Deadly Derecho

In my last blog on July 7th (I was on vacation last week), I wrote about derechoes and their contributions to July being the month that -- on average -- has the most reports of damaging thunderstorm winds. Derechoes are widespread damaging windstorms associated with a cluster of thunderstorms, often in the shape of a bow echo. The swath of damage can be tens of miles wide and hundreds of miles long.

One of those July derechoes formed on Sunday night July 20 and continued into the morning of July 21. Its appearance as a bow echo on radar is shown below from 06:58 AM CDT, in the red-cored storms that is about 70 miles in north-south extent (courtesy of GRLevelX). Strong winds are on the leading (east or southeast) edge of the bow echo's precipitation as the bow moves eastward.

The figure below (courtesy of National Weather Service Storm Prediction Center) shows its path, with those blue dots indicating damaging winds and the black squares indicating wind gusts of at least 75 mph. The derecho began in northeast Nebraska shortly after midnight on July 21, rolled across Iowa, and into northwest Illinois through 7AM CDT. The second diagram shows the rest of the derecho's path across northern Illinois and Indiana during the rest of the morning before it dissipated. The derecho's path was about 500 miles long!

Winds gusts were reported to 100 mph at Dawson, Iowa, 94 mph at Moline-Quad City Airport, and above 70 mph in many locations. Trees were downed in dozens of counties, some falling onto homes, and winds blew off a few roofs. 50% of the trees were damaged and 30% downed in Jamaica, IA. Many communities were left without power because of downed power lines. The windstorm turned tragic in Colona, IL, where a 4-year-old was crushed and killed by a tree that fell on his family's tent at a campground.

The Weather Channel Weather Warrior Troy Berg was located at Armington, IL (between Lincoln and Bloomington) along the south edge of the derecho and took the photo below. The storm and strong winds were moving from right to left, as he is looking toward the south. The sharp cloud edge just above the right side of the barn likely marks the leading edge of the strong winds blowing from right to left. This is a form of shelf cloud, formed as air is pushed upward as the denser, rain-cooled air from the bow echo "plows" into it.

The clouds don't look as deep/tall here as most shelf clouds. They look kind of flat. There appears to be a reason for that. I've shown below the radar image from about 7:31 AM CDT, about the time when I think the photo was taken. The head of the arrow points to Armington, IL where the photo was taken. Note that this location was well south of the strongest storms with the bow echo. And the bow echo seems to have weakened in the half hour since the previous radar figure.

What you see near the photographer's site, however, is a thin purple/blue line running southwest from near Armington to just north of ILX (the radar site at Lincoln, IL) and southwestward. This is called a "fine line" in radar terminology and depicts the gust front - the leading edge of rain-cooled outflow blowing southeastward from the yellow- and red-cored storms to the northwest. The outflow here was apparently not deep enough to trigger deep thunderstorms right along its leading edge. And that's what Mr. Berg's photo showed!

I love to show and analyze Weather Warrior photos. They often show that you can really learn about the weather by "reading the sky."