Water from snow melt and rainfall continued to work through the Big Sioux River early in May. Flooding remained generally minor with impacts limited to agricultural areas near the river.
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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.
Water from snow melt and rainfall continued to work through the Big Sioux River early in May. Flooding remained generally minor with impacts limited to agricultural areas near the river.
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 Apr 30, 9:45 PM CST, 2 hours 15 minutes earlier.
Area Forecast Discussion National Weather Service Sioux Falls SD 1045 PM CDT Mon Apr 30 2018 .SHORT TERM...(This evening through Tuesday) Issued at 254 PM CDT Mon Apr 30 2018 Our first true severe weather risk of the season will slowly unfold late this afternoon and into the evening hours. A stalled frontal boundary stretching from central Nebraska into central and northeastern South Dakota and northern Minnesota, will continue to serve as the focal point for convection later today. At the surface, we're continuing to see very slow moisture advection northward, however this increase in moisture is nearly being offset by the deeper mixing and mixdown of drier air aloft. The result is that prefrontal dew points may struggle to climb any past the mid 50s. Winds also seem to be overachieving this afternoon, with gusts up to 45 to 50 mph already reported in portions of the Tri-State area. Later today and tonight: Really watching two areas for convective development later today, an area along a sharp thermal nose in north central Nebraska, and an area of higher moisture convergence and strong warm advection just to the north of the CWA. By late afternoon most high resolution guidance as well as the HREF, suggest an elongated corridor of ~1500 J/KG MLCAPE from northeastern SD into central Nebraska along the frontal boundary. Soundings however show an increased capping inversion centered around 700 mb that will hold strong into the late afternoon hours. It's not until a 700 mb shortwave begins to enter the Plains that we'll begin to cool this layer and enhance mid-lvl lapse rates. This should spark several areas of scattered convection, with a few more concentrated areas. One more concentrated area of development will stretch from Broken Bow northeast along the sfc front towards ONeil and Lake Andes, and a second is likely to focus north of the Watertown and Huron areas where better sfc convergence develops. In between these areas, convection could remain more scattered in nature. Effective shear this evening sits around 35 to 45 knots which could support a few rotating storms within the initial development axis, however the primary mode will end up being multi-cell clusters with several bowing segments as storms are nudged eastward. Soundings indicate high LCLs, with DCAPE values approaching 1000 J/KG so feel that for most of the area there will be an initial large hail threat, but a more predominate wind threat with the potential for winds upwards of 70 mph or greater. While 0-1 helicity and shear values do approach sufficient values for tornadoes, we're really limited here by the poor boundary layer moisture levels along with the high LCLs. Initially broken cells will begin to undergo mergers quickly this evening, with one or more clusters moving towards the I-29 corridor late this evening. Once the LLJ becomes involved after 02Z, a somewhat organized MCS should develop over northern Nebraska and SE South Dakota and move into northern Iowa overnight. The one thing noted in models today is a very sharp instability gradient east of I- 29, and along with quick boundary layer stabilization this could limit the largest severe weather risks to areas west of I-29. Ensemble updraft helicity plots support this idea, but that said, despite weakening MUCAPE values, elevated DCAPE values continue eastward, which could support a strong wind risk further east. In fact, several high resolution models already show the development of an outflow boundary ahead of convection, supporting a continued wind risk into northern Iowa but also perhaps indicating an unbalance of shear and instability as storms move east. Cold pools should really prevent any widespread flash flood risk, but there are a few concerns along the MO river valley and into northwestern Iowa for training storms especially as the LLJ begins to turn perpendicular to the 850 mb front. .LONG TERM...(Tuesday night through Monday) Issued at 254 PM CDT Mon Apr 30 2018 Tuesday: Convection risks remain uncertain on Tuesday, and are highly dependent on tonight's convection. At this point, expecting the surface front to drop just southeast of the forecast area by mid- day Tuesday, but will elevated instability along and ahead of the 850 mb trough holding back northwest, I wouldn't be surprised to see a few stronger elevated post frontal storms in NW Iowa Tuesday afternoon. In fact, given MUCAPE plots, it's possible that the marginal and slight risks could be pulled slightly further northwest. Wednesday-Thursday: A somewhat disorganized upper trough will slowly cross the Plains through the middle of the week. With the frontal boundary well south of the CWA, this should really put us on the cooler side of the system, more supportive of a few showers and less thunder. Friday-Saturday: We should finally dry out for the end of the week and into next weekend as a broad upper trough sweeps into the eastern half of the CONUS. Canadian high pressure will bring slightly cooler temperatures into the region, but nothing overly extreme. && .AVIATION...(For the 06Z TAFS through 06Z Tuesday night) Issued at 1040 PM CDT Mon Apr 30 2018 Showers and thunderstorms will continue to push eastward across the area through the late night hours. Southerly winds will continue to diminish overnight, gradually becoming northwesterly as a frontal boundary slides through the region. Additional thunderstorms will be possible on Tuesday afternoon, possibly affecting KFSD and KSUX. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && SHORT TERM...Dux LONG TERM...Dux AVIATION...JM
Hazardous Weather Outlook National Weather Service Sioux Falls SD 508 PM CDT Mon Apr 30 2018 IAZ001>003-012>014-020>022-031-032-MNZ072-080-081-089-090-NEZ013- 014-SDZ071-012215- Lyon IA-Osceola-Dickinson-Sioux-O'Brien-Clay IA-Plymouth-Cherokee- Buena Vista-Woodbury-Ida-Lyon MN-Murray-Cottonwood-Nobles-Jackson- Dixon-Dakota-Union- 508 PM CDT Mon Apr 30 2018 This Hazardous Weather Outlook is for portions of northwest Iowa, west central Iowa, southwest Minnesota, northeast Nebraska, and southeast South Dakota. .DAY ONE...Tonight Scattered showers and thunderstorms will be possible this evening and overnight. A few of the storms could produce dime sized hail as well as wind gusts to 55 mph. Minor to moderate flooding will continue along many rivers and streams, including the James and Big Sioux rivers. .DAYS TWO THROUGH SEVEN...Tuesday through Sunday Showers and thunderstorms will be possible again late Tuesday afternoon and Tuesday evening. A few of these storms could be strong. Minor to moderate flooding is forecast to continue through mid to late week on many rivers and streams due to the recent snowmelt. .SPOTTER INFORMATION STATEMENT... Spotter activation will not be needed. MNZ071-097-098-SDZ038>040-050-052>070-012215- Lincoln MN-Pipestone-Rock-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- 508 PM CDT Mon Apr 30 2018 This Hazardous Weather Outlook is for portions of southwest Minnesota, central South Dakota, east central South Dakota, south central South Dakota, and southeast South Dakota. .DAY ONE...Tonight An area of thunderstorms should develop this evening near the James River Valley to just west of Interstate 29. These storms will shift east through the night. A few severe thunderstorms will be possible, especially between 6 pm and 11 pm, with 70 mph wind gusts and half dollar sized hail the main threats. Minor to moderate flooding will continue along many rivers and streams, including the James and Big Sioux rivers. .DAYS TWO THROUGH SEVEN...Tuesday through Sunday Minor to moderate flooding is forecast to continue through mid to late week on many rivers and streams due to the recent snowmelt. .SPOTTER INFORMATION STATEMENT... Spotter activation will not be needed.
PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SIOUX FALLS SD
830 PM CDT MON APR 30 2018
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0823 PM HAIL 3 NW AVON 43.04N 98.11W
04/30/2018 E1.00 INCH CHARLES MIX SD PUBLIC
&&PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SIOUX FALLS SD
837 PM CDT MON APR 30 2018
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0835 PM TSTM WND GST 4 E AVON 43.00N 97.99W
04/30/2018 E64 MPH BON HOMME SD STORM CHASER
&&The national radar mosaic for the moment the first event began.
2 flood. Local standard time, as filed.
A slow fall through minor flood stages persisted along the Des Moines River during much of the first week of May, with impacts generally limited to flooding of agricultural areas near the river. Flooding ended near Windom (gage site WDOM5) on May 5.
A slow fall through minor flood stages persisted along the Des Moines River during much of the first week of May, with impacts generally limited to flooding of agricultural areas near the river. Flooding ended near Windom (gage site WDOM5) on May 5.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Minnesota in 2018Every state in 2018This day in weather historyWarnings in Minnesota, 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: 761237, 761239.