Synoptically the set up was nothing impressive. However, with 1000-2000 J/kg MUCAPE, 30-40 kts effective bulk shear, and weak surface convergence, a cluster of storms was able to develop in west central Minnesota.
Every event with a place to put it. Tap one for its entry below.
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.
Synoptically the set up was nothing impressive. However, with 1000-2000 J/kg MUCAPE, 30-40 kts effective bulk shear, and weak surface convergence, a cluster of storms was able to develop in west central Minnesota. Said cluster of storms propagated SSE into parts of northern Iowa and was able to produce a single severe hail report in Calhoun county.
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 Aug 18, 5:36 PM CST, 4 hours 2 minutes earlier.
Area Forecast Discussion National Weather Service Des Moines IA 636 PM CDT Fri Aug 18 2017 ...Updated for 00z Aviation Discussion... .SHORT TERM.../Tonight through Saturday/ Issued at 404 PM CDT Fri Aug 18 2017 Bottom Line up Front... A few strong to potentially severe thunderstorms are possible this evening primarily from central to northwestern Iowa. The storms should generally be out of our area by midnight, and then fog will impact the state of Iowa, including southwestern Iowa, through sunrise. Rest of today through tonight... 20z Visible satellite has shown generally clear skies across northwestern Iowa throughout today...now gradually being replaced by bubbling cu. 20z Water vapor imagery clearly shows an upper low pushing through eastern Minnesota into northwestern Wisconsin. Hi- res models picking up on a subtle 1012 sfc high propagating along the IA/MN border throughout today..adding a SE component to the wind. Meanwhile, another subtle sfc high was located over SE Nebraska. Coupled with westerly flow aloft, this has produced a line of slight sfc convergence oriented from NW to SE in northwestern IA...with a veering vertical profile. Around 1500 j/KG 0-6km MUCAPE starting to become tapped into in NW IA. With low-level lapse rates reaching 8 C/km and dcape values upwards of 1,500 J/kg, large hail and damaging winds will be the primary threats. Challenge will be timing...once daytime heating lost, cap will likely win out and severe potential will wane. Strong subsidence and clearing behind this system will lead to skc overnight. With minimal dwpt depression and potential moisture from precip, radiational fog seems a given across much of Iowa, including southwestern Iowa. The need for a Dense Fog Advisory cannot be ruled out north of Highway 30. Clear skies on Saturday with another day of near-normal temperatures likely. .LONG TERM.../Saturday night through Friday/ Issued at 404 PM CDT Fri Aug 18 2017 Bottom Line up Front... Temperatures warm back up Sunday through Tuesday as heat index values during this time should generally be into the 90s across the DMX CWA. Cooler and drier conditions look to move in from the middle of next week and beyond. Lastly, viewing conditions for the solar eclipse on Monday are beginning to look dicey. Sunday through Tuesday... Models remain consistent with overall setup. A deep low beings to develop over the intermountain west by sunday afternoon, with a boundary/WAA making it into southern MN by peak heating. LLJ will start to push moisture into Iowa from west to east. This LLJ will overspread Iowa during the day Monday. Fcst soundings respond, showing a decent stratocu field across Iowa in the warm, moist sector, which is very plausible. Thus, solar eclipse viewing may be very hampered across Iowa. Have 60 to 70 percent sky coverage in grids...which if anything, is underdone. Severe potential exists both Sunday and Monday evenings, though there are some negatives on each day. Sunday evening will feature "off the charts" MUCAPE values across central to northern Iowa and strong low-level lapse rates. Location of boundary will be key, as this would serve as good focusing mechanism to make storms more sfc- based versus elevated. Problem for Monday will be cloud cover and precip potential all day...this may keep environment capped. Tuesday and beyond... Long-range models have accelerated CAA as large high pressure to set up over the intermountain west. Thus, have lowered Tuesday temperatures versus previous forecasts. 850mb temps look to drop back to the +10C to +15C range...which is slightly below normal...suggesting 70s and low 80s the rest of the week...with dry air and subsidence winning out. && .AVIATION.../For the 00Z TAFS through 00Z Saturday evening/ Issued at 636 PM CDT Fri Aug 18 2017 Main aviation concerns this TAF period are thunderstorms that will dive southeastward out of Minnesota this evening and fog development thereafter through early Saturday morning. Convective allowing/short range models along with present forward movement provided timing of VCTS groups. Looking upstream over Minnesota, will likely need amendments for impacts of storms with the main concern being strong wind gusts. The only terminal without mention of VCTS was KALO as storms may stay west. After the storms pass through, expect widespread fog to form as skies are clear upstream of the storms. Rainfall that does fall from the storms will provide sufficient low level moisture along with light winds for IFR and LIFR visibility restrictions. The only exception at this point is KDSM, where MVFR restrictions are currently forecast. The fog will dissipate a few hours after sunrise with VFR conditions prevailing at all terminals through the end of the TAF period. && .DMX WATCHES/WARNINGS/ADVISORIES... NONE. && SHORT TERM...Kotenberg LONG TERM...Kotenberg AVIATION...Ansorge
Hazardous Weather Outlook National Weather Service Des Moines IA 342 PM CDT Fri Aug 18 2017 IAZ004>007-015>017-023>026-033>037-045>048-192045- Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo- Pocahontas-Humboldt-Wright-Franklin-Sac-Calhoun-Webster-Hamilton- Hardin-Carroll-Greene-Boone-Story- 342 PM CDT Fri Aug 18 2017 This Hazardous Weather Outlook is for portions of central Iowa. .DAY ONE...This afternoon and tonight Strong to potentially severe thunderstorms are possible this evening. Large hail and damaging winds are the primary threats. Areas of fog are possible tonight...especially north of Highway 30. .DAYS TWO THROUGH SEVEN...Saturday through Thursday Thunderstorms are possible Sunday afternoon into Tuesday evening. A few storms may be strong to possibly severe northwest and west Sunday and Monday afternoon and evenings. Moderate to heavy rainfall is possible through Tuesday evening. .SPOTTER INFORMATION STATEMENT... Spotter activation cannot be ruled out this evening...Sunday...and Monday evenings. IAZ027-028-038-039-044-049-050-057>062-070>075-081>086-092>097- 192045- Butler-Bremer-Grundy-Black Hawk-Crawford-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- 342 PM CDT Fri Aug 18 2017 This Hazardous Weather Outlook is for portions of central Iowa. .DAY ONE...This afternoon and tonight Scattered thunderstorms are expected this evening into the overnight. A few storms may produce small hail and gusty winds. Areas of fog are possible tonight...especially north of Highway 30. .DAYS TWO THROUGH SEVEN...Saturday through Thursday Thunderstorms are possible Sunday night into Tuesday evening. A few storms on Sunday and Monday afternoon and evenings may be strong. There may be moderate to heavy rainfall north and northeast. .SPOTTER INFORMATION STATEMENT... Spotter activation is unlikely today and Sunday...and cannot be ruled out for Monday evening. Kotenberg
Special Weather Statement National Weather Service Des Moines IA 846 PM CDT Fri Aug 18 2017 IAZ023-033-034-190400- Pocahontas-Sac-Calhoun- Including the cities of Pocahontas, Laurens, Rolfe, Fonda, Gilmore City, Sac City, Lake View, Odebolt, Wall Lake, Schaller, Early, Rockwell City, Manson, Lake City, and Pomeroy 846 PM CDT Fri Aug 18 2017 ...Strong thunderstorms approaching... At 8:45pm, National Weather Service Doppler Radar was tracking strong thunderstorms near Spirit Lake to west of Spencer. The storms west of Spencer have had a history of producing large hail and wind damage. The storms will reach northern Pocahontas and northern Sac Counties within the next 45 minutes to hour and fifteen minutes. This area remains under a Severe Thunderstorm Watch until 1 am Saturday. Please continue to monitor local media or NOAA Weather Radio for the latest weather information and for any warnings should they become necessary. Ansorge
Special Weather Statement National Weather Service Des Moines IA 850 PM CDT Fri Aug 18 2017 IAZ004-005-015-190400- Emmet-Kossuth-Palo Alto- Including the cities of Estherville, Algona, and Emmetsburg 850 PM CDT Fri Aug 18 2017 ...Strong thunderstorms moving into northern Iowa... At 8:50pm, National Weather Service Doppler Radar was tracking strong thunderstorms from Fairmont, Minnesota to near Spirit Lake. These storms have had wind gusts up to 40 mph within the last hour. Wind gusts up to around 45 mph will be possible with these storms as they move to the southeast. This area remains under a Severe Thunderstorm Watch until 1 am Saturday. Please continue to monitor local media or NOAA Weather Radio for the latest weather information and for any warnings should they become necessary. Ansorge
The Des Moines radar (KDMX), 108 km away, was scanning. The volume nearest the first event began at 03:37 UTC.
1 hail. Local standard time, as filed.
Trained spotter reported quarter sized hail lasting about 3 to 5 minutes, resulting in some damage to vehicles.
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, August 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: 714609.