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SPC MCD Watch Probability

17 Apr 2025 05:30 AM
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  

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SPC Watch Issuance Confidence

29 Mar 2017 05:36 AM
Continuing with the theme of this week being Severe Weather Awareness Week in Iowa, today's featured plot looks at an analysis of convective watch issuance verification based on probabilities included in Mesoscale Convective Discussions (MCD) issued by the Storm Prediction Center (SPC). The SPC issues the MCD product to provide a short term update and analysis of the storm scale environment during active weather. Prior to most tornado or severe thunderstorm watch issuance, the SPC typically issues a MCD to highlight an area of interest and provides a confidence percentage on if a new watch may be soon issued. The association of an issued watch and the MCD product is not exact, so a simple algorithm was used with reasonable defaults to provide a verification of these percentages. The algorithm being the new watch needed to be issued with 2.5 hours and at least 50% of the watch polygon had to overlap with the MCD polygon. What is the moral of the story? Well, the high confidence MCDs typically get an issued watch, the lower confidence ones typically do not. The next time you read a MCD product from the SPC, check out the confidence noted and see if a subsequent watch is issued!

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Tags:   nws   spc   mcd   watcfh