textproduct: Des Moines

KEY MESSAGES

- Locally heavy rainfall and potential for isolated flash flooding through Wednesday night. This risk is offset by dry soils and mature crops in rural areas with a relative higher risk for flash flooding in urban areas. The severe weather threat is very minimal.

- Drier conditions later in the week, with warming into the weekend.

SHORT TERM /THROUGH WEDNESDAY NIGHT/

Issued at 324 AM CDT Tue Aug 4 2026

A tight, upper level low pressure is seen on GOES-East water vapor imagery this morning over the Red River Valley of the North within a more broad, low amplitude trough from the Pacific Northwest to the western Great Lakes. A line of GLM detected lightning flashes are nearing Lake Superior and stretching back to Siouxland and the central High Plains ahead of this tight, upper level spin. These storms are associated with a surface cold front that has moved into northwestern Iowa. With limited effective shear and instability, this points to storms that will be less organized, but could produce wind gusts up to 50 mph per recent Spencer observation as scattered storms and showers move over north central Iowa before sunrise. As this first wave passes and the low level jet veers off this morning, coverage is expected to become much more spotty later this morning into some of this afternoon with any forcing more nebulous. In the meantime, atmospheric moisture transport will continue into the state with precipitable water (PWAT) values building to between 1.5 to around 2 inches ahead of the front. These PWAT values are around the 90th percentile in the NAEFS and ECMWF data, which while anomalous are not like what we saw late last week near record values. As a shortwave trough moves through the base of the broader trough later this afternoon into tonight and the low level jet ramps up and points into the northeast to southwest draped front over southern Iowa by this time, will see showers and thunderstorms become more widespread on the cool side of the boundary. This will initially be over northern Iowa and then spreading into central Iowa this evening/overnight before settling into southern Iowa either late tonight or Wednesday morning, perhaps aided by outflow surges moving in some southward fashion.

The severe risk will be greatly minimized by skinny CAPE profiles with 500-1000 J/kg of most unstable instability (MUCAPE), weak lapse rates, and marginal effective shear of around 25 to 30 knots. Thus, the top concern is locally heavy rainfall and potential flash flooding tonight given the reservoir of seasonally high PWATs coupled with efficient rain rate warm cloud depths and quasi- parallel 850-300mb flow around 25 to 30 knots. Deterministic, ensemble, and AI models along with WPC forecast are favoring generally south of US Highway 30 for the highest rainfall totals. For the deterministic models, rain totals by Wednesday morning are between 1 and 3.5 inches with the aggressive FV3 showing localized amounts up to around 6 inches. Meanwhile, the latest HREF/REFS and WPC forecast rainfall have between 1 to 3 inches with the HREF/REFS localized probability matched mean (LPMM) showing upper end of 4 to 5 inches. There is some question in the southern extent of the heaviest rainfall with deterministic models tending to have lesser amount south of US 34.

So, what is the flash flood risk for tonight into Wednesday morning? The short answer is urban areas have a relative higher flash flood risk with less infiltration capacity with rural areas having dry soils and mature agricultural crops to absorb much of the rainfall. The longer answer is that NASA SPoRT's soil moisture analysis has changed little since yesterday given the dry conditions on Monday with western Iowa into southern Iowa at or below the 30th percentile for the 0-16 inch layer. Northwest and west central Iowa are the driest with soil moisture below the 5th percentile. Eastern Iowa's higher soil moisture percentile above the 70th has shrunk, but north of I-80 and east of I-35 will be more prone to heavy rainfall. WPC continues with their marginal risk of excessive rainfall outlook (ERO) today/tonight. Further, the National Water Center's Flood Hazard Outlook (FHO) has a limited area over all of the southern two- thirds of Iowa, though their discussion from Monday afternoon points to more concerns over eastern Iowa and points east due to the aforementioned rainfall from late last week. This (eastern Iowa) is where the National Water Model forced by the GFS is showing up to a 25% chance of rapid onset flooding. This leads to the conclusion outlined above that the flash flood risk for rural areas is minimized by the dry soils allowing for capacity for much of this rainfall. That is, until rates start to exceed 3 or so inches per hour overwhelming infiltration capacity or totals start to top 5 if not 6 inches. For urban areas, the flash flood risk is higher due to less ability to infiltrate the rainfall. A flood watch was considered, but given the dry antecedent conditions this lower the concerns for widespread flash flooding at this time.

There may be a lull after daybreak Wednesday until later in the afternoon when the same process of more coverage of showers and storms occurs as shortwave energy moves through the base of the trough and the nocturnal low level jet kicks in. Depending on where rain falls before then, this could heighten the flash flood risk in southern Iowa, which lines up well with the WPC day 2 slight risk ERO, if a second round of rain falls over the same area. As for longer term river flooding potential, the latest 72 hour contingency forecasts and the experimental, 3 day Hydrologic Ensemble Forecasting Service (HEFS) are showing below action stage. Further, the National Water Model forced by the NBM and GFS show within bank rises.

LONG TERM /THURSDAY THROUGH MONDAY/

Issued at 305 PM CDT Mon Aug 3 2026

As the boundary gradually departs Wednesday, dry conditions are expected to return through the evening. The trend Thursday and beyond shows a period of dry conditions as ridging generally builds across the Western CONUS, with gradual warming occurring as highs return into the upper 80s to low 90s by the weekend. Could see a few shortwaves ride the broader ridge late week and more so later in the weekend that could introduce some potential for showers and storms, though this is low confidence given this being in the long range period of the current forecast, and will continue to keep an eye on trends over the next several days.

AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 645 AM CDT Tue Aug 4 2026

Primary challenge this forecast period was shower and storm chances and their timing and impacts on a given terminal. Showers and a few storms are ongoing from MCW to FOD to northwest of DSM. These should be moving out of MCW/FOD shortly with activity expected to diminish in coverage. Outside of a brief window of IFR ceiling this morning, VFR conditions will prevail until tonight when rain and storms become more widespread spreading from northern into central and southern Iowa. This will bring SCT/BKN ceilings down below FL030 to DSM/OTM/ALO area while northern Iowa will return or be in VFR conditions as the rain moves away from those areas.

DMX WATCHES/WARNINGS/ADVISORIES

None.