THE JULY 5, 1921 PROGRESSIVE DERECHO

A long-lived, widespread damaging wind event blasted areas from Wisconsin to at least Kentucky. This occurred amidst a torrid heat wave with air temperatures to 104.

Posted: Jul. 4, 2018 3:33 PM
Updated: Jul. 4, 2018 10:31 PM


On July 5, 1921, a significant, long-lived, widespread damaging wind event swept a swath of nearly 600 miles in length over the Great Lakes, WLFI viewing area & Ohio Valley. 

A derecho, this likely bow reportedly began to produce wind damage near Green Bay, Wisconsin & then picked up speed & blasted south-southeastward at 60 mph.  Classified as a progressive derecho, such type usually occurs on the periphery of a significant heat wave in the "Ring of Fire" as hot, humid, unstable air feeds it at the surface, while dry air impinging from the Plains at mid-levels enhances the downdrafts of rain-cooled air.  This aides in the development of a cold pool, which causes the initial storms to begin to surge forward & accelerate.  Typically, an initial disturbance like a shortwave (cold pocket) or MCV (old vortex from previous clusters of storms) serves as the trigger & initial stronger winds & shear develop & maintain it before it is "off to the races".

1921 was a torrid summer, especially around the time of this derecho.  Drought gripped the area after an unusually warm, dry spring & an early start to the hot summer with multiple days of +100 degrees.

Much like the June 2012 derecho that mainly affected areas to our north & northeast to southeast, this 1921 event occurred on the periphery of temperature near or exceeding 100 degrees.  Unlike the 2012 one, however, the 1921 bow tracked southeast & then due south, which I have seen derechos do in multiple instances.  Take the June 29, 2018 marginal derecho to our southwest & two other derechos to our south (three over a two-day period!).  They turned more to the due south & actually southeast with time on the periphery of the hot upper ridge.  Progressive derechos are by-products of torrid heat waves.

June 28-29, 2018:

https://www.youtube.com/watch?v=x5yV4FNHf8s

June 29, 2012:

These are plotted storm reports from U.S. Weather Bureau at the time & newspapers. 

In the WLFI viewing area wind damage was reported in Jasper, White, Fulton, Cass, Clinton, Miami, Carroll & Howard counties.  There was probably other damage &/or severe gusts, but in 1921, the network of observers & that communication between those observers in rural areas & media to Weather Bureau was limited.

The worst damage reported in our area was centered around Lake Cicott & southeastward in Cass County in a strip of intense wind damage to structures, forests, fence & crops.  Kokomo was also hit particularly hard with extensive wind damage.  One person was killed at Wolcott in the storm.

Based on the damage reports, preliminary analysis suggests a macroburst* with winds up to 110 mph in Cass County around Lake Cicott to the southern part of the county.  Length was around 20 miles with maximum width around 5 miles.  Damage suggest gust to 85 mph in Kokomo from microburst*.  Elsewhere in the track, limited reports suggest winds of 55-70 mph.

*Note:  There are two types of downbursts; macroburst & microburst.  Diameter of macroburst is more than 2.5 miles.  Microburst is at or less than 2.5 miles.

So damaging was the derecho to the network of powerlines across northern Indiana that the Indiana & Michigan Electric Company took out a full page add in the South Bend News-Times on July 12, 1921.

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