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                   Thursday, May 24, 2012 - 12:05AM CST

Chase forecast update for May 24-June 5

By DAN ROBINSON
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The upcoming few days will feature simultaneous Midwest and Plains chase targets, so this post will combine the two regions into one. Chasing is likely in one or both regions over the next 5 days! Shortwave troughs are shown ejecting into the Plains and Midwest through this weekend, spreading strong southwesterly to westerly flow aloft overtop of regions of instability. This will provide a few chase setups in both regions. The low-level moisture situation should be improving slightly with time, although still not ideal. The details for each day look to be too unpredictable to narrow down in advance, but it appears that opportunities will exist both along the dryline in the Plains and along a frontal boundary in the Midwest.

At this stage, I'm optimistic about getting out for at least two expedition days during this system - the first being today (Thursday the 24th). Some of the Midwest plays look just as good, if not better, to me both in parameters and convenience than the dryline out west. Today (Thursday), good upper support will overspread a skinny instability axis in southern Iowa and northern Missouri. The instability axis is shown as being very narrow, which will limit the time frame for storms to produce. However, I believe it's likely the best play of the day anywhere, and worth taking a vacation day for the relatively short drive north of STL. If storms can develop farther south, the action could be even closer to home in a wider instability axis, but with a little less upper support.

Work will tie me down on Friday and Saturday during a couple of decent-but-conditional Plains setups out west. If the cap can break, those two days could be good scores for expeditioners. A small chance of local nighttime lightning expeditions will be present here around STL during those days. Sunday, another dual-region setup takes shape, again with a dryline play out west and possibly a warm frontal setup in the lower Midwest. My general rule for a 'dual region' tornado setup is to always choose the Midwest, not only for the convenience but for the likely low observer numbers and the chance to score a home-region tornado. As it's currently shown by models, the Missouri/Iowa/Illinois tri-state region could be the target again on Sunday. With that being my normal day off, an expedition is a no-brainer.

Monday, another normal day off, looks like it could be another Midwestern day. The upper system still has strong midlevel flow over the Midwest and Plains. The best shear/instability combination is shown in Missouri, though low-level winds could be too badly veered for a good tornado day. The best low-level wind profile should be in Oklahoma or Kansas along the dryline. With Monday being another free-to-travel day, I'm not ruling out the Plains option if the Midwestern one looks too marginal. Tuesday is similar to Monday, but with northwest flow on the backside of the shortwave and no clear surface focus (aside from the triple point, again in the Plains). I've also scheduled another vacation day for Tuesday, providing for a possible hybrid Midwest-Plains weather expedition from Sunday through Tuesday.

I expect the targets for Sunday-Tuesday to change considerably due to there being multiple potential plays and the setups being several days out in model-land. I'd also expect these days to be highly dependent on morning analysis and real-term data.

Beyond Tuesday, models continue to show another major ridge building into the Plains/Midwest, shutting down tornado season for another extended interval. However, northwest flow could remain over the Midwest, providing for some various chase opportunities for as long as upper support can remain over good instability.

The following tables plot the chances of Plains and Midwest expeditions happening in a particular date range:

2012 Plains Storm Expeditions - Probabilities as of May 24
May 24-260%
May 27-2950%
May 30-June 510%

2012 Midwest Chases - Probabilities beginning May 24
May 2495%
May 25-2615%
May 27-3090%
May 31-June 540%

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