Synoptically speaking, widespread severe weather potential was not in the cards for the Iowa area with a fairly stationary boundary well to the south and little to speak of with regards to surface features. That said, elevated storms were possible across southern Iowa with low level jet initiated storms each of the very early morning hours of August 31st and then again during the evening hours.
Every event with a place to put it. Tap one for its entry below.
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 DMX wrote it up.
Synoptically speaking, widespread severe weather potential was not in the cards for the Iowa area with a fairly stationary boundary well to the south and little to speak of with regards to surface features. That said, elevated storms were possible across southern Iowa with low level jet initiated storms each of the very early morning hours of August 31st and then again during the evening hours. The initial early morning round propagated slowly northward after growing from storms in northeast Kansas, while the evening round initiated across southwest Iowa.
Environmentally, there was some support for at least brief organization with effective bulk shear in excess of 30 to 40 kts and MUCAPE values in excess of 2000 to 3000 J/kg during both events. When all was said and done, only a small handful of hail reports were received, with the large being 2 inch in diameter.
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 30, 10:58 PM CST, 4 hours 17 minutes earlier.
Area Forecast Discussion National Weather Service Des Moines IA 1158 PM CDT Thu Aug 30 2018 ...Updated Aviation Discussion... .SHORT TERM.../Tonight through Friday/ Issued at 329 PM CDT Thu Aug 30 2018 Clouds have spread across most of the area today, helping hold down temperatures in modest southeasterly surface flow. However, warm air advection has set in aloft and surface dewpoints have begun to creep upward, a harbinger of things to come as we enter what appears to be a May or June-like weather pattern heading into September. A modest shortwave impulse is moving eastward across the High Plains currently within modest zonal flow aloft. The broad forcing ahead of this impulse will reach our forecast area later tonight, coincident with the development of a nocturnal low-level jet. This will be sufficient to generate at least scattered thunderstorms, however confidence in location and intensity of precipitation is relatively low. Earlier high-resolution model output last night/this morning indicated two regions of primary convective development, one over southern Minnesota and northern Iowa and the other over southwest Iowa into northern Missouri, however, more recent solutions have indicated somewhat more widespread convection across much of our forecast area, including central Iowa. Instability and shear parameters are supportive of a non- zero severe weather threat, probably in the form of marginal hail, which is outlined well by SPC products. Locally heavy rain may also be possible, but, unlike subsequent nights, the storms should be scattered and progressive enough to mitigate this threat for the most part. It is expected that any lingering overnight storms will clear the area and/or dissipate quickly after sunrise Friday morning. Southerly flow will increase by late morning and through the rest of the day, aiding more robust warm air and especially moisture advection up across Iowa. A synoptic frontal boundary will remain nearly stationary from around central Nebraska up to western Minnesota during the day, well northwest of our area. Some guidance suggests isolated/scattered thunderstorm development on Friday afternoon, especially the NAM which is uniquely aggressive in this regard, but forecast soundings indicate a fairly stout cap for much of the day, which will be dependent on the extent of morning convection. For now have deferred to the broader consensus and opted to maintain 20-40 POPs during the afternoon, reserving higher chances for Friday night and onward. It should be noted that if strong updrafts can develop on Friday afternoon, the environment would certainly be supportive of a severe weather threat. .LONG TERM.../Friday night through Thursday/ Issued at 329 PM CDT Thu Aug 30 2018 The long term forecast is very wet, with a prolonged period of unseasonably high dewpoints and instability and multiple rounds of thunderstorms, several with the potential for heavy rainfall. This is primarily the result of a 500 mb blocking pattern with a ridge entrenched over the eastern seaboard and a broad trough stuck over the Rockies. This will produce persistent west southwesterly flow over Iowa. With a synoptic boundary waffling around near/northwest of our forecast area and no change in airmass, it will be a wet, muggy week for most of Iowa. Thunderstorms will be generated periodically by multiple mechanisms including shortwave impulses aloft, diurnal heating, and repeated development of the nocturnal low-level jet. Confidence in details of timing/location/magnitude decreases with each successive round of convection, as each round will be altered by the one before it. On Friday evening thunderstorms are likely to develop near the synoptic boundary, probably northwest of our area up toward Siouxland. Brisk mid-level flow will then steer these storms eastward toward northern and central Iowa. Given the expected strong instability and developing LLJ, upscale growth into a more linear system with an attendant strong wind threat is very possible. While much of the prognostic guidance indicates such a convective complex would move east or even east northeast, given the orientation of the moisture/instability pool it appears more likely that the storms would propagate/develop more southeastward toward central and eastern Iowa. This has resulted in a large slight risk area outlined by SPC. However, there is uncertainty regarding the location of various boundaries due to Friday morning convection, and the degree of instability in the evening depending on if the NAM is correct in its depiction of earlier afternoon convection as discussed in the short term discussion above. In any event, as the LLJ becomes active after dark Friday night, any ongoing storms should be fairly persistent and this will lead to a risk of locally heavy rainfall, especially given the unseasonably high precipitable water values in place. By tomorrow, when we should have a better handle on Friday night convective details due to the Friday morning convection having already occurred, we may need to ramp up wording of flash flood potential for Friday night and/or Saturday night. However, the threat is too conditional to do so at this time. Each of the days from Saturday through at least Monday, and likely well beyond, will see repeated chances for thunderstorms as there is very little change in the weather pattern. The 500 mb steering flow will remain essentially unchanged, surface dewpoints will remain in the lower 70s, the synoptic boundary will remain nearby, and the nocturnal LLJ will continue to be active. Storms will be most likely around the evening and again overnight each day. While the severe weather threat should generally decrease with time, due to repeated convective overturning and resulting effects on instability, the threat of flooding/flash flooding should gradually ramp up with each round of storms. This will lead to a very active holiday weekend and beyond, and we will likely be using increased messaging to highlight these threats for many days, well into next week. No significant change in the weather pattern is foreseen through at least the next seven days. && .AVIATION.../For the 06Z TAFS through 06Z Friday night/ Issued at 1159 PM CDT Thu Aug 30 2018 Latest hires models have pulled back the timing of convection until past 10z Friday. Looks to be mainly along I-35 and then push east, so diminished the amount of time for thunder mentioned at FOD. Higher confidence storms will impact DSM, MCW, and ALO during the morning hours. A break is likely late in the morning Friday before redevelopment seems reasonable during peak heating over the south. After 18z Friday, low confidence with timing and duration of convection in the afternoon and slightly higher confidence with storms developing with the LLJ during the evening hours Friday. && .DMX WATCHES/WARNINGS/ADVISORIES...NONE. && SHORT TERM...Lee LONG TERM...Lee AVIATION...Podrazik
Hazardous Weather Outlook National Weather Service Des Moines IA 316 AM CDT Fri Aug 31 2018 IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075- 081>086-092>097-011100- 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-Marshall-Tama-Audubon-Guthrie-Dallas-Polk-Jasper- Poweshiek-Cass-Adair-Madison-Warren-Marion-Mahaska-Adams-Union- Clarke-Lucas-Monroe-Wapello-Taylor-Ringgold-Decatur-Wayne- Appanoose-Davis- 316 AM CDT Fri Aug 31 2018 This Hazardous Weather Outlook is for portions of central Iowa. .DAY ONE...Today and tonight Strong to possibly severe storms will be possible at times through the day, with chances increasing this evening into the late night hours. Large hail and damaging wind are the main threats along with locally heavy rainfall. A tornado chance cannot be ruled out later today. .DAYS TWO THROUGH SEVEN...Saturday through Thursday Periodic storm chances continue Saturday through Thursday. There is a chance for strong to possibly severe storms Friday through Sunday night. Large hail and damaging winds will be the main threats. Localized heavy rainfall will be possible with all of the storm chances. River flooding and flash flood threats could increase into next week with several rounds of possibly heavy rainfall expected through the weekend into next week. .SPOTTER INFORMATION STATEMENT... Spotter activation may be needed today and tonight, and possibly into the weekend at times. MJB
The Des Moines radar (KDMX), 145 km away, was scanning. The volume nearest the first event began at 09:17 UTC.
5 hail. Local standard time, as filed.
Trained spotter reported quarter sized hail along with periods of heavy rainfall including 1.2 inches of rain in about 30 minutes.
Trained spotter reported a few hail stones quarter in size, along with a lot of pea sized hail.
Trained spotter reported ping pong ball sized hail.
Public reported quarter sized hail.
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, August 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: 777807, 779236, 777808, 777809, 777810.