An upper-level system over the Ozarks lifted north and interacted with a northern stream shortwave moving across the area, amplifying forcing. High precipitable water values of nearly two inches were in place.
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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.
An upper-level system over the Ozarks lifted north and interacted with a northern stream shortwave moving across the area, amplifying forcing. High precipitable water values of nearly two inches were in place. While the system was somewhat progressive in nature, an area of enhanced reflectivity set up over east-central to northeast Iowa, resulting in prolonged heavy rain in the area.
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 27, 10:49 PM CST, 4 hours 11 minutes earlier.
Area Forecast Discussion National Weather Service Des Moines IA 1149 PM CDT Wed May 27 2020 ...Updated Aviation Discussion... .DISCUSSION.../Tonight through Wednesday/ Issued at 237 PM CDT Wed May 27 2020 Not nearly as active this afternoon but still some worries over the next few hours. Surface vorticity remains good along boundary across western Iowa and have a report of a funnel near Dawson earlier. Limiting factor in having additional funnels or even a landspout or two so far has been very limited 0-3KM CAPE. However, this may increase over the next hour or two and may still have some limited threat of seeing a funnel or even brief landspout near the boundary prior to decreased heating later today. Otherwise, scattered showers are approaching from the southeast in response to renewed forcing and moisture push as upper system over the Ozarks begins to lift northeast. This forcing becomes more amplified overnight as interaction with a weak shortwave to the north increases with showers and embedded thunderstorms becoming likely overnight across the southeast half to two-thirds of the forecast area. PWATS are expected to be quite high with values of 1.5-1.75" across the east into tonight and should help in rain production. Once again, precipitation should be somewhat generous but for the most part, remain below flash flood potential outside something localized in an urban setting. Drier air will be working into northwest Iowa overnight as the boundary begins moving eastward with lifting ceiling. Precipitation persist in the southeast into Thursday but slowly gets suppressed southeastward as the drier air pushes across Iowa. Meanwhile, increasing subsidence is expected to lead to clearing farther northwest by afternoon with much more pleasant humidity levels. Surface ridging into Thursday night and Friday provides quiet weather along with mild temperatures and little cloudiness. Guidance continues to show and southeast moving shortwave passing through the upper Midwest on Saturday into Saturday evening. Warm advection tries to sneak back into the southwest half of the state by Saturday with the threat of showers and theta-e advection increases. However, the surface ridge is expected to be stuck across northeast Iowa feeding drier air from the east and northeast back over the state. Some light showers and even a thunderstorms or two may occur during the day into the evening in the western half of Iowa, but any such precipitation appears light at this time. A more significant push of warm air and moisture are expected later Sunday into early next week as upper ridging builds into the central United States. Still some difference in the models on the handling of the amplitude of the ridge, therefore confidence remains rather low for the outcome, especially precipitation-wise into the middle of next week. For now, certainly seems warmer and more humid into the week. && .AVIATION.../For the 06Z TAFS through 06Z Thursday night/ Issued at 1143 PM CDT Wed May 27 2020 Delayed onset of showers overnight along with the MVFR ceilings. Hires models in good agreement with showers developing past 10z Thursday and lasting through around midday with them drifting east. Winds look to shift more north by the end of the period and increase to 10-20 knots. && .DMX WATCHES/WARNINGS/ADVISORIES...NONE. && DISCUSSION...Cogil AVIATION...Podrazik
Hazardous Weather Outlook National Weather Service Des Moines IA 316 PM CDT Wed May 27 2020 IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075- 081>086-092>097-282030- 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- 316 PM CDT Wed May 27 2020 This Hazardous Weather Outlook is for portions of central Iowa. .DAY ONE...This afternoon and tonight Showers and isolated thunderstorms are possible into tonight. Locally heavy rainfall is possible. Localized flooding issues may occur. .DAYS TWO THROUGH SEVEN...Thursday through Tuesday Additional showers and isolated thunderstorms are expected on Thursday. Locally heavy rainfall is again possible, with localized flooding issues possible. Additional thunderstorms are possible Saturday into Saturday night, as well as on Monday and Tuesday night. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected. Zogg
The Des Moines radar (KDMX), 54 km away, was scanning. The volume nearest the first event began at 09:00 UTC.
2 heavy rain, 2 flash flood. Local standard time, as filed.
WOI replayed a public report of 2.25 inches of rain. Beginning and end time roughly estimated from radar.
CO-OP report of 3 inches of rain in 3 hours.
Water over many secondary gravel roads in and around Sumner.
Emergency manager reported water running over gravel roads across the northern and eastern parts of the county.
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, May 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 2026-09-24. Each event’s entry at NCEI: 890979, 890980, 890981, 890982.