Late on the evening of October 11th a line of thunderstorms developed along a cold front that passed across Iowa. This was a high shear, low CAPE environment with MLCAPE of 500 to 1200 J/kg and deep layer shear of 40 to 50 knots.
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Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS DMX wrote it up.
Late on the evening of October 11th a line of thunderstorms developed along a cold front that passed across Iowa. This was a high shear, low CAPE environment with MLCAPE of 500 to 1200 J/kg and deep layer shear of 40 to 50 knots. This environment with steep low level lapse rates and DCAPE of 1000+ J/kg yielded a damaging wind threat. While no damage reports were received, many observation stations reported gusts over 60 mph. Storms exited the area to the east in the early morning hours of October 12th.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Des Moines, IA wrote its last area forecast discussion before the first event at Oct 11, 5:24 PM CST, 5 hours 22 minutes earlier.
AREA FORECAST DISCUSSION National Weather Service Des Moines IA Issued by National Weather Service Quad Cities IA IL 624 PM CDT Sun Oct 11 2020 ...AVIATION UPDATE... .Discussion... A rather wild ride from a very warm windy day today to a cooler pleasant day tomorrow is expected tonight. In the short term, the wind is certainly the story, as wind gusts continue to reach near advisory levels in the northwestern counties, while 15 to 25 mph are more common in the east. This wind will decrease with the loss of mixing depth by early evening/sunset, which will begin the reduction in the over all severe weather shear profile. Tonight, the cold front will arrive in the far west around 8 PM and exit far east around 4 AM. That will be the advertised severe weather time frame, but certainly the available DCAPE and low level shear interaction for any storm to interact with will be mainly in the far northwest with a general decay in storm intensity the farther east and late into the night we go. Thus, this is more of a "can't rule out" severe weather night, than one capable of becoming a widespread wind producer. Still, wind gusts to 40-50 mph see very much possible with evening storms, and overnight, some gusts to 45 mph still be possible, given the strong winds aloft. Rainfall tonight, should be in the quarter to half inch range for locations northwest of a Fort Dodge to Mason City line, and up to a quarter inch southeast of that axis. Looking at Monday through Wednesday, very pleasant conditions are expected with dry air, and highs reaching the upper 60s to mid 70s. A strong cold front will arrive late Wednesday, with a pattern change toward much cooler weather to finish the week. A deep and broad upper trof is forecast over the Great Lakes, with highs in the 50s, and lows in the upper 20s to mid 30s. This pattern is likely to bring a killing freeze to much of the Upper Midwest if they haven't seen it yet. ERVIN && .AVIATION...(For the 18Z TAFS through 18Z Monday Afternoon) ISSUED AT 1223 PM CDT Sun Oct 11 2020 A breezy south to southeast wind at 15 to 25 kts, gusting to 30 kts at times will continue into the early evening, with VFR/clear to partly cloudy conditions. A cold front with thunderstorms will arrive in the 03-05Z time frame and bring a 1-2 hour period of rain and gusty winds, with some threat of lightning. This front will bring a line of showers and thunderstorms east across Iowa, with a period of MVFR/brief IFR weather with it. A quick wind change to west to northwest will occur, and following the rain, a period of strong northwest winds are expected which will see gusts over 25 kts. By 10-12Z Monday, winds will decrease somewhat and VFR weather should extend through the day Monday. ERVIN && .DMX WATCHES/WARNINGS/ADVISORIES... NONE. && DISCUSSION...Ervin AVIATION...Ervin
Hazardous Weather Outlook National Weather Service Des Moines IA Issued by National Weather Service Quad Cities IA IL 436 AM CDT Sun Oct 11 2020 IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075- 081>086-092>097-120945- Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo- Pocahontas-Humboldt-Wright-Franklin-Butler-Bremer-Sac-Calhoun- Webster-Hamilton-Hardin-Grundy-Black Hawk-Crawford-Carroll-Greene- Boone-Story-Marshall-Tama-Audubon-Guthrie-Dallas-Polk-Jasper- Poweshiek-Cass-Adair-Madison-Warren-Marion-Mahaska-Adams-Union- Clarke-Lucas-Monroe-Wapello-Taylor-Ringgold-Decatur-Wayne- Appanoose-Davis- 436 AM CDT Sun Oct 11 2020 This Hazardous Weather Outlook is for portions of central Iowa, north central Iowa, northeast Iowa, northwest Iowa, south central Iowa, southeast Iowa, southwest Iowa, and west central Iowa. .DAY ONE...Today and Tonight A line of thunderstorms will move across Iowa this evening and overnight. The primary threat is damaging winds, mainly along and west of Interstate 35 where the Storm Prediction Center has placed a slight risk for severe weather. The storms are anticipated to weaken overnight as they move east of Interstate 35, where a marginal risk for severe weather exists. .DAYS TWO THROUGH SEVEN...Monday through Saturday The probability for widespread hazardous weather is low. .SPOTTER INFORMATION STATEMENT... Spotter activation may be needed tonight.
The Des Moines radar (KDMX), 102 km away, was scanning. The volume nearest the first event began at 04:45 UTC.
3 thunderstorm wind. Local standard time, as filed.
Law enforcement report of a one foot diameter tree limb down.
RWIS recorded a 61 mph wind gust.
ASOS recorded a 63 mph wind gust.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Iowa in 2020Every state in 2020This day in weather historyWarnings in Iowa, October 2020
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-10-01. Each event’s entry at NCEI: 924372, 924373, 924374.