A slow moving boundary across southern Iowa provided the impetus for training storms along the Iowa Missouri border. Several inches of rain fell and resulted in flash flooding in Appanoose county.
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The episode as NWS DMX wrote it up.
A slow moving boundary across southern Iowa provided the impetus for training storms along the Iowa Missouri border. Several inches of rain fell and resulted in flash flooding in Appanoose county. Later in the afternoon as the front lifted into central Iowa scattered storms developed within the warm sector. While storms remained below severe thresholds, they did produce two brief funnel clouds in Jasper and Poweshiek counties.
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 12, 5:30 AM CST, 5 hours 48 minutes earlier.
Area Forecast Discussion National Weather Service Des Moines IA 630 AM CDT Thu Aug 12 2021 .SHORT TERM.../Today through Friday/ Issued at 256 AM CDT Thu Aug 12 2021 Confidence: Medium to High While still in the midst of a rather warm and energetic pattern across the central CONUS, we are now seeing some transition toward a a quieter and more seasonable pattern coming up by tomorrow and into the weekend. Tonight the forecast weak cool front that was expected to track across the state has made it to its present destination and and now stalled over southern Iowa and northwest Missouri. Though the H700 cap held over most of the region today, tonight the column aloft has cooled enough and with the expected weak cross H850-H700 frontal flow with the nocturnal jet, we are seeing scattered convection in far southern Iowa as of 04z which continues presently at 0630z. Remarkably, the surface front has allowed for much drier air to filter into northern Iowa where dewpoints are now in the mid 50s compared to 75 to 80 along the boundary itself in southern Iowa. Today will be our last "warm to hot day" in a while. A upper level trough over the Rockies and a surface reflection over South Dakota will track east today as a cold front sweeping into the Northern Plains from a rather pronounced and strong upper level system over Hudson Bay catches up with the weaker Central Plains system in southern Iowa this afternoon and evening. The net result will be some return south southwest flow yet today ahead of the main cold front...allowing for highs to reach the lower to mid 80s north and in the upper 80s to lower 90s in the south. With evapotranspiration ongoing again today, dewpoints will recover in the south just ahead and along the boundary into the lower to mid 70s by mid to late afternoon. Across much of the north half of the area from about US 30 to the MN border, dewpoints will have already dropped into the lower 50s along the border to the mid 60s along US 30. Our early morning convection now along the IA MO border will diminish by 12z as the H850 flow becomes more parallel to the boundary. A good share of the day today will be dry as the airmass recovers over the south ahead of the approaching cold front. By this afternoon, MLCAPE will increase to about 1000 to 3000 j/kg across the far south into Northern Missouri. Increasing 0-6km shear to 30 to 40 kts and a sharpening H700 trough will aid in thunderstorm development by 21-00z in the south, which will further increase in coverage after 00z. As with most boundaries that get caught up our south with exiting systems in Canada, we will see an uptick in convection between 02 and 08z across Missouri into Southern Iowa. There appears to be a small window in the afternoon across the far south southeast when a few storms may reach severe limits in the far south, with just scattered storms for the remainder of the evening. Severe threats will mainly include some hail and possible wind gusts in the south today. Depending on the push of the storms, we may see the activity end up a bit farther south into Missouri but as the low level jet again kicks in for a portion of the overnight, the south still has a better chance of some storms. Tonight the cooler and drier air will continue to drive south across the region with overnight lows in the lower 50s north to the lower to mid 60s south. Tomorrow the rain will be out of the area by early in the morning with ample sunshine as high pressure builds across the region for the remainder of the afternoon. A rather comfortable end to a rather unusually humid and hot week over the area; highs Friday will reach the lower 80s over most of our area with a light northwest wind during the day. .LONG TERM.../Friday Night through Wednesday/ Issued at 256 AM CDT Thu Aug 12 2021 Confidence: High Much of the weekend into early next week remains quiet. Though there remain some differences within the various medium range solutions, a common theme with cooler air dropping south into the Great Lakes accompanied by high pressure is that it generally takes a while for the pattern to shift east enough to bring the warmer air back into Iowa and the Great Lakes/Ohio River Valley. This happened last week and despite the rather hot/humid combination, 7 days ago the models suggested 100+ highs this past weekend into Monday. So, with that in mind, a more gentle and slower approach to warming into next week as well as moisture return is most prudent. Looking aloft at the upper level flow, a large gyre over Central Canada will push cool air into MN southeast across the Great Lakes through Saturday evening, with the most pleasant H850 temperatures moving over Iowa tomorrow into Sunday. By Sunday, a slow increase in H500 heights and warming H850 temperatures will be forced north northeast across the Northern Plains and Canada prior to a more eastward shift into Iowa by next week. At the same time, the lower level H850 moisture plume is likely to be diverted farther west in its northerly track, keeping the deeper moisture channel focused into Nebraska and the lee of the Rockies. From Monday through Wednesday, H850 temperatures warm up from 14C to about 18-19C across the region. This will bring highs up from the lower 80s Monday to the mid to upper 80s by Wednesday. Meanwhile, a trough making some progress east will enter the Western High Plains/Northern Plains on Wednesday with a shift in the moisture plume into Iowa around Thursday. Eventually, the heat and humidity returns for a bit along with at least a better chance for precipitation. Wind fields look fairly relaxed until the mid to late week trough, so expect another period of rather tranquil flow as well until late Wednesday...with stronger westerlies then forecast into the end of next week. && .AVIATION.../For the 12Z TAFS through 12Z Friday morning/ Issued at 629 AM CDT Thu Aug 12 2021 Small cluster of storms in southern Iowa now will spread bkn250 over KOTM through 14z...while approaching front brings sct-bkn110 across remainder of area through 21z along with sct060 with daytime heating. Scattered storms develop aft 20z KOTM and continue through 08z with storms exiting entire area by 12z. Remainder of sites likely not affected by convection this period. /rev && .DMX WATCHES/WARNINGS/ADVISORIES... NONE. && SHORT TERM...REV LONG TERM...REV AVIATION...REV
Hazardous Weather Outlook National Weather Service Des Moines IA 834 AM CDT Thu Aug 12 2021 IAZ049-050-059>062-070>075-081>086-092>097-131345- Marshall-Tama-Dallas-Polk-Jasper-Poweshiek-Cass-Adair-Madison- Warren-Marion-Mahaska-Adams-Union-Clarke-Lucas-Monroe-Wapello- Taylor-Ringgold-Decatur-Wayne-Appanoose-Davis- 834 AM CDT Thu Aug 12 2021 This Hazardous Weather Outlook is for portions of central Iowa. .DAY ONE...Today and tonight A few thunderstorms, some strong, are ongoing in the far southeast this morning. Isolated to scattered thunderstorms are possible late this afternoon into the evening hours. The best potential for severe storms is expected over far southern Iowa where large hail and damaging winds look to be the main threat. .DAYS TWO THROUGH SEVEN...Friday through Wednesday No hazardous weather is expected at this time. .SPOTTER INFORMATION STATEMENT... Spotter activation will not be needed. IAZ004>007-015>017-023>028-033>039-044>048-057-058-131345- Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo- Pocahontas-Humboldt-Wright-Franklin-Butler-Bremer-Sac-Calhoun- Webster-Hamilton-Hardin-Grundy-Black Hawk-Crawford-Carroll-Greene- Boone-Story-Audubon-Guthrie- 834 AM CDT Thu Aug 12 2021 This Hazardous Weather Outlook is for portions of central Iowa. .DAY ONE...Today and tonight No hazardous weather is expected at this time. .DAYS TWO THROUGH SEVEN...Friday through Wednesday No hazardous weather is expected at this time. .SPOTTER INFORMATION STATEMENT... Spotter activation will not be needed.
Special Weather Statement
National Weather Service Des Moines IA
738 AM CDT Thu Aug 12 2021
IAZ097-121330-
Davis IA-
738 AM CDT Thu Aug 12 2021
...A strong thunderstorm will impact portions of Davis and eastern
Appanoose Counties through 830 AM CDT...
At 737 AM CDT, Doppler radar was tracking a strong thunderstorm near
Bloomfield, moving east at 25 mph.
HAZARD...Winds in excess of 30 mph and nickel size hail.
SOURCE...Radar indicated.
IMPACT...Gusty winds could knock down tree limbs and blow around
unsecured objects. Minor damage to outdoor objects is
possible.
This strong thunderstorm will be near...
Bloomfield around 750 AM CDT.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
If outdoors, consider seeking shelter inside a building.
&&
LAT...LON 4063 9264 4082 9264 4087 9218 4063 9218
TIME...MOT...LOC 1237Z 261DEG 22KT 4072 9253
MAX HAIL SIZE...0.88 IN
MAX WIND GUST...30 MPH
PodrazikThe Des Moines radar (KDMX), 141 km away, was scanning. The volume nearest the first event began at 17:16 UTC.
2 funnel cloud, 1 flash flood. Local standard time, as filed.
Delayed report of flooding in multiple areas, including highway 5 where 6 to 7 inches of water was over the road. Several other roads impassable.
KCCI viewer shared a picture looking south from a truck stop along Interstate 80. Location edited based on likely thunderstorm location from radar.
Public report of funnel cloud spotted along Interstate 80 looking south.
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, 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: 978575, 978576, 978577.