An eventful day played out across Iowa, carried out in two parts. First was a wave of strong to severe storms during the morning hours across much of northern Iowa, followed by a subsequent wave of storms across southern Iowa.
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.
An eventful day played out across Iowa, carried out in two parts. First was a wave of strong to severe storms during the morning hours across much of northern Iowa, followed by a subsequent wave of storms across southern Iowa. Respectively, the storms produced primarily severe hail and then severe winds and heavy rainfall.
Environmentally, the setup was relatively prime for organized convection and to a degree, training storms. A relatively stationary front (slowly drifting southward later in the day) laid across Iowa from southwest to northeast throughout the day on the 28th. On the warm side of the boundary, MUCAPE values were in excess of 2000 J/kg already early in the morning, growing to 3000-4000 J/kg by the afternoon. Other supporting factors for organized convection were present as well, including 40 to 50+ kts of effective bulk shear, hail CAPE in excess of 600 J/kg (especially in the morning round), and effective storm relative helicity around or in excess of 200 m2/s2. Understandably, composite indices such as the significant hail parameter and the supercell composite parameter returned values that raised concerns for severe weather potential.
At the end of the day, reports were split into two distinct areas. The first was large hail over northern Iowa with the early morning convection, followed by damaging winds and heavy rainfall with the afternoon and evening convection over southern Iowa.
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 27, 10:26 PM CST, 1 hour 34 minutes earlier.
Area Forecast Discussion National Weather Service Des Moines IA 1126 PM CDT Mon Aug 27 2018 .DISCUSSION.../Tonight through Monday/ Issued at 253 PM CDT Mon Aug 27 2018 Active weather pattern through much of the forecast period. Initial concerns for severe weather are focused on the remainder of today over northern Iowa later this afternoon and into the early evening then again over the southeast Tuesday. Another strong short wave is lifting northeast through central Nebraska this afternoon with thunderstorms developing over northwest Iowa and into northeast Nebraska/southeast South Dakota. The storms are continuing to strengthen as the boundary layer continues to destabilize. A trough axis precedes the upper level system through northwest Iowa and into western Minnesota and will be the focus for stronger instability and destabilization as the upper level support arrives. Effective deep layer shear across the north is supportive of organized and sustained updrafts and certainly some supercell structures. Large hail, damaging winds and a few tornadoes are all possible over the far north between 3-8 pm today with the initial supercell structures before then entire system grows upscale as it lifts into Minnesota and Wisconsin. The remainder of the evening is expected to be quiet until late tonight into early Tuesday as the primary cold front enters the state across the north. Much of the northern activity will be post surface frontal. Also a few storms will be possible over the south through the warm advection zone. Expect strong instability to develop over southern/southeast Iowa by the afternoon assuming the area does not become contaminated by convective debris, which it shouldn't with deep layer flow from the southwest. Supercell development could once again occur with an attendant very large hail, damaging wind and isolated tornado threat. The boundary will move south by late Tuesday night then high pressure will arrive for Wednesday. Wednesday will be cool with highs in the low to mid 70s. The active pattern arrives Thursday night as a strong upper level westerlies develop and persist through the weekend. This flow will help keep a boundary lingering in the area. There does appear to have some severe weather potential and potential more so a heavy rain threat. There are hints of MCS development Friday night that could develop into a damaging wind threat but a bit too far out at this point to narrow focus to overall evolution during this period as overnight cold pools will impact next day potential. && .AVIATION.../For the 06Z TAFS through 06Z Tuesday night/ Issued at 1126 PM CDT Mon Aug 27 2018 Minor changes from previous package. Main area of forcing /isentropic lift just east of KMCW now with some potential for a few back-building cells overnight. Another wave approaching from South Dakota will provide additional support for scattered storms from 08-14z across the north sites KMCW/KFOD. As main wave and front begins to cross area from 16z through 03z will expect to see increasing coverage of storms - especially from 18 through 03z along with cigs dropping to MVFR/IFR along the frontal zone. Have brought most sites into categorical thunder aft 18z with vicinity prior. Some MVFR br also expected through 14z. /rev && .DMX WATCHES/WARNINGS/ADVISORIES... NONE. && DISCUSSION...Donavon AVIATION...REV
Hazardous Weather Outlook National Weather Service Des Moines IA 323 PM CDT Mon Aug 27 2018 IAZ004>007-015>017-023>028-033>039-282030- Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo- Pocahontas-Humboldt-Wright-Franklin-Butler-Bremer-Sac-Calhoun- Webster-Hamilton-Hardin-Grundy-Black Hawk- 323 PM CDT Mon Aug 27 2018 This Hazardous Weather Outlook is for portions of central Iowa. .DAY ONE...This afternoon and tonight Strong to severe thunderstorms are forecast over northern Iowa through the evening hours. The primary threats are damaging winds and large hail, but an isolated tornado or two cannot be ruled out. The greatest severe threat remains along and north of Highway 20. .DAYS TWO THROUGH SEVEN...Tuesday through Sunday Thunderstorms are again possible Tuesday into Tuesday night. A strong storm may be possible with gusty winds and small hail. Another chance for storms arrives late Thursday night into Friday. .SPOTTER INFORMATION STATEMENT... Spotters are likely to be needed through the evening and again on Tuesday. IAZ044>050-057>062-070>075-081>086-092>097-282030- 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- 323 PM CDT Mon Aug 27 2018 This Hazardous Weather Outlook is for portions of central Iowa. .DAY ONE...This afternoon and tonight No hazardous weather is expected at this time. .DAYS TWO THROUGH SEVEN...Tuesday through Sunday Widespread thunderstorms are anticipated on Tuesday into Tuesday night. Some strong to possibly severe storms remain possible with the primary threats of hail, damaging winds, and even a tornado or two. The location of greatest severe threat is in the southeast portions of Iowa. More storms are possible Friday into Saturday. However, the severe potential remains uncertain at this time. .SPOTTER INFORMATION STATEMENT... Spotters might be needed on Tuesday. KLP
The Des Moines radar (KDMX), 127 km away, was scanning. The volume nearest the first event began at 06:01 UTC.
22 heavy rain, 21 thunderstorm wind, 17 hail. Local standard time, as filed.
Ottumwa Regional Airport ASOS recorded heavy rainfall of 4.07 inches over the last 24 hours.
CoCoRaHS observer recorded heavy rainfall of 2.04 inches over the last 24 hours.
CoCoRaHS observer recorded heavy rainfall of 2.43 inches over the last 24 hours.
CoCoRaHS observer recorded heavy rainfall of 2.08 inches over the last 24 hours.
Public reported up to around baseball or larger sized hail. Time estimated from radar.
CoCoRaHS observer recorded heavy rainfall of 3.50 inches over the last 24 hours.
Ottumwa Regional Airport ASOS recorded heavy rainfall of 4.19 inches over the last 24 hours.
Public reported baseball sized hail. Time estimated from radar.
CoCoRaHS observer recorded heavy rainfall of 3.46 inches over the last 24 hours.
CoCoRaHS observer recorded heavy rainfall of 3.00 inches over the last 24 hours.
CoCoRaHS observer recorded heavy rainfall of 2.50 inches over the last 24 hours.
Coop observer recorded heavy rainfall of 2.20 inches over the last 24 hours.
CoCoRaHS observer recorded heavy rainfall of 2.18 inches over the last 24 hours.
Emergency manager reported windows broken on houses and cars by hail. This is a delayed report and time estimated from radar.
CoCoRaHS observer recorded heavy rainfall of 3.92 inches over the last 24 hours.
Public reported quarter to golf ball sized hail.
Trained spotter reported tennis ball sized hail.
Public reported, through local media, golf ball sized hail. Time estimated from radar.
CoCoRaHS observer recorded heavy rainfall of 3.41 inches over the last 24 hours.
Coop observer recorded heavy rainfall of 2.40 inches over the last 24 hours.
Public reported, through local media, hail larger than baseball in size. Time estimated from radar.
Public reported hail around ping pong ball in size.
Law enforcement reported a tree downed onto a house in Thornton. Time and location estimated from radar.
Law enforcement reported a semi-truck blown over by winds on Interstate 35. One individual was transported to the hospital. Time estimated from radar.
Mesonet station recorded heavy rainfall of 2.02 inches over the last 24 hours.
Mesonet station recorded heavy rainfall of 2.06 inches over the last 24 hours.
Trained spotter reported heavy rainfall of 2.25 inches from the afternoon storms.
Mesonet station recorded heavy rainfall of 2.06 inches over the last 24 hours.
Mesonet station recorded heavy rainfall of 2.45 inches over the last 24 hours.
Mesonet station recorded heavy rainfall of 3.19 inches over the last 24 hours.
Trained spotter reported heavy rainfall of 2.71 inches so far today. Report came via social media.
Trained spotter reported heavy rainfall of 2.10 inches.
Amateur radio operator reported around 4 minutes of time to penny sized hail.
Trained spotter reported 3 minutes of dime to quarter sized hail.
Trained spotter reported quarter sized hail on the north side of Latimer at about 1255 pm.
Public reported golf ball sized hail.
Trained spotter reported half dollar sized hail.
Public reported, relayed by local media, hail around 2 inches in diameter. Time and location estimated from radar and this is a delayed report.
Public reported quarter sized hail.
Public reported golf ball sized hail.
Emergency manager reported several tree limbs and power lines down. Time estimated from radar.
Emergency manager reported several trees and power lines down throughout town. Time estimated from radar.
A large egg laying building was destroyed south of Knoxville along Highway 14. The storm moved the south end of the building roughly 40 feet off of its foundation. The walls were anchored into the concrete floor. Time estimated from radar.
Large 1 to 2 foot diameter tree branch blown between two homes. Report relayed from KNIA/KRLS radio.
KNIA/KRLS Radio reported, via social media, several large trees snapped on the Pella Golf and Country Club. Power lines were also reported down throughout Pella.
Emergency manager reported several trees and power lines down south of Knoxville. Two trailers were also blown over. Time estimated from radar.
Emergency manager reported that one residence had about 300 square feet of shingles blown off their roof.
Emergency manager reported power lines and several tree limbs down. Time estimated from radar.
Public reported that a large section of mature sweet corn was completely flattened. Time estimated from radar.
Emergency manager reported that a large tree fell on a residence, causing significant damage to the roof and interior rooms. Time estimated from radar.
Law enforcement reported a large tree limb covering Market Street and several smaller limbs down around town. Time estimated from radar.
Law enforcement reported 2 to 3 large trees down. Time estimated by radar.
Trained spotter reported up to 60 mph wind gusts, with small tree branches down.
Law enforcement reported a large tree down blocking the road near the intersection of N 9th Street and B Ave East. Time estimated from radar.
Ottumwa Regional Airport ASOS recorded a 68 mph wind gust.
Large, tall tree roughly 2 feet in diameter knocked over. This is a delayed report and time estimated from radar.
Trained spotter reported that the small town of Blakesburg had a lot of tree damage, including several trees falling on houses. Extent of damage was unknown at the time. Report came via social media.
Trained spotter reported 60 to 70 mph wind gusts. Several large trees and 2 to 6 inch diameter tree limbs were down as a result. Report came in via social media.
Public reported, via the mPING app, 3 inch tree limbs and power poles broken.
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, August 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: 777785, 777786, 777787, 777788, 777745, 777789, 777790, 777746, 777791, 777792, 777793, 777794, 777795, 777747, 777796, 777748, 777750, 777751, 777797, 777798, 777752, 777754, 777755, 777756, 777800, 777801, 777783, 777802, 777803, 777804, 777784, 777799, 777757, 777758, 779235, 777759, 777761, 777762, 777763, 777764, and more.