A cluster of thunderstorms moving from southeast South Dakota into northwest Iowa intensified briefly with the help from an increasing low-level jet, tapping elevated instability of up to 2000 J/kg. Linear organization led to a brief wind enhancement.
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The episode as NWS FSD wrote it up.
A cluster of thunderstorms moving from southeast South Dakota into northwest Iowa intensified briefly with the help from an increasing low-level jet, tapping elevated instability of up to 2000 J/kg. Linear organization led to a brief wind enhancement.
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 Aug 6, 10:48 PM CST, 2 hours 12 minutes earlier.
AREA FORECAST DISCUSSION National Weather Service Sioux Falls SD 1148 PM CDT Fri Aug 6 2021 .SHORT TERM...(This Evening through Saturday) Issued at 242 PM CDT Fri Aug 6 2021 Forecast Highlights: 1. Uncertain severe weather risk overnight. 2. Higher confidence on thunderstorm risks and severe weather Saturday. 3. Convection risks remain in the region into early next week. THIS AFTERNOON: Surface warm front continues to arch across the Missouri River valley Friday afternoon. Temperatures north of the boundary are holding into the lower to middle 80s, with areas south of the front approaching 90. Most of the area remains capped this afternoon, with an absence of large scale lift, feel convection should stay minimal into early evening. That said, already seeing reflection of increasing 850:700 theta-e advection from central South Dakota into eastern Nebraska, in the form of elevated showers. This Evening and Overnight: Not great model initialization this morning with mid-lvl shortwave chain moving from the Northern Rockies into the Western Dakotas. NAM perhaps has the best initialization, and has some support from other CAMS into the overnight. That said, passage of mid-lvl elevated moisture layer northeast will increase mid-lvl lapse rates and enhanced MUCAPE over the area (but especially eastern SD, southwest MN, into northern Iowa), with values approaching 2000 J/KG. Further west, should also see weakening convection that comes off the High Plains reach central South Dakota later this evening. This convection could lead to some gusty winds in decaying echoes over the far west. Greater concern would be increasing convergence on the LLJ inducing strong to severe elevated convection near or after 10 pm. Effective shear approaching 40 knots could lead to supercell structures with golf ball or larger hail at times into the early morning of Saturday into the Buffalo Ridge areas. Saturday: A fairly potent severe weather setup remains in place for Saturday. There may likely be convection ongoing over the far east initially in the period, with some potential for trailing non- severe isolated convection southwest into the axis of the LLJ through northeast Nebraska in the morning. It may take a bit of time for this convection and cloud cover to clear the area in the morning, but it should eventually clear enough to allow strong heating to take place. A deepening shortwave entering the western Dakotas and Nebraska will serve to deepen return flow northward and result in sfc dew points in the 70s ahead of an elongated surface front stretching from north to south over central Nebraska into northeast South Dakota. MLCAPE values may push 3000 J/KG along and ahead of this boundary, but environmental shear does remain rather weak at 25 to 30 knots near initiation time. Speaking of initiation, strong upper forcing moves into the area by mid-afternoon ahead of an impressive mid-lvl trough. This forcing should be enough to bust through the small capping inversion still present in soundings around 4pm. Initial convection may be loosely supercellular given the modest shear. That said, if mid-lvl winds do increase around the base of the trough, this may favor more supercell development initially. Eventually storm mode may transition towards more multicellular clusters and bowing segments as activity slides east towards Minnesota and Iowa in the evening. As far as hazards are concerned, given mid-lvl lapse rates and high CAPE and favorable CAPE in the hail growth zone, would lead to potential for hail 2"+ during initial development state. Eventually collapsing updrafts should lead to a heightened wind risk with storms moving east. The other concern we'll have is with some tornadic potential both early in storm life but also as we begin to see enhancement of the 850 mb winds in the evening. Some backing of the sfc winds along and near the Missouri River during initial storm development may locally enhance sfc helicity. Secondly, increasing low lvl jet does increase 0-2 km shear towards the 20 knot mark, but alignment isn't completely perpendicular to cluster motion. Still this could produce an increased risks over NE Nebraska and further east into Iowa for the early overnight. Would also anticipate some localized heavy rain risks. Pre-storm PWATS approach 1.5-1.75" and slow storm motion initially could lead to some localized 2-3" amounts. .LONG TERM...(Saturday Night through Friday) Issued at 242 PM CDT Fri Aug 6 2021 Sunday: Quiet weather should return behind the upper trough on Sunday, with cooler temperatures in the region. Weak cold advection aloft could increase lapse rates enough for some scattered afternoon showers over NW Iowa. Highs otherwise will be in the 80s east to 90s west. Monday: Mid-lvl shortwave ridging will keep things quiet on Monday, but temperatures respond to that ridging as well. HIghs climb into the 90s in many areas with heat index readings approaching heat index criteria. Tuesday-Wednesday: Models are consistent on bringing large scale troughing into the northern US Tuesday into Wednesday. Another front will pass through the area, and could produce scattered strong to severe convection. The concern would be the degree of capping ahead of the boundary. Highs will be in the 80s both days. Thursday-Friday: Warm and dry conditions return for the second half of the week with the upper flow again transitioning towards longer wave pattern over the CONUS. This should favor warming temperatures into next weekend. && .AVIATION...(For the 06Z TAFS through 06Z Saturday night) Issued at 1128 PM CDT Fri Aug 6 2021 Showers and thunderstorms will continue to develop and push off to the north/northeast through the early overnight Saturday. Some of these storms could be strong to severe, with the main threats gusty winds to 50 knots and large hail to the size of ping pong balls, with the greatest risk of severe weather for KHON. Expect shower and storm activity to dissipate toward sunrise. Additional showers and thunderstorms are expected to develop Saturday afternoon and continue into the evening and early overnight. Some thunderstorms could be strong to severe once again, with the greatest threats damaging winds and large hail. Given low confidence in solutions regarding timing/location of development, have omitted convection from this TAF package. Folks with plans to fly Saturday should remain aware of the forecast as confidence increases. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...NONE. MN...NONE. IA...NONE. NE...NONE. && SHORT TERM...Dux LONG TERM...Dux AVIATION...SG
Hazardous Weather Outlook National Weather Service Sioux Falls SD 822 PM CDT Fri Aug 6 2021 IAZ001>003-012>014-020>022-031-032-MNZ071-072-080-081-089-090-097- 098-NEZ013-014-SDZ038>040-050-052>071-080130- Lyon-Osceola-Dickinson-Sioux-O'Brien-Clay-Plymouth-Cherokee- Buena Vista-Woodbury-Ida-Lincoln-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-Bon Homme- Yankton-Union- 822 PM CDT Fri Aug 6 2021 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...Tonight A line of thunderstorms will move into central South Dakota around midnight and towards interstate 29 by 3 am. Winds of 70 mph and half dollar hail will be possible. The better chances for severe weather should be west of Interstate 29. Parts of southwest Minnesota and northwest Iowa may see some stronger storms late tonight but the chances are lower. .DAYS TWO THROUGH SEVEN...Saturday through Thursday Severe weather may return to the area on Saturday afternoon. The main threats with the strongest storms are half dollar hail, damaging wind gusts up to 65 mph, locally heavy downpours, and isolated tornadoes. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected at this time.
The Sioux Falls radar (KFSD), 79 km away, was scanning. The volume nearest the first event began at 07:01 UTC.
1 thunderstorm wind. Local standard time, as filed.
Thunderstorm winds ripped off portions of the roofs on two hog barns. Property damage costs are a rough estimate. Crop damage is an estimate from insured losses. Information provided by the United States Department of Agriculture.
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Every storm in Iowa in 2021Every state in 2021This day in weather historyWarnings in Iowa, August 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: 982047.