Classic Severe MCS “Not-crossing” Case this morning

No detailed review here, but thought I’d just take this opportunity since we had a strong MCS (mature bow echo) approaching the west side of the southern Appalachians this morning, with a history of severe weather (see below), to remind folks about some past research associated in part with previous CSTAR efforts. WFO RNK and SPC originally began looking at several years of severe MCS events approaching the Appalachians (50 cases total) to see if we could determine any important environmental clues that would help determine the likelihood of severe weather with these events crossing east of the mountains. A link to those original efforts is here:

http://intranet.wrnk.noaa.gov/reference/MCS/main%20page/severe_mcs_reference%20guide.htm

Following this, and based in part on this database RNK identified, Casey Letkewicz and Matt Parker looked at this problem more thoroughly, including some numerical simulation work. The link to a Wea and Fcstg article on this work is available here:

http://journals.ametsoc.org/doi/abs/10.1175/2010WAF2222379.1

The gist of this is that in most cases, downstream instability is the most important parameter in determining if the MCS is able to continue producing severe weather east of the mountains, but weaker wind shear downstream may also play a role. As such, climatology favors these systems crossing as severe during the afternoon and evening hours, vs. the early morning hours. A couple of exceptions were noted where a severe MCS did not make it across during the day when there was some degree of damming to the east, and another where a severe MCS did make it across overnight when instability remained high due to anomalous moisture pooling along an outflow boundary.

This morning, the MCS/bow echo was classic in appearance with an apparently strong cold pool and trailing stratiform region, and several wind and hail reports across KY/TN, but with sfc-based instability zero with a nocturnal inversion…very few reports were observed even in the higher elevations, and really none at any appreciable distance east of the mountains (a couple in the Asheville area).

 

If one simply viewed the loop of the reflectivity (not shown here), despite a weakening to the convective leading edge and apparent rear-to-front flow withing the MCS, one could argue that the system itself effectively crossed the mountains, yet clearly severe weather did not cross, and that was the focus of the above research efforts.

Mat Parker may have some additional thoughts on the findings of the above studie, and previous work he has done on MCSs in this region.

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One Response to Classic Severe MCS “Not-crossing” Case this morning

  1. mdparker says:

    Good call, Steve. I was watching this case with interest… a classic example of a threatening looking base scan in the lee, but no severe reports. Another appealing aspect of this case was the existence of an MCV (at least as inferred from radar data). Early in the “Appalachian MCS crossing” project, one of the working hypotheses was that an MCV could be generated by an MCS on the windward side of the mountains, and then (because it is a mid-level feature) it could be advected over the mountains into the lee even if the cold pool were partly (or totally) blocked by the Appalachians. We were never able to truly test this hypothesis because of data limitations as well as the necessity of a limited domain size in our numerical experiments. But, it’s still of interest. Perhaps someone can come along and try to understand the degree to which mid-level MCVs can assist in the survival of convection (if not severe convection) in the lee in such cases.

    All the best,
    Matt

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