A cold front was situated just north of the state during the morning of the 18th, slowly progressing into the state during the afternoon and evening hours. Ahead of the front, temperatures were warm and relatively humid with temperatures in the upper 80s to low 90s and dew point values around 70.
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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 cold front was situated just north of the state during the morning of the 18th, slowly progressing into the state during the afternoon and evening hours. Ahead of the front, temperatures were warm and relatively humid with temperatures in the upper 80s to low 90s and dew point values around 70. Atmospheric conditions yielded MUCAPE values around 2000 J/kg, hail CAPE around 500 J/kg, bulk effective shear around 40 kts, and DCAPE values around 1200 J/kg. While conditions warranted concern for the potential for multiple severe storms, they just could not tap into their full potential yielding low end severe wind and hail.
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 17, 11:38 AM CST, 2 hours 34 minutes earlier.
Area Forecast Discussion National Weather Service Des Moines IA 1238 PM CDT Mon Sep 17 2018 .SHORT TERM.../Today through Tonight/ Issued at 310 AM CDT Mon Sep 17 2018 Upper ridging across the central United States was undergoing flattening early this morning as shortwave passes through southern Canada. This is allowing a front to sink south through the Dakotas and Minnesota early this morning. This boundary will settle toward far northern Iowa by this afternoon. Meanwhile, the state will remain in the warm sector today with south winds as thermal ridging in the Plains presses back into the state. Temperatures should respond well given decent mixing and sunshine for much of the state today and highs will be around 90 in most locations. When combined with dewpoints approaching 70, heat index values in the low to mid 90s will be common by early to mid afternoon for an unseasonably warm day once again. By later this afternoon as the front nears northern Iowa, MUCape from 1500-2500 J/KG are expected across northern Iowa. While convergence will be somewhat weak given a col across the north in the surface winds, some convection is anticipated near the boundary by mid to late afternoon. The decent instability should produce strong enough updrafts for marginally severe hail this afternoon and evening in the far north, and a few severe storms remain possible despite the relatively weak low level shear. The low level jet will point into the boundary tonight albeit weak to moderate flow at best but it should fuel convection into the overnight. The main concern will transition to localized heavy rainfall with PWATs of 1.5-2.0" and warm cloud depth of 10-12kft. This first round of precipitation will likely act to saturate soil although an isolated flash flood threat will exist. Farther south, not much threat for precipitation is expected south of Highway 30 with temperatures remaining well above normal. .LONG TERM.../Tuesday through Sunday/ Issued at 310 AM CDT Mon Sep 17 2018 The front lingers near northern Iowa on Tuesday into early Wednesday morning before lifting northward into Minnesota ahead of an approaching wave. Thunderstorms are forecast to redevelop across northern into central Iowa on Tuesday afternoon into the evening with little severe threat. The main concern will continue to focus on heavy rainfall from Tuesday into Wednesday across northern Iowa and WPC has highlighted these areas for the potential of excessive rainfall. Given the heavy rain over the past month, the is certainly the threat of renewed river flooding in the river basins, particularly the Iowa and Cedar. Temperatures will be held in check farther north by clouds and precipitation threat but are expected to remain above normal across the south where insolation and weak south flow is expected to persist. Close attention will need to be directed to frontal placement on Wednesday evening/night as the low level inflow will be strongest on that night ahead of the approaching surface wave. The high PWATs and strong moisture transport will lead to heavy rainfall on this night from southern Minnesota and possibly northern Iowa and after a couple of prior days of rainfall, may set up to be a very hydrologically active night. The front will sweep southeast through the state on Thursday with the threat of storms as it progresses through the state. Friday appears cooler and dry statewide with high pressure passing through the upper Midwest. There is significant divergence in the models into the weekend with the Euro indicating the remnants of a tropical system from the eastern Pacific passing through the southern half of the state. The GFS remains drier for much of the weekend and will continue to monitor trends. && .AVIATION.../For the 18Z TAFS through 18Z Tuesday afternoon/ Issued at 1238 PM CDT Mon Sep 17 2018 Several concerns this period. Convection expected to gradually impact the area over the next 24 to 36 hours with some influence at KMCW by 01-02z then building southeast through 15z affecting KALO/KOTM/KFOD. KDSM may escape the bulk of convection through 15z due to warm dome aloft over western/central Iowa now. Cigs also lower as second round of convection passes into region aft 10z. This will have a tendency to bring KMCW/KFOD/KALO to MVFR category. Some uncertainty regarding coverage of convection overnight. Timing has already been delayed a bit north. Will continue to make adjustments as needed. /rev && .DMX WATCHES/WARNINGS/ADVISORIES... NONE. && SHORT TERM...Cogil LONG TERM...Cogil AVIATION...REV
Hazardous Weather Outlook National Weather Service Des Moines IA 400 AM CDT Mon Sep 17 2018 IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-181100- 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- 400 AM CDT Mon Sep 17 2018 This Hazardous Weather Outlook is for portions of central Iowa. .DAY ONE...Today and tonight Today will once again be unseasonably hot and humid, with heat index values in the low to mid 90s this afternoon. Those engaging in strenuous outdoor activities should take appropriate precautions. Thunderstorms are forecast to spread from northern Iowa late today down toward central Iowa tonight. A few strong to severe storms may be possible north of Highway 20, with large hail the primary threat. Locally heavy rainfall is also possible. .DAYS TWO THROUGH SEVEN...Tuesday through Sunday Several rounds of thunderstorms are expected between Tuesday and Thursday, especially across northern and northeastern Iowa. The overall severe weather chances are low, however, there is potential for heavy rain especially in the north and northeast. This could lead to renewed rises on some rivers and streams later in the week. .SPOTTER INFORMATION STATEMENT... Spotter activation is not anticipated at this time. IAZ081>086-092>097-181100- Adams-Union-Clarke-Lucas-Monroe-Wapello-Taylor-Ringgold-Decatur- Wayne-Appanoose-Davis- 400 AM CDT Mon Sep 17 2018 This Hazardous Weather Outlook is for portions of central Iowa. .DAY ONE...Today and tonight Today will once again be unseasonably hot and humid, with heat index values in the low to mid 90s this afternoon. Those engaging in strenuous outdoor activities should take appropriate precautions. .DAYS TWO THROUGH SEVEN...Tuesday through Sunday Thunderstorms will be possible at intervals between Tuesday and Thursday. Severe weather is unlikely, but locally heavy rainfall may occur. It will also remain unseasonably hot and humid during this time frame, with heat index values approaching or exceeding 90 degrees each afternoon. .SPOTTER INFORMATION STATEMENT... Spotter activation is not anticipated at this time. Lee
Special Weather Statement National Weather Service Des Moines IA 214 PM CDT MON SEP 17 2018 IAZ004>006-015-016-172015- Palo Alto IA-Kossuth IA-Winnebago IA-Emmet IA-Hancock IA- 214 PM CDT MON SEP 17 2018 ...Wind Gusts in Excess of 40 mph and Small Hail Possible... At 213 PM CDT, Doppler radar was tracking strong thunderstorms along a line extending from 8 miles west of Elmore to near Linn Grove. Movement was east at 35 mph. Half inch hail and winds in excess of 40 mph will be possible with these storms. Locations impacted include... Estherville, Algona, Emmetsburg, Britt, Swan Lake, Five Island Lake, Ingham Lake, Iowa Lake, Union Slogh, High Lake, Lost Island Lake, Eagle Lake, Okamanpedan Lake, Silver Lake, Rush Lake, Armstrong, Buffalo Center, Graettinger, West Bend and Ruthven. PRECAUTIONARY/PREPAREDNESS ACTIONS... To report severe weather, contact your nearest law enforcement agency. They will relay your report to the National Weather Service office in Des Moines . && LAT...LON 4350 9368 4291 9364 4291 9491 4350 9492 TIME...MOT...LOC 1913Z 261DEG 32KT 4354 9426 4294 9523 Kotenberg
The national radar mosaic for the moment the first event began.
1 hail, 1 thunderstorm wind. Local standard time, as filed.
Trained spotter reported quarter sized hail along with heavy rain and estimated 45 mph winds.
Hancock County Sheriff reported power lines down.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Iowa in 2018Every state in 2018This day in weather historyWarnings in Iowa, September 2018
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: 780290, 780291.