Note, a portion of this event began on August 31st (heavy rainfall namely) and carried over into September 1st. The general setup for the day was less so for severe weather and more so for potential heavy rainfall.
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.
Note, a portion of this event began on August 31st (heavy rainfall namely) and carried over into September 1st.
The general setup for the day was less so for severe weather and more so for potential heavy rainfall. Synoptically, there were no major systems affecting the Plains, though a weak warm frontal boundary found itself slowly moving northward into the state, eventually giving way to a weak cold front later in the day. MUCAPE values ramped up throughout the day on the heels of strong warm air advection and moisture advection, reaching the 2000-3000+ J/kg during the evening and overnight hours. Effective bulk shear shear values were marginally supportive, sporadically reaching 30 to 40 knots. As hinted at previously, strong warm air advection and moisture advection at and around the 850 mb level boosted widespread lift and raised precipitable water values around and above 2.0 inches.
The primary result of the storms was heavy rainfall, though a few were able to form strong enough updrafts and/or organize long enough to produce brief periods of marginally severe hail and damaging winds. The greatest rainfall amounts were around and in excess of 4 to 6 inches.
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 31, 11:27 PM CST, 33 minutes earlier.
Area Forecast Discussion National Weather Service Des Moines IA 1227 AM CDT Sat Sep 1 2018 ...Updated 06z Aviation... .SHORT TERM.../Tonight through Saturday/ Issued at 316 PM CDT Fri Aug 31 2018 A swath of showers and thunderstorms currently extends from west central Iowa down into Missouri and is slowly progressing eastward. Convection within this swath has been more or less steady state all day as it has approached from Nebraska and has thus far shown no signs of organization. Despite fairly high instability ahead of the rain across central and eastern Iowa, mesoanalysis indicates substantial convective inhibition due to warm temperatures aloft. This, combined with a relative lack of deep shear, is likely what is inhibiting any intensification. While the storms bear watching as they progress across our forecast area in the next several hours, the odds of severe weather from them appear low overall. Short-term model guidance has consistently predicted secondary development of additional thunderstorms across parts of western Iowa after sunset tonight, around 01-02Z, which then move fairly quickly eastward as a multi-cell cluster or possibly bowing line. This appears to be the result of modest low-level convergence upstream interacting with the developing nocturnal low-level jet. However, very few of the models are depicting the current western Iowa rain in their initializations, and it is difficult to determine what effect this will have on the veracity of their solutions. Nevertheless, given the persistence of the schedule and the stronger forcing of the developing LLJ, have maintained POPs that reflect this scenario. If the storms do develop as expected then they will have an attendant threat of strong winds as the progress eastward across central Iowa tonight. Meanwhile, well to our north/northwest across Minnesota and Dakotas, a separate area of intense thunderstorms will form near the synoptic boundary this evening. This will likely produce an elongated outflow boundary that will travel east southeastward overnight, accompanied by at least a broken band of convection. This feature will likely impact at least our northern and northeastern counties between midnight and sunrise, again with an attendant threat of strong winds. .LONG TERM.../Saturday night through Friday/ Issued at 316 PM CDT Fri Aug 31 2018 The long term forecast period remains unusually active and wet as Iowa will reside beneath very persistent west southwesterly flow aloft, allowing a nearly continuous fetch of moisture advection into a quasi-stationary surface boundary draped across the Upper Midwest. Overnight thunderstorms may linger into Saturday morning in our eastern counties, otherwise there may be a brief quiet period from mid-morning into mid-afternoon Saturday as it will take some time to destabilize behind the overnight convection, which will also have left behind an outflow boundary somewhere over central Iowa. Diurnal heating will result in a high to extreme degree of instability south of this boundary, probably roughly south of Highway 30, which should be enough to generate robust thunderstorm development near the boundary in the late afternoon/evening. Have carried increased POPs to account for this, but they are necessarily broad-brushed in deference to uncertainty regarding the location of the boundary. While the degree of instability in proximity to the boundary will support a severe weather threat, it should be fairly localized and a lack of deep-layer shear will likely result in a quick transition to more of a heavy rain/flooding threat, which will become the primary hazard for the remainder of the event. Most of the forecast area has been devoid of heavy rains lately, but if the storms manage to persist over a metro or flood-prone area then flooding concerns will be enhanced. On Saturday night the nocturnal LLJ will develop once again, and it's more southerly orientation along with somewhat more southwesterly flow aloft may act to push the boundary and storms northward. Model solutions diverge somewhat on this point, but it is likely that the jet will keep storms redeveloping for much of the night, so if they are able to move northward then that will spread out the heavy rain threat, while a more stationary boundary would result in more localized but much higher flooding potential. This will certainly bear watching as we head into the weekend. From Sunday through next week the overall pattern will remain essentially unchanged. One or more boundaries will waffle around the area, being repeatedly activated by mid-level shortwave impulses, diurnal heating, and/or LLJ redevelopment. This will result in a continued intermittent threat of severe weather and heavy rain, with the heavy rain and flooding concerns being a higher threat for the most part. There is no significant pattern change foreseen until at least late next week, and possibly longer. && .AVIATION.../For the 06Z TAFS through 06Z Saturday night/ Issued at 1217 AM CDT Sat Sep 1 2018 The main concern throughout the period will be the development of rain showers and thunderstorms as front lifts northward across Iowa throughout Saturday morning. Rain shower and thunderstorms will likely develop in clusters, and gradually increase in coverage throughout the period. This will result in MVFR conditions across the terminals as the morning progresses. IFR conditions may briefly develop if thunderstorms move directly over the terminals. The other concern overnight into the morning will be low-level wind shear at ALO, DSM, and OTM as the low- level jet continues to pass through the area. This is the beginning to what will be a rather wet weekend. && .DMX WATCHES/WARNINGS/ADVISORIES... NONE. && SHORT TERM...Lee LONG TERM...Lee AVIATION...Krull
Hazardous Weather Outlook National Weather Service Des Moines IA 232 PM CDT Fri Aug 31 2018 IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075- 081>086-092>097-011945- 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- 232 PM CDT Fri Aug 31 2018 This Hazardous Weather Outlook is for portions of central Iowa. .DAY ONE...This evening and tonight Strong to possibly severe thunderstorms are possible this evening into tonight. Large hail and damaging wind are the main threats along with locally heavy rainfall. A tornado chance cannot be ruled out this evening. .DAYS TWO THROUGH SEVEN...Saturday through Thursday There will be periods of thunderstorms, especially during the overnight hours, through Thursday. Though widespread severe weather is not likely, a few storms may be strong. Damaging winds and will be the main threats with any stronger storms. Localized heavy rainfall will be possible with all of the storm chances. River flooding and flash flood threats could increase Sunday into next week. .SPOTTER INFORMATION STATEMENT... Spotter activation may be needed this evening into tonight. Rainfall reports are appreciated. Kotenberg
Special Weather Statement
National Weather Service Des Moines IA
1001 PM CDT FRI AUG 31 2018
IAZ057-070-010400-
Audubon IA-Cass IA-
1001 PM CDT FRI AUG 31 2018
...Strong Storms near Atlantic...
At 1000 PM CDT, Doppler radar was tracking a strong thunderstorm near
Atlantic, moving northeast at 25 mph.
Nickel size hail and wind gusts up to 50 mph will be possible with
this storm.
Locations impacted include...
Atlantic, Lewis, Cumberland, Brayton, Marne, Wiota, Griswold Park,
Cold Springs State Park and Atlantic Municipal Airport.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
Persons in campgrounds should consider seeking sturdy shelter until
this storm passes.
&&
LAT...LON 4145 9515 4151 9505 4151 9504 4152 9503
4159 9491 4129 9470 4128 9470 4123 9512
4127 9516
TIME...MOT...LOC 0300Z 239DEG 20KT 4135 9508
SmallPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
1028 PM CDT FRI AUG 31 2018
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
1021 PM HAIL MARNE 41.44N 95.11W
08/31/2018 E1.00 INCH CASS IA TRAINED SPOTTER
A FEW QUARTERS WITH A LOT OF PEA SIZE HAIL.
&&
AKPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
1032 PM CDT FRI AUG 31 2018
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
1027 PM HAIL MARNE 41.44N 95.11W
08/31/2018 E2.00 INCH CASS IA TRAINED SPOTTER
&&
AKPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
1141 PM CDT FRI AUG 31 2018
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
1134 PM HAIL 4 SSE LYNNVILLE 41.52N 92.77W
08/31/2018 E1.00 INCH JASPER IA PUBLIC
&&
AKThe Des Moines radar (KDMX), 79 km away, was scanning. The volume nearest the first event began at 06:02 UTC.
29 heavy rain, 4 hail, 2 flash flood, 1 thunderstorm wind. Local standard time, as filed.
Trained spotter reported heavy rainfall of 3.56 inches. Due to database entry limitation, beginning time coded as midnight CST. Actual estimated start time of 215pm CST on August 31st.
Coop observer recorded heavy rainfall of 4.65 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
Waterloo Regional Airport ASOS recorded heavy rainfall of 3.02 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
Coop observer recorded heavy rainfall of 3.87 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
Lamoni Municipal Airport recorded heavy rainfall of 2.61 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
Coop observer recorded heavy rainfall of 3 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
Ames Municipal Airport ASOS recorded heavy rainfall of 2.03 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
Coop observer recorded heavy rainfall of 3.07 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
Coop observer recorded heavy rainfall of 2.71 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
Coop observer recorded heavy rainfall of 6.57 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
Trained spotter reported heavy rainfall of 3.80 inches. Due to database entry limitation, beginning time coded as midnight CST. Actual estimated start time of 220pm CST on August 31st.
Public reported heavy rainfall of 4.5 inches. Due to database entry limitation, beginning time coded as midnight CST. Actual estimated start time of 200pm CST on August 31st.
CoCoRaHS observer recorded heavy rainfall of 2.43 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
CoCoRaHS observer recorded heavy rainfall of 2.23 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
CoCoRaHS observer recorded heavy rainfall of 2.17 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
CoCoRaHS observer recorded heavy rainfall of 3.83 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
CoCoRaHS observer recorded heavy rainfall of 3.83 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
CoCoRaHS observer recorded heavy rainfall of 3.95 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
CoCoRaHS observer recorded heavy rainfall of 3.25 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
Trained spotter reported heavy rainfall of 3.00 inches. Due to database entry limitation, beginning time coded as midnight CST. Actual estimated start time of 200pm CST on August 31st.
Trained spotter reported heavy rainfall of 6.57 inches. Due to database entry limitation, beginning time coded as midnight CST. Actual estimated start time of 1120pm CST on August 31st.
CoCoRaHS observer recorded heavy rainfall of 4.59 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
CoCoRaHS observer recorded heavy rainfall of 4.65 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
Trained spotter reported heavy rainfall of 3.50 inches. Due to database entry limitation, beginning time coded as midnight CST. Actual estimated start time of 1130pm CST on August 31st.
Coop observer recorded heavy rainfall of 4.00 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
Trained spotter reported heavy rainfall of 2.24 inches. Due to database entry limitation, beginning time coded as midnight CST.
Trained spotter reported heavy rainfall of 3.60 inches. Due to database entry limitation, beginning time coded as midnight CST. Actual estimated start time of 530pm CST on August 31st.
Trained spotter reported heavy rainfall of 2.04 inches.
Trained spotter reported penny sized hail.
NWS Davenport relayed a trained spotter reported of quarter sized hail.
Law enforcement reported power lines down and a power outage in Leon. Basement flooding was also reported at some locations in the area. This is a delayed report.
Trained spotter reported nickel sized hail.
Trained spotter reported quarter sized hail and winds estimated up to 60 mph.
County Sheriff's Office reported that the bridge on M Ave over Wolf Creek was under water due to the heavy rains overnight. Unknown beginning time of water over the bridge.
County Sheriff's Office relayed reports of fairly widespread flooding in and around Traer. Reports included water in basements, standing water over roads, sewers backing up, etc... Time recorded as report time. Unknown start time of the issues.
CoCoRaHS observer recorded heavy rainfall of 2.70 inches over the last 24 hours. Due to database entry limitation, beginning time coded as midnight CST.
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: 779402, 779403, 779405, 779406, 779409, 779411, 779412, 779413, 779414, 779415, 779420, 779421, 779433, 779435, 779437, 779438, 779439, 779440, 779441, 779442, 779445, 779447, 779448, 779450, 779453, 779454, 779456, 779451, 779392, 779393, 779396, 779417, 779418, 779430, 779432, 779434.