A few evening thunderstorms developed along a cold front in portions of northwest Iowa. Fairly narrow CAPE profiles and lack of deep layer shear kept storms mainly disorganized, and the strongest updrafts resulted in some sub-severe hail.
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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 few evening thunderstorms developed along a cold front in portions of northwest Iowa. Fairly narrow CAPE profiles and lack of deep layer shear kept storms mainly disorganized, and the strongest updrafts resulted in some sub-severe 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 Sioux Falls, SD wrote its last area forecast discussion before the first event at May 2, 5:30 PM CST, 2 hours 7 minutes earlier.
AREA FORECAST DISCUSSION...UPDATED National Weather Service Sioux Falls SD 630 PM CDT Sun May 2 2021 .SHORT TERM...(This Evening through Monday Night) Issued at 257 PM CDT Sun May 2 2021 Near-term forecast revolves around scattered showers and thunderstorms into the overnight period. Surface observations show the now almost stationary boundary near Vermillion SD, to Orange City IA and up towards Spirit Lake IA, further confirmed by looping radar reflectivity depicting a very fine line across the same area. Models have been struggling with placement of this boundary (along with the morning and afternoon showers), which has implications for where storms will be initiating later this afternoon. On Satellite, a developing cumulus field is forming on the southern side of the boundary, indicating that there is some instability to be realized there, likely initiating after 5-6 pm as a shortwave reaches the area. Questions remain if these storms will be able to be surface-based, which would give them access to more instability on the order of ~1500 J/kg, whereas if they were elevated the CAPE values drop to around 1000 J/kg. Sounding profiles across NW IA indicate effective shear values around 25-30 knots, but overall the flow itself aloft is rather weak so storms will likely have trouble sustaining themselves, let alone staying organized to produce anything larger than small hail. Skinny CAPE profiles are also not indicative of anything other than small hail, but DCAPE values are in the 500-1000 J/kg range hinting at there being somewhat of a wind threat. All of this to say, severe weather is possible just not very likely. As the low pressure system associated with the stationary front tracks eastwards south of the area, expect the precipitation to move out along with it. By Monday morning, chances for rain will largely be confined to areas in NW Iowa, which should see the rain move out of the area by mid-afternoon. Behind the aforementioned low pressure the surface pressure gradient will tighten overnight, largely in part due to a surface high pressure coming in from the north, leading to breezy winds throughout the day tomorrow. North/northwesterly winds will likely be sustained in the 20-25 mph range with gusts around 35-40 mph. As for temperatures, northerly winds will keep us in the upper 50s and lower 60s. For the overnight hours, the surface pressure gradient should weaken as the surface high gets closer to the area, relaxing the winds a bit and leading to a cool morning. .LONG TERM...(Tuesday through Sunday) Issued at 257 PM CDT Sun May 2 2021 Early morning temperatures will feel quite brisk, though should remain above freezing for locations along the highway 14 corridor. Winds will likely become weak enough for patchy frost to develop, but if winds remain a bit stronger than frost will not be likely. Elsewhere temperatures will be warmer getting closer to the 40 degree mark. Tuesday itself should be fairly quite as west/northwesterly flow aloft sets up to bring our next shortwave to the region, with the surface high pressure keeping temperatures on the cooler side a couple degrees below normal. It will likely also be a little breezy as the pressure gradient does tighten a bit during the day, but will weaken again as winds begin to turn ahead of our next system. While the models have been consistent in showing scattered showers on Wednesday, timing differences amongst the various models don't allow for great confidence on the arrival of the next system. Because of this, have stuck with the populated guidance which shows chances for precipitation starting early morning for central SD along the MO River valley and spreading eastwards throughout the day. Chances for rain dwindle by Thursday morning, with northwesterly flow keeping us dry through Friday. Warmer southerly winds return on Friday out ahead of a developing low pressure, which will likely result in scattered chances for precipitation on Saturday. && .AVIATION...(For the 00Z TAFS through 00Z Monday evening) Issued at 620 PM CDT Sun May 2 2021 VFR conditions will prevail across much of the region this evening. However isolated showers will continue across southeast SD and southwest MN with a concentrated area of scattered thunderstorms and showers continuing to redevelop further south, along and east of a front extended from near SUX to SPW. Thunderstorms will have lightning with potential for small hail and gusty, erratic winds. Current ceilings with mid level stratus will deteriorate to MVFR later this evening and below 2 kft overnight. Once developed, MVFR stratus will continue well into Monday, perhaps with IFR ceilings developing in northwest IA late tonight. North winds will increase overnight, sustained at near 20 kts with gusts up to 35 kts during daylight hours Monday. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...NONE. MN...NONE. IA...NONE. NE...NONE. && SHORT TERM...APT LONG TERM...APT AVIATION...BP
Hazardous Weather Outlook National Weather Service Sioux Falls SD 155 PM CDT Sun May 2 2021 IAZ001-002-012-MNZ071-072-080-081-089-090-097-098-SDZ038>040-050- 052>070-031900- Lyon-Osceola-Sioux-Lincoln-Murray-Cottonwood-Nobles-Jackson- Pipestone-Rock-Beadle-Kingsbury-Brookings-Gregory-Jerauld-Sanborn- Miner-Lake-Moody-Brule-Aurora-Davison-Hanson-McCook-Minnehaha- Charles Mix-Douglas-Hutchinson-Turner-Bon Homme-Yankton-Clay- 155 PM CDT Sun May 2 2021 This Hazardous Weather Outlook is for northwest Iowa, southwest Minnesota, central South Dakota, east central South Dakota, south central South Dakota and southeast South Dakota. .DAY ONE...This afternoon and tonight. Isolated thunderstorms will be possible by late afternoon. At this time, severe weather is not expected. .DAYS TWO THROUGH SEVEN...Monday through Saturday. Hazardous weather is not expected at this time. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected at this time. IAZ003-013-014-020>022-031-032-NEZ013-014-SDZ071-031900- Dickinson-O'Brien-Clay-Plymouth-Cherokee-Buena Vista-Woodbury-Ida- Dixon-Dakota-Union- 155 PM CDT Sun May 2 2021 This Hazardous Weather Outlook is for northwest Iowa, west central Iowa, northeast Nebraska and southeast South Dakota. .DAY ONE...This afternoon and tonight. Scattered thunderstorms are expected to develop from very late this afternoon into the evening. While severe weather is generally not anticipated, an isolated storm could produce wind approaching 60 mph and small hail. .DAYS TWO THROUGH SEVEN...Monday through Saturday. Hazardous weather is not expected at this time. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected at this time.
The Omaha radar (KOAX), 148 km away, was scanning. The volume nearest the first event began at 01:39 UTC.
1 hail. Local standard time, as filed.
Crop damage is an estimate from insured losses. Information provided by the United States Department of Agriculture.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Iowa in 2021Every state in 2021This day in weather historyWarnings in Iowa, May 2021
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: 962106.