Tuesday, July 1, 2014

2 Derechos to end June?

Shelf Cloud, Monmouth IL 6/30/14 -- Jeff Last

The word derecho has been thrown around a lot ever since late June 2012 when the national media grabbed hold of the term. While many forecasts have included the term over the past 2 years, very few storms have actually verified as a true derecho. But on Monday, June 30th it could be argued that two derechos struck the Midwest. 

First, let's review the true criteria for a derecho.

While many MCS (Mesoscale Convective Systems) have come and gone with widespread wind damage, many times it is the high end wind damage (75 mph or greater) reports that are lacking to define a storm as a derecho. That was not the case with either of these thunderstorm complexes. 

The first had 7 confirmed reports of 75 mph winds or greater between 1 PM and 4 PM. (Reports are via SPC filter storm reports)

1810 81 HARDIN IA
1949 80 LINN IA
2007 80 CLAYTON IA
2017 80 LINN IA
2045 90 DUBUQUE  IA
2048 80 SCOTT IA
2100 78 GRANT WI

The second line of thunderstorms that develop also produced widespread wind damage between central Illinois and near Detroit, MI. Over the 240 mile base line criteria. 

303 82 WILL IL
320 85 COOK IL
327 75 COOK IL
330 75 COOK IL
331 75 WILL IL
335 85 BEAVER OK
342 86 LAKE IN
500 75 CASS MI

While these two complex of thunderstorms went over the parts of the same geographic areas, they were two distinct line of storms. The first squall line affected more square miles, but the second resulted in more significant wind reports. So the question is asked, do we classify one of these as a derecho or both? 

It should also be noted and not debated the job of those who work at local NWS offices and the SPC for their job with predicting this severe weather outbreak. Below is a verification of the SPC 1630z severe weather outlook.