Monday night.

I will post an update later this week on the forecast for Sunday and Monday as the event draws closer.

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