Iowa is forecast to have a chance of severe weather today and into the overnight hours. As the event unfolds, the Storm Prediction Center (SPC) issues Mesoscale Convective Discussions (MCD)s [example MCD text] that provide up to the minute forecast details and thinking from the experts at the SPC. The MCD product sometimes portends an upcoming Tornado or Severe Thunderstorm Watch. As a part of that process, the MCD will contain a watch issuance probability (a fixed percentage value of 5, 20, 40, 60, 80, or 95 percent). So the featured chart compares this forecast value to if a watch was issued in the near term future (within 2.5 hours) for a spatially coincident area outlined by the MCD. The archive for this probability exists back to 2012 and there's an IEM portal to download the archive. Remarkably, the 60, 80, and 95 percent thresholds seem in high agreement with only a few percentage points difference between the two. The 20 percent threshold may be suffering from this arbitrary time and space threshold combination or maybe it is not well calibrated, this crude and unofficial IEM analysis wishes not to state anything absolutely.

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Tags:   spc   mcd