An intense line of thunderstorms ripped, tore, screamed, blasted, and raced through the state of Iowa this morning, producing wind gusts exceeding 90 MPH in spots. Right or wrong, the storm is being touted as a “derecho” in the media; regardless of the name, the storms were brutal.


The squall line developed in northwestern Iowa, straddling the Iowa/Minnesota border as it produced a wide swath of wind damage. The area hardest-hit by the storms seem to be the corridor between Waterloo and Dubuque, along U.S. 20 northeast of Des Moines. The above radar snapshot shows the squall at 8:30 AM CDT, as it was nearing peak intensity.

Infrared satellite imagery from around the same time (shown at the top of this post) also shows just how strong these storms were—the deep grays and whites over Iowa show updrafts shoving clouds into the bitterly cold upper levels of the atmosphere, with the system’s vast anvil encircling the system (in black).

Residents had plenty of warning that these storms were coming. The severe thunderstorm warning issued ahead of the strongest part of the squall included the line “THESE ARE VERY DANGEROUS STORMS.” They were moving at 70 MPH and the alert warned residents of wind gusts to 80 MPH. That’s pretty strong language, and unfortunately, the winds played out as expected.

The above map shows storm reports received by the Storm Prediction Center between 5:00 AM and 12:00 PM CDT; blue dots indicate a report of wind damage or winds gusting higher than 58 MPH, while green dots indicate reports of hail the size of quarters or larger. As we see with most squall lines, the main hazard here was wind damage as opposed to hail.

A weather station located at a high school in Independence, Iowa, measured a wind gust of 94 MPH as the storm came through, and there were about 70 reports of wind damage collected by the Storm Prediction Center as of the writing of this post. The damage was much more extensive—this just accounts for damage reported to the agency. Officials in Sheldon, Iowa, report that the town’s airport sustained nearly half a million dollars’ worth of damage when winds gusting up to 95 MPH tore through the town. The storm destroyed five small aircraft and several structures on the airport grounds.

Reports of destroyed structures and costly damage are common across the area, both to structures, trees, and especially crops. There are still about 25,000 power outages across Iowa thanks to the storms, and storms this afternoon could serve to delay restoration of service to those stuck in the dark.

Some injuries were also reports as a result of the damage. Robert and Ursula Cole were inside their two-story home when the winds applied so much force to the structure that the first floor collapsed, trapping the 73- and 78-year old inside. Ursula is in the hospital this afternoon, and Robert was rescued without serious injury, according to KCCI-TV.

Damage to crops is something that happens every year, whether by wind or hail. Miles and miles of corn fields mowed down by the strong winds isn’t an uncommon sight across Iowa this afternoon. It remains to be seen how much of it is salvageable—sometimes if the damage isn’t too severe, the plants are able to continue growing even if they’re bent or leaning over.

More Storms Coming (Maybe)

The atmospheric setup was perfect for a destructive line of storms to tear through Iowa this morning. At the time of the storms, we had an upper-level trough moving through the Dakotas, strong winds in the mid-levels of the atmosphere, the edge of a pool of high dew points sitting over Iowa, and a warm front at the surface that served to focus the line of storms.

Areas hit by the strong storms this morning aren’t in the clear just yet, either. The atmosphere is slowly recovering from this morning’s storms, rebuilding the instability necessary for new thunderstorms to develop and tap into the favorable environment for severe weather. If enough instability is able to build back in, there’s an enhanced risk for severe weather across portions of the Midwest and Great Lakes, including cities like Chicago, Detroit, and Cleveland.

Due to strong wind shear in the lower levels of the atmosphere, there’s also an elevated risk for tornadoes across much of the enhanced risk zone this afternoon. The greatest risk for tornadoes lies along any outflow boundaries that exist across the area; fronts, even those as small as a storm’s outflow boundary, are excellent focal points for severe weather to develop.

A 10% risk for tornadoes doesn’t sound like much, but given that values of 2% to 5% usually warrant concern, 10% is nothing to sneeze at. (See this post for a deeper explanation for these risk values.)

Derecho? Squall? Squallrecho Death Storm 2015?

The big question among weather geeks today, though, is whether or not the storm constitutes a dreaded “derecho.”

A derecho is an intense squall line that produces extensive wind damage over a path at least 250 miles long. Derechos are the polar vortices of summer in that most people only first heard the term a couple of years ago, and the media preyed on this unfamiliarity in order to trumpet these storms as some new, powerful demon we all must confront lest it kill us in our sleep.

The most widely-known derecho in recent years was the one that tore a path from Indiana to Washington D.C. and Baltimore back on June 29, 2012—since the derecho hit such a heavily populated area, the news was all over it, and this was the first time the term “derecho” really made it into the vocabulary of people who aren’t weather geeks. Some of us jokingly refer to a derecho as “the ‘d’ word” for its ability to whip people up into a panic.

Of course, nerds have to ruin the whole thing by arguing over semantics. According to sticklers, if that swath wind damage doesn’t reach the magic, arbitrary 250-mile marker, it’s not a derecho, period, the end. More lenient folks are willing to give storms a pass if they produce extensive damage over a widespread area that’s a bit smaller than 250 miles.

If you measure the main swath of widespread wind damage, the path only comes out to about 180 miles—70 short of the definition of a derecho. However, if you consider a smaller trace of wind reports that extends out toward Madison, Wisconsin, the path just surpasses that 250-mile mark, making the system a derecho by the definition.

Correction: I was wrong when I initially published this post. The main swath of wind damage associated with the squall line started in southeastern South Dakota near Sioux Falls, not in central Iowa. When I downloaded the storm data into Google Earth to measure distance, the data started at 7:00 AM CDT, leaving out at least two hours of damage that extended well to the west toward South Dakota. Sorry about that.

The swath of damage is arguably longer if you include the original complex of storms that formed farther west in South Dakota, but just this main squall alone produced a swath of wind damage that was more than 300 miles long. The SPC reports that there was a 122 MPH wind gust reported by a home sensor in central South Dakota near Hayes, back before the main squall line in question developed. I’m skeptical of that value, though—home sensors have a tendency to not measure winds accurately once they get up that high.

Here’s a radar loop showing the squall from about the time it congealed in southeastern South Dakota around 4:30 AM CDT, until it crossed the Mississippi and lost most of its punch.

Again, it’s mostly a matter of semantics and what will gather the most clicks and ratings when reporters discuss the storm in the coming hours. The people who lost their homes this morning don’t really care if it was technically a derecho or not. By the definition, though, this was indeed a derecho.

If you live in the area, continue keeping an eye on the Storm Prediction Center and your local National Weather Service office. If forecasts pan out, you’re not out of the woods just yet.

[Images: NASA, author, Google Earth]

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