During the day of the 20th, a cold front made its way southward through Iowa before stalling out during the evening of the 20th/early morning hours of the 21st. No severe weather occurred, but with an unstable MUCAPE environment of 1000-2000 J/kg low level jet initiated storms trained over the same area for a number of hours.
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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.
During the day of the 20th, a cold front made its way southward through Iowa before stalling out during the evening of the 20th/early morning hours of the 21st. No severe weather occurred, but with an unstable MUCAPE environment of 1000-2000 J/kg low level jet initiated storms trained over the same area for a number of hours. The areas the received the heaviest rainfall ran west to east from roughly Albia to Ottumwa and Fairfield.
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 Sep 20, 5:45 PM CST, 4 hours earlier.
Area Forecast Discussion National Weather Service Des Moines IA 645 PM CDT Wed Sep 20 2017 .SHORT TERM.../Tonight through Thursday/ Issued at 303 PM CDT Wed Sep 20 2017 As of 20z a cold front was slowly sagging east through the CWA, evident on radar and surface obs stretching northeast from near Creston, through the DSM metro, to just west of Waterloo. Northwest winds behind the front have ushered in a much drier airmass with dewpoints falling into the 50s. Warm and humid conditions continue out ahead of the front. Increasing instability and convergence along the frontal axis has triggered a few updrafts in northeast Iowa. A strong mid-level cap will likely limit the intensity and coverage of this activity through early evening. Hi-res models advertise more convective potential after 00z further south along the front into south central and southeast Iowa. Again, widespread coverage should be limited by the cap and weak height rises aloft. The unstable airmass and 20-30 kts of effective layer shear may be enough to support a few strong to severe storms, capable of producing hail and strong wind gusts. The cold front stalls out over our southeastern zones tonight. Rich theta-e low level flow lifting over the boundary will maintain the potential for a few showers and storms through Thursday morning, mainly south of Interstate 80. The boundary retreats northward as a warm front tomorrow morning in response to mid-level height falls over the high Plains. The reprieve from the unseasonably high humidity will be short lived as upper 60s/low 70s dewpoints advect back northward through the state. A hot and humid afternoon is expected with heat indicies in the low to mid 90s. Despite the development of a highly unstable airmass, a strong cap and lack of focus for convergence should suppress any late day convective potential. .LONG TERM.../Thursday night through Wednesday/ Issued at 303 PM CDT Wed Sep 20 2017 First half to two thirds of the longer term will be dominated by the amplifying upper pattern. The deepening western will lead to an large upper ridge across the eastern United States. Will see surface front enter the Plains on Thursday and becoming nearly stationary as upper flow becomes nearly parallel with the boundary. This remains the case until Monday when the boundary finally slips eastward into the state. In the meantime, deep southerly flow is expected across Iowa from Thursday night into the weekend. Being in the warm sector, forcing will be limited with a very warm to hot airmass in place for the state. Temperatures will be well above normal with highs in the mid to upper 80s with even some 90s on Friday into Saturday and likely Sunday as well. In addition, moisture will flow north into the state with dewpoints into the mid 60s to even lower 70s. Heat index values well into the 90s if not approaching 100 are expected on Friday if not into the weekend as well. The front will begin to approach western Iowa late Sunday and gradually move into the state on Monday. Increased convergence and lift along the front will help to spread showers and storms into the forecast area during the day. Both GFS and Euro have continued to slow the progression of the boundary with thunderstorms chances persisting well into Tuesday before gradually moving eastward by the end of day 7. Temperatures will cool to normal or below by the middle of next week as the boundary passes through the state. && .AVIATION.../For the 00Z TAFS through 00Z Thursday evening/ Issued at 645 PM CDT Wed Sep 20 2017 A boundary over central Iowa will move slowly southeast overnight before stalling. Expect light winds in vicinity of the boundary with northerly winds over northern Iowa. A few thunderstorms are possible across Iowa through this evening and may be possible in vicinity of KOTM but threat is look enough to not mention at this time. A band of MVFR to local IFR stratus is possible over central and northern Iowa late tonight into early Thursday in advance of a warm front lifting back north. The stratus should clear by the afternoon. Expect gusty south winds as the boundary passes through. && .DMX WATCHES/WARNINGS/ADVISORIES... NONE. && SHORT TERM...Martin LONG TERM...Cogil AVIATION...Donavon
Hazardous Weather Outlook National Weather Service Des Moines IA 333 PM CDT Wed Sep 20 2017 IAZ028-038-039-049-050-059>062-070>075-081>086-092>097-211100- Bremer-Grundy-Black Hawk-Marshall-Tama-Dallas-Polk-Jasper- Poweshiek-Cass-Adair-Madison-Warren-Marion-Mahaska-Adams-Union- Clarke-Lucas-Monroe-Wapello-Taylor-Ringgold-Decatur-Wayne- Appanoose-Davis- 333 PM CDT Wed Sep 20 2017 This Hazardous Weather Outlook is for portions of central Iowa. .DAY ONE...Tonight Isolated to scattered thunderstorms are expected into tonight. A few of the storms may approach severe limits with marginally severe hail and wind gusts. Localized heavy rain will also be possible. .DAYS TWO THROUGH SEVEN...Thursday through Tuesday The combination of heat and humidity will push heat index values into the mid 90s with some locations approaching 100 on Friday into Saturday. A few isolated non-severe storms are possible Thursday morning and then again Thursday evening. Otherwise, more widespread showers and thunderstorms are expected to mainly arrive over northwestern Iowa by Sunday and over central and eastern Iowa by early Monday lasting through much of Tuesday. It is too early to determine the severe weather threat this weekend into early next week. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected at this time. IAZ004>007-015>017-023>027-033>037-044>048-057-058-211100- Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo- Pocahontas-Humboldt-Wright-Franklin-Butler-Sac-Calhoun-Webster- Hamilton-Hardin-Crawford-Carroll-Greene-Boone-Story-Audubon- Guthrie- 333 PM CDT Wed Sep 20 2017 This Hazardous Weather Outlook is for portions of central Iowa. .DAY ONE...Tonight No hazardous weather is expected at this time. .DAYS TWO THROUGH SEVEN...Thursday through Tuesday The combination of heat and humidity will push heat index values into the mid 90s with some locations approaching 100 on Friday into Saturday. A few isolated non-severe storms are possible Thursday morning and then again Thursday evening. Otherwise, more widespread showers and thunderstorms are expected to mainly arrive over northwestern Iowa by Sunday and over central and eastern Iowa by early Monday lasting through much of Tuesday. It is too early to determine the severe weather threat this weekend into early next week. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected at this time.
The national radar mosaic for the moment the first event began.
3 heavy rain. Local standard time, as filed.
A personal weather station on the SW side of Ottumwa recorded heavy rainfall of 5.30 inches.
River gage along the Des Moines river measured heavy rainfall of 6.20 inches.
Personal weather station recorded heavy rainfall of 6.08 inches on the SW side of Ottumwa.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Iowa in 2017Every state in 2017This day in weather historyWarnings in Iowa, September 2017
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: 717417, 717418, 717419.