A weak, relatively stationary, boundary found itself across portions of western and central Iowa during the evening and overnight hours of the 1st into the 2nd. While surface observations were not tremendous, especially for early September, in the upper 70s to mid 80s, SBCAPE values approached 4000+ J/kg in areas of southwest and south-central Iowa concurrent with dew points into the upper 70s.
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.
A weak, relatively stationary, boundary found itself across portions of western and central Iowa during the evening and overnight hours of the 1st into the 2nd. While surface observations were not tremendous, especially for early September, in the upper 70s to mid 80s, SBCAPE values approached 4000+ J/kg in areas of southwest and south-central Iowa concurrent with dew points into the upper 70s. Support for organized storms was marginal to weakly supportive with bulk effective shear generally less than 40 kts and hail CAPE around 800 J/kg. Heavy rain was also favored with the slow moving boundary, expected 850mb jet, and strong moisture advection into the area.
Resulting weather involved initially discrete cells producing severe hail, quickly morphing into multicellular convection and messier convection as the low level jet began to influence conditions. Early on, severe hail dominated reports, eventually giving way to heavy rainfall and flash flooding. The most impressive reports included up to 2 inch hail on the north side of the Des Moines metro area and heavy rainfall in excess of 6 inches in portions of Poweshiek 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 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), 100 km away, was scanning. The volume nearest the first event began at 06:02 UTC.
14 hail, 13 heavy rain, 8 flash flood, 3 flood. Local standard time, as filed.
Trained spotter reported heavy rainfall of 6.48 inches for the last 24 hour period. Due to limitations in the database entry, beginning time coded as midnight CST.
Newton Municipal Airport recorded heavy rainfall of 4.14 inches over the last 24 hours.
Coop observer (radio station KCOB) recorded heavy rainfall of 2.90 inches over the last 24 hours.
Coop observer recorded heavy rainfall of 5.16 inches over the last 24 hours.
Grinnell Regional Airport recorded heavy rainfall of 3.20 inches over the last 24 hours.
Coop observer recorded heavy rainfall of 4.54 inches over the last 24 hours.
Ames Municipal Airport recorded heavy rainfall of 2.14 inches over the last 24 hours.
CocoRaHS observer recorded heavy rainfall of 3.44 inches over the last 24 hours.
Trained spotter reported heavy rainfall of 3.5 inches.
Trained spotter reported heavy rainfall of 4.25 inches since 7pm.
Trained spotter reported heavy rainfall of 7.5 inches.
Trained spotter reported heavy rainfall of 5.65 inches near Highway 14 and Interstate 80.
Public reported quarter sized hail.
Trained spotter reported hail larger than golf balls.
Trained spotter reported quarter sized hail.
Public relayed report of half dollar sized hail.
Trained spotter reported ping pong ball sized hail at Saydel High School.
Public reported heavy rainfall of 2.10 inches on the north side of Ames.
Delaware Township Fire Department reported golf ball sized hail.
Public reported quarter sized hail.
WHO relayed reports of 2 inch hail in the Norwoodville Neighborhood. Time estimated from radar.
Public reported golf ball sized hail.
Dallas Center Fire Department reported nickel sized hail.
Public reported golf ball sized hail via the mPING app. Time estimated from radar.
Local fire department reported a motorist rescue along Xavier Ave, where water was up to the hood of the vehicle.
Local dispatch reported a gravel road washed out along 120th St between 310th and 320th St.
Poweshiek County Sheriff's Office reported a gravel road washed out; 120th Street between 320th and 310th streets.
Public stated that Highway 17 northbound is underwater and closed approximately 1 mile north of Granger.
Public reported nickel sized hail.
Iowa DOT reported that Highway 141 between Grimes and Granger was essentially a sheet of water, with speeds of only 10 mph possible.
Iowa DOT reported foot deep water along Highway 30 east of Highway 65. Story County Sheriff also reported Highway 30 on-ramp to Highway 65 under water.
Iowa DOT reported Highway 65 at Old Lincoln Highway was completely under water. The local fire department was blocking the road.
Marshall County 911 reported the shoulder washed out on Hwy 146 south of 290th St.
Law enforcement reported several county and rural roads across the county were under water from the heavy rainfall.
Law enforcement reported several county and rural roads across the county were under water from the heavy rainfall.
Marshall County 911 reported the shoulder washed out of Hwy 146 south of 290th St.
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: 779564, 779568, 779569, 779570, 779571, 779572, 779573, 779576, 779554, 779560, 779575, 779577, 779543, 779544, 779545, 786309, 779547, 779546, 779574, 786311, 779549, 779548, 779550, 779551, 779552, 779553, 779556, 779557, 786310, 779559, 779561, 779563, 779566, 779567, 786730, 786295, 786296, 786731.