A persistent overrunning pattern produced periods of moderate to heavy rain from May 8 to May 11, bringing a widespread 2 to 4 inches rainfall to the middle portions of the Big Sioux River. On top of already high flows, minor flooding resulted at many sites which just recently had pulled out of minor flood conditions.
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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 FSD wrote it up.
A persistent overrunning pattern produced periods of moderate to heavy rain from May 8 to May 11, bringing a widespread 2 to 4 inches rainfall to the middle portions of the Big Sioux River. On top of already high flows, minor flooding resulted at many sites which just recently had pulled out of minor flood conditions.
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 Sioux Falls, SD wrote its last area forecast discussion before the first event at May 11, 9:51 PM CST, 4 hours 9 minutes earlier.
Area Forecast Discussion National Weather Service Sioux Falls SD 1051 PM CDT Fri May 11 2018 .SHORT TERM...(This evening through Saturday) Issued at 256 PM CDT Fri May 11 2018 Our cool and occasionally showery/drizzly pattern looks to continue for another day or so. A thick stratus cover has kept afternoon temperatures largely in the mid 40s to 50s today with much of the same expected for Saturday. These temperatures are anywhere from 10 to 20 degrees below normal for this time of year. Additionally, the 850 mb boundary wobbles north and south through the area and will keep sporadic shower chances around. Could even see a few rumbles of thunder south of I-90 later this evening although elevated instability values in place are less than impressive so don't expect anything too noteworthy. Better combined instability and shear profiles exist to the south of our area along I-80 in NE/IA where a somewhat better chance of an organize strong storm threat exists. Relatively similar day expected for Saturday with saturated low levels and the mid level baroclinic boundary still present across the area. This will likely keep some occasional shower/drizzle chances around for the daytime hours, although certainly not a washout. An east or northeast wind along with extensive cloud cover will keep temperatures chilly and lead to another fairly raw feeling day. .LONG TERM...(Saturday night through Friday) Issued at 256 PM CDT Fri May 11 2018 Things finally look to dry out by Sunday as surface high pressure and upper level shortwave ridging begins to build in. This should help thin out some of our cloud cover as well, especially north of I- 90. This will also allow for some warmer temperatures with afternoon highs into the mid 60s to low 70s. Large scale upper level pattern remains fairly stagnant through much of next week with the upper level trough over the intermountain west breaking down early in the week but then quickly reinforced by another incoming trough by mid week. Without any significant shortwave to shift our quasi-stationary mid level boundary, it continues to hang around our region. This combined with some varying levels of instability will allow for occasional shower and thunderstorm chances. A lack of any convincing upper level dynamics or forcing though keep plenty of uncertainty about precipitation coverage or intensity on any single day. Given this uncertainty, didn't make any changes to the populated blended guidance and will just cover with some chance POPs for now until daily details can be better addressed. Afternoon high temperatures will be somewhat dependent on the eventual precip and cloud cover but largely look to be in the 70s to low 80s across the region. && .AVIATION...(For the 06Z TAFS through 06Z Saturday night) Issued at 1044 PM CDT Fri May 11 2018 Timing precipitation continues to be very tricky tonight and Saturday, so there is very low confidence on current VCSH and rain timing in the TAFs. However, there is high confidence in a very moist lower atmosphere keeping low clouds decks in the IFR to low MVFR range for most locations in the tri- state area through mid- day Saturday. Daytime mixing should raise ceilings, but sunshine will be rare amidst multiple mid-level cloud layers. Light northeast surface winds persist, gusting 15 to 20 kt during the daylight hours. Thunderstorms look unlikely over the next 24 hours given a lack of instability. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && SHORT TERM...Kalin LONG TERM...Kalin AVIATION...VandenBoogart
Hazardous Weather Outlook National Weather Service Sioux Falls SD 444 AM CDT Fri May 11 2018 IAZ001>003-012>014-020>022-031-032-MNZ071-072-080-081-089-090-097- 098-NEZ013-014-SDZ038>040-050-052>071-120945- Lyon IA-Osceola-Dickinson-Sioux-O'Brien-Clay IA-Plymouth-Cherokee- Buena Vista-Woodbury-Ida-Lincoln MN-Lyon MN-Murray-Cottonwood- Nobles-Jackson-Pipestone-Rock-Dixon-Dakota-Beadle-Kingsbury- Brookings-Gregory-Jerauld-Sanborn-Miner-Lake-Moody-Brule-Aurora- Davison-Hanson-McCook-Minnehaha-Charles Mix-Douglas-Hutchinson- Turner-Lincoln SD-Bon Homme-Yankton-Clay SD-Union- 444 AM CDT Fri May 11 2018 This Hazardous Weather Outlook is for portions of northwest Iowa, west central Iowa, southwest Minnesota, northeast Nebraska, central South Dakota, east central South Dakota, south central South Dakota, and southeast South Dakota. .DAY ONE...Today and Tonight No hazardous weather is expected at this time. .DAYS TWO THROUGH SEVEN...Saturday through Thursday No hazardous weather is expected at this time. .SPOTTER INFORMATION STATEMENT... Spotter activation will not be needed.
The national radar mosaic for the moment the first event began.
3 flood. Local standard time, as filed.
Runoff from periods of heavier rainfall on top of already high river levels produced minor flooding of lowland agricultural areas along lower portions of the Big Sioux River. River levels reached 2.8 feet above flood stage above Hawarden (gage site HWDI4) on May 13.
Runoff from periods of heavier rainfall on top of already high river levels produced minor flooding of lowland agricultural areas along lower portions of the Big Sioux River. River levels reached 1.4 feet above flood stage above Hawarden (gage site AKRI4) on May 14.
Runoff from periods of heavier rainfall on top of already high river levels produced minor flooding of lowland agricultural areas along lower portions of the Big Sioux River. River levels reached 1.0 feet above flood stage near Richland (gage site BSRS2) on May 14.
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, May 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: 761218, 761219, 761220.