textproduct: Des Moines

KEY MESSAGES

- Showers and thunderstorms passing from west to east through the day today, followed by a brief lull in the afternoon and early evening.

- Widespread, prolonged period of rainfall expected over Iowa from tonight through Wednesday and Thursday. Rainfall amounts of 2 to 4 inches likely, with higher amounts possible in areas that see less breaks from the rain. Flooding and flash flooding will be possible.

- Rainfall chances ending north and northwest on Thursday, ending in the south Friday morning. Cooler and drier conditions into the weekend.

UPDATE

Issued at 949 AM CDT Tue Sep 29 2026

The Flood Watch has been updated to now include Wapello, Appanoose and Davis counties.

SHORT TERM /THROUGH WEDNESDAY NIGHT/

Issued at 336 AM CDT Tue Sep 29 2026

After finally getting to see some sunshine across Iowa today, cloud cover is already increasing from the west. These clouds are foreshadowing what will be a wet couple of days for Iowa. A slow moving 500 mb trough and remnants from tropical activity in the Pacific will bring a steady surge of moisture up into Iowa today through Thursday. Visible satellite and water vapor imagery highlight this broad mid and upper level moisture plume well, stretching up from Hurricane Polo in Baja California, up through New Mexico, Colorado, Kansas, Nebraska, the Dakotas and Minnesota. Through the day today, a northern stream 500 mb shortwave will push this moisture plume eastward into Iowa, while also providing weak synoptic ascent. The result will be rain chances beginning as soon as 9 to 10 am today in western Iowa, spreading across the area throughout the day.

A small amount of instability will accompany this activity, but instability values generally under 1000 J/kg, poor lapse rates, and minimal shear should keep any thunderstorm activity below severe limits. Warm cloud processes and increasing layer moisture/precipitable water values will promote efficient rainfall with activity today, and really through the entirety of the event. Storms during the day today should generally be progressive enough to mitigate flash flood concerns, barring a poorly placed downpour over an urban area. However, with rainfall generally expected to continue into the overnight hours and beyond, flooding concerns will be increasing tonight through Wednesday.

While today's initial push of rain will generally be associated with the passing northern shortwave, the pattern quickly transitions to a broad theta-e advection/overrunning setup as Hurricane Polo lifts northward into the plains tonight and parks itself over Kansas on Wednesday. The coincident moisture from the hurricane remnants will overrun a cooler air mass over Iowa, keeping persistent rainfall over Iowa through Wednesday. The combination of anomalously high PWATs for this time of year (2"+), storm motions roughly parallel to the boundary, warm cloud processes, and persistent lift will result in a prolonged, widespread moderate to heavy rainfall over Iowa tonight through Wednesday night and potentially Thursday.

It appears likely we will see one or multiple areas of 2 to 4 inch rainfall amounts falling from tonight through Wednesday night. Embedded within this band, locally higher amounts closer to 5 inches will be possible, especially in areas where convective processes drive up rain rates, or where breaks in the rain are minimal. That said, exactly where the axis of heaviest rainfall will occur is still somewhat up for debate as models continue to waver on QPF placement. On previous shifts, the heaviest band looked to setup from western into north central Iowa. On this shift, the QPF band has seemed to shift southward slightly, albeit not completely removed from northern Iowa. More on that in a second. Recent deterministic guidance, namely the GFS, ECMWF, and RRFS, are showing the highest QPF footprint roughly from southwest into central Iowa. This southerly trend is being picked up by the NBM as well, with the highest probabilities for exceeding 4" of rainfall (~40 to 50%) now from the southwest corner of Iowa up through Atlantic and Creston to roughly Ames and Des Moines. However, while the heaviest band is trending southward, the AI GFS and GFS output still show a second band along that Fort Dodge to Mason City line in north central Iowa. This band is in addition to the southern band, resulting from a northward push of the rainfall overnight Wednesday, before rain/storm activity is shunted back south and east again. That all to say, there is considerable uncertainty with the evolution of the rainfall and where the heaviest rain will fall. What is certain is that prolonged, widespread rainfall will be ongoing from tonight through Wednesday, and will likely lead to flooding and flash flooding concerns as water is added to the system. The Flood Watch remains in effect from this evening through Thursday evening, although have expanded it southeastward to account for the southeastward trend in the QPF. For more on flood concerns, see the hydro discussion below.

Instability through Wednesday remains fairly low (under 1000 J/kg), which should help to negate severe weather concerns. However, will be watching how the pattern evolves Wednesday, as some better instability will be in place to our south and west in the warm sector of the low. It seems unlikely this will get too far up into Iowa, but a few stronger storms could certainly scrape southern Iowa if this pushes farther north Wednesday evening. At this time, the storm prediction center just has general thunder over the area, which is valid given the environment in our current guidance.

LONG TERM /THURSDAY THROUGH MONDAY/

Issued at 336 AM CDT Tue Sep 29 2026

Moving into Thursday and 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 /12Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 622 AM CDT Tue Sep 29 2026

VFR conditions prevail at all sites this morning, as high level clouds begin to move across the area. Showers and isolated thunderstorms will begin to track from west to east today, potentially impacting all sites. A lull in activity is expected through the evening hours, before more widespread rain spans at least the southern half of Iowa tonight, impacting KDSM and KOTM. Northern sites (KFOD, KMCW, KALO) may also be impacted by overnight showers/storm activity, but confidence is lower in northern extent. Thunder will be possible with any activity today and tonight, but should be sporadic, so have left out of TAFs at this time. Finally, in addition to rain, IFR to potentially LIFR ceilings develop tomorrow morning.

DMX WATCHES/WARNINGS/ADVISORIES

Flood Watch from 7 PM CDT this evening through late Thursday night for IAZ004>007-015>017-023>028-033>039-044>050-057>062- 070>075-081>086-092>097.