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                   Thursday, November 29, 2012 - 11:33PM CST

Chase forecast update for December 1-8

By DAN ROBINSON
Editor/Photographer
25 Years of Storm Observing
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Yes, you read that right. That's a typical spring/summer blog title being invoked in December! This month could be one of the more active convective Decembers that the Midwest has seen in quite some time, and possibly the best I've personally experienced in terms of photo opportunities and number of expedition days. An anomalously north-positioned series of upper troughs will be traversing the middle part of the US during the first half of the month, providing at least three early spring-like thunderstorm events in the lower Midwest. Right now I don't see any notable severe weather with these - yet. However, the chances for a few good after-dark lightning displays exists with each setup. That's enough to get me excited, especially in December.

Upper trough passages are not uncommon in the winter months in the Midwest, but usually what limits (or eliminates) the thunderstorm potential with them is the lack of low-level moisture available for instability. That won't be an issue with the upcoming systems, as plenty of Gulf moisture should be drawn northward in advance of the troughs.

On Monday, strong cold front is shown pushing through upper 50°Fs dewpoints underneath a potent swath of 40 to 50 knot west-southwesterly midlevel flow associated with the trough, lighting up convection along a cold front towards dark on Monday evening. A similar setup is shown for next Thursday/Friday by both the GFS and Euro models. On Saturday night, the arrival of the deeper moisture could also produce a round of thunderstorms marking the first potential chase of the series.

The primary limiting factor with all three of these setups is the anticipated thick cloud cover, which could eliminate any solar heating to boost instability for the afternoons/evenings. Even without that, however, the dynamics and temperature profiles will still support thunderstorms. The wild card to watch for will be if those clouds manage to clear out significantly. That will introduce instability into the equation that, combined with the wind profiles, could produce severe storms. A couple of tornadoes would not be out of the question if that happens, but the primary risk would be damaging winds with stronger cells.

From a personal perspective, this series of setups has the potential to 'clinch' an unprecedented lightning-stills-in-every-month of a single calendar year. Lightning stills are already in the bag from January to November this year, and rounding that out with some December catches would add another highlight to this already amazing chase year. Stay tuned!

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