textproduct: Des Moines

KEY MESSAGES

- High confidence in a prolonged period of rain with rumbles of thunder from late Tuesday through late Thursday. Rain totals of 2 to 4 inches are forecast in a southwest to northeast oriented band across central Iowa.

- Flood Watch is in effect for much of central Iowa due to concerns for flash flooding and areal flooding. Rises on rivers are likely and some river flooding may result.

- Conditions turning overall drier late this week into next weekend.

SHORT TERM /THROUGH WEDNESDAY/

Issued at 241 PM CDT Mon Sep 28 2026

We've made it to another short period of dry conditions today with high pressure over northern Wisconsin ushering in drier air on the light easterly low level flow. This dry air finally helped to break up the stratus clouds over southern Iowa, which has pushed northwestward through the day as shown on GOES-East visible imagery. This has allowed temperatures to be seasonable well into the 60s to middle 70s. While conditions will remain dry tonight, mid and high level clouds will begin to stream northeastward over the northwestern half of the state tonight. This moisture from the remnants of Odalys will arrive over the region ahead of an amplified longwave trough that will be over the Southwest Tuesday morning. As the moisture plume moves northeastward through the day paired with increasing forcing, off and on showers and storms will trickle into our northwestern forecast area from Emmet to Sac Counties late morning if not Tuesday afternoon. Showers and storms will become less common through Tuesday night over northern Iowa, but there will be new development moving in from southwest Iowa into northeastern Iowa as the low level jet increases into Iowa. Far portions of southern and southeastern Iowa may still remain dry though. Given this will be the beginning of the prolonged wet period with very atypical efficient rainfall parameters in conjunction with wet soil conditions, have issued a Flood Watch from Tuesday evening through late Thursday evening for areas along and north of a roughly Lamoni to Oskaloosa line.

LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/

Issued at 241 PM CDT Mon Sep 28 2026

Overview: A wet period will continue through Wednesday night with conditions very slowly turning drier from northwest to southeast across the state from Thursday into Friday morning. Looking at the weather elements to monitor from yesterday's discussion, it does seem that the first surge of atmospheric moisture plays more with the southern Canadian prairie shortwave and thus limits its northward potential. In part as a result of this is why the last few runs of the AIFS/AIGFS have trended a bit southward in the area of highest rainfall.

Details: For Wednesday, there will be a steady stream of rainfall with rumbles of thunder ongoing over a good portion of Iowa outside of some southern areas as waves of strong low level thermodynamic forcing and steady kinematic forcing prevail over the state. Further, the remnant moisture from Hurricane Polo will arrive and keep precipitable water values above the 90th percentile and likely above sounding climatology record values with warm cloud depths unseasonably high for late September/early October. To compare, these heavy rainfall parameters are more reminiscent of summer time values. While the 850-300mb flow will be on the faster side (around 30-40 knots), there looks to be a corridor where rainfall will repeat. As mentioned, there have been some shifts southward in the AI guidance as well as the deterministic and ensemble. However, there remain outliers in the heavy rainfall axis with the REFS/AIGFS (despite shifting southward recently) farther north over northwest and north central Iowa while the ECMWF/EPS is farther south over southern Iowa with the remainder of guidance in-between. So while the rounds of rainfall will waver and wiggle in their placement at times, there will be a band of 2 to 4 inches and locally 5 inches or more in some sort of southwest to northeast orientation. Looking at this rainfall probabilistically through Thursday morning, our forecast chances of 2" or more are above 80% around an axis from near Mason City to Fort Dodge to Omaha with 4" or more around an axis from Omaha that ends near Fort Dodge. This rainfall may lead to flash flooding or at a minimum low lying and poor drainage flooding. In addition, areal or overland flooding may result as the soils continue to lose soil infiltration capacity.

Of course, this rainfall will route to area streams and rivers. As briefly mentioned in the short term discussion, soil conditions are wet with NASA SPoRT showing above the 70th percentile in the top 4" (rapid response layer) and 16" (river response layer) over most areas south of Highway 3 in central Iowa. Most rivers are above their baseflows (+2 feet above) from the start of the month, mainly areas along Highway 30 and southward in the state, but also much of the Cedar Basin. This is shown nicely by the National Water Model's 14 day streamflow anomaly analysis. Thus, rivers and streams in north central into northwest Iowa have more capacity to handle the incoming rainfall and its runoff and routing compared to elsewhere. This includes the Winnebago, much of the upper Iowa and upper Des Moines (above Fort Dodge/FODI4) basins, and parts of the Skunk Basin (above Ames/AESI4). While these latter basins have more capacity, the 72 hour contingency hydrologic forecasts show rises across central Iowa with about a dozen forecast points rising above flood stage. This is also shown in last evening's experimental Hydrologic Ensemble Forecasting Service (HEFS), which uses ensemble GFS QPF, with several points exceeding flood stage at the 50% chance exceedance level. If looking at the 30% chance exceedance level (so higher QPF), this increases to over a dozen points above at least flood stage.

Moving later Thursday into Friday, the longwave trough deamplifies and largely shears out as an approaching northern stream wave aids in pushing this system away from the area. For sensible weather, this will mean the rain ending from early Thursday or perhaps as soon as Wednesday night over northwestern Iowa and our northwest forecast area. This drying trend moves slowly southeastward ending the rain in our southeastern forecast area by Friday morning.The approaching northern stream wave will move through Saturday and will bring a return of some clouds and atmospheric moisture return, but showers seem more limited and would certainly be progressive. Behind this system, northwesterly flow looks to take over and with a zone of high pressure moving in, this may spell a change as we continue into October with some drier conditions.

AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 1145 PM CDT Mon Sep 28 2026

Impacts begin after 15z with the arrival of showers and perhaps a few thunderstorms into central Iowa. Exact timing is still unknown, though a long period of showers is highly certain, especially after 18z. Current trends indicate higher chances of showers at KDSM/KALO after 18z, and will be monitored for an addition of prevailing showers. Precip may decrease for a time into the evening and CIGs will likely lower at that time. Falls to LIFR CIGs, with fog potentially building in as well, appear to be possible but onset may be beyond 06z.

DMX WATCHES/WARNINGS/ADVISORIES

Flood Watch from Tuesday evening through late Thursday night for IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>074- 081>083-092>094.