A large area of high pressure dominated the eastern US with low pressure settling across the Pacific northwest. The resultant southerly flow allowed fro deep moisture transport from the Gulf into the Midwest.
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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.
A large area of high pressure dominated the eastern US with low pressure settling across the Pacific northwest. The resultant southerly flow allowed fro deep moisture transport from the Gulf into the Midwest. A subtle disturbance in the southerly flow moved into Iowa through the afternoon and evening hours with widespread shower activity. The moisture rich environment allowed for LCLs below 500 m, combined with 0-3 km CAPE near 100 J/kg and 0-1 km SRH near 200 m2/s2. Many of the showers exhibited rotation and a couple did produce low end tornadoes and wind damage. Interestingly, the deep warm clouds within this tropical air mass resulted in no lightning with any storm the entire night.
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 May 20, 5:35 PM CST, 2 hours 19 minutes earlier.
Area Forecast Discussion National Weather Service Des Moines IA 635 PM CDT Thu May 20 2021 ...Updated for 00z Aviation... .DISCUSSION.../Tonight through Thursday/ Issued at 215 PM CDT Thu May 20 2021 A highly amplified synoptic pattern remains in place over the conus with a deep trough out west and an downstream ridge out east. The local region lies under deep southerly flow which continues to stream a rich supply of Gulf moisture northward. Widespread cloud cover and a few areas of drizzle and light showers are evident on satellite and radar which has hindered warming today with early afternoon temps generally in the low 70s. Shower and thunderstorm coverage will increase again as a remnant MCV approaches from the south later this afternoon and evening. Low level wind field will be supportive of a few rotating storms should any robust updrafts materialize. Surface winds steadily back a bit and become more SSE in response to the approaching wave leading to 0-1km SRH values ~ 100 m2/s2. Although deep layer instability is weak due to limited insolation and nearly moist adiabatic lapse rates, most instability that does exist is rooted near the surface and could still be favorable for low level parcel acceleration and stretching. Thus a marginal threat for a tornado or two exists along and south of I-80 through this evening. Hires model runs today have supported this general idea with isolated to scattered weak UH tracks associated with deeper convection. Otherwise much of the area looks to receive more rainfall later today into tonight with some isolated spots possibly taking on another 1 to 2 inches. Recent rains have produced little to no response on local waterways and thus no concern for excessive rainfall at this time. Chances for showers and a few rumbles of thunder linger into Friday, but a gradual decrease in precipitation coverage is expected as the axis of deeper moisture and kinematic support shifts east. Drier air filtering in from the west eventually begins to break up cloud cover and brings sunshine to portions of the area before the day is over. Warmer temperatures build for the weekend as the eastern upper ridge retrogrades west. NBM is considerably warmer than most model guidance, however highs in low to mid 80s appear reasonable given h85 temps warming to near 15c. Cloud cover uncertainties cast a little doubt as models still develop some scattered shower or storm activity. Any forcing or low level impetus for convection is nebulous at the moment, but it wouldn't take much to get some activity going given the anomalously moist airmass in place. Low chance PoPs are appropriate for now. Mean western trough axis swings north of Sunday and is absorbed into the mean flow as it tries to crest the ridge early next week. CIPS analogs highlight this period for potential strong to severe storms, but uncertainties surrounding precip timing and possible storm modes hinder forecast confidence at this time range. && .AVIATION.../For the 00Z TAFS through 00Z Friday evening/ Issued at 635 PM CDT Thu May 20 2021 Widespread rain across much of the area will impact all sites through the evening. Cigs initially above 3kft will diminish to MVFR/IFR overnight into Friday morning. Showers gradually taper off through midday Friday, however cigs will largely remain at MVFR throughout the day. && .DMX WATCHES/WARNINGS/ADVISORIES... NONE. && DISCUSSION...Martin AVIATION...Hagenhoff
Hazardous Weather Outlook National Weather Service Des Moines IA 236 PM CDT Thu May 20 2021 IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075- 081>086-092>097-211100- 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- 236 PM CDT Thu May 20 2021 This Hazardous Weather Outlook is for portions of central Iowa. .DAY ONE...This afternoon and tonight Widely scattered thunderstorms are forecast at intervals through tonight. There is a low risk of severe weather through the evening hours. Isolated strong wind gusts are the primary threat, but an isolated tornado cannot be ruled out in southern Iowa. .DAYS TWO THROUGH SEVEN...Friday through Wednesday Isolated thunderstorms will remain possible from Friday through the weekend, with a low threat of severe weather. More widespread thunderstorm chances return early next week, mainly from late Monday through Tuesday night, when severe weather may be more possible at times. .SPOTTER INFORMATION STATEMENT... Spotter activation is unlikely through the weekend. Lee
The Des Moines radar (KDMX), 51 km away, was scanning. The volume nearest the first event began at 01:54 UTC.
2 tornado, 1 thunderstorm wind. Local standard time, as filed.
A public report of wind damage within the town of Winterset with sheds blown over near the county fairgrounds. A separate personal weather station in the area reported a wind gust of 73 mph.
Tornadic Debris Signature observed on KDMX WSR-88D Doppler radar but no reports of damage, hence the EF-Unknown rating.
Weak tornado occurred south of Woodward producing minor tree damage along its path.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Iowa in 2021Every state in 2021This day in weather historyThe tornado archive for Iowa, 2021Warnings in Iowa, May 2021
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 2026-09-24. Each event’s entry at NCEI: 957607, 957627, 957628.