textproduct: Des Moines

KEY MESSAGES

- Ongoing showers and thunderstorms will prime the area through the remainder of the afternoon into the late 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.

SHORT TERM /THROUGH WEDNESDAY NIGHT/

Issued at 306 PM CDT Tue Sep 29 2026

The latest surface analysis shows high pressure centered over the eastern Ohio River Valley, with its influence extending westward toward Iowa. However, a weak surface low centered over northern Minnesota and its associated cold front are moving through the area, bringing showers and isolated rumbles of thunder across central Iowa. Due to this ongoing shower activity, temperatures currently range from the mid-60s where rain is falling to the low-to-mid 70s ahead of the showers under clearer skies. Our primary future weather makers are currently centered over the Desert Southwest and the Gulf of California.

Through the remainder of the afternoon and into the overnight hours, this initial round of showers and isolated thunderstorms will push through the area. The cold front will stall as upper-level dynamic support pulls away, allowing the boundary to become stationary and serve as the focal point for repeated rounds of heavy rainfall overnight. Forecast soundings for overnight precipitation depict deep-layer saturation up to 200 mb alongside a tall, skinny CAPE profile yielding 500 to 1,000 J/kg of CAPE. Precipitable water values exceed 2.00 inches, which sits at climatological maximums in model ensembles as well as SPC sounding climatology. These combined factors support a WPC Day 1 Slight ERO and the initial issuance of a CWA-wide Flood Watch.

Beginning late tonight, an upper-level trough will absorb the tropical moisture and dynamics associated with the remnants of Hurricane Polo (currently driving weather in the Desert Southwest and Gulf of California) and eject the system northeastward toward the local area. As this energy moves northeast, it will interact with the remnant stationary boundary, sparking a redevelopment of widespread showers and isolated thunderstorms through Wednesday. Similar to today, forecast parameters are aligning to support another period of heavy rainfall. Because tropical moisture from the remnants of Hurricane Polo is being drawn directly into the region, a joint WFO/WPC Day 2 Moderate ERO was introduced for the expected flood threat Wednesday into Thursday. The flooding threats are further highlighted in the hydrologic discussion below. Should this system overperform, September 2026 could become the wettest September on record and climb higher on the list of all-time wettest months. The all-time wettest September record stands at 14.81 inches, while DSM currently sits at 13.05 inches with heavy rainfall expected to persist through midnight Thursday.

A cold front moving through the area will stall overnight and allow for multiple rounds of heavy rainfall. Multiple rounds of heavy rainfall are expected again Wednesday into Thursday as an upper- level trough absorbs Hurricane Polo and moves northeast toward the area. Given saturated atmospheric profiles and precipitable water values near climatological maximums, WPC EROs (Slight Day 1, Moderate Day 2) and a CWA-wide Flood Watch are in effect. Additional heavy rain could push September 2026 toward the all-time monthly precipitation record for Des Moines.

LONG TERM /THURSDAY THROUGH TUESDAY/

Issued at 306 PM CDT Tue Sep 29 2026

The extended forecast begins with decreasing coverage of showers and thunderstorms as a cold front pushes the slow-moving surface low out of the region, bringing an end to the prolonged period of heavy rainfall that has affected the area since Tuesday. Behind the front, cold air advection will support cooler, much more autumn-like temperatures starting Friday and persisting through much of the extended period. In addition to the cooler temperatures, surface high pressure will build across the region, keeping conditions predominantly dry. However, long-term deterministic models indicate a minor shortwave passing through the area Saturday night into Sunday, though measurable precipitation associated with this feature will likely be confined to northern portions of the CWA. Otherwise, surface high pressure rapidly reestablishes itself, ensuring a continued period of seasonable autumn weather across the region.

A passing cold front will clear the slow-moving surface low out of the region, putting an end to the persistent heavy rainfall seen since Tuesday. High pressure and post-frontal cold air advection will bring cooler, dry, autumn-like conditions from Friday through the extended forecast period, with only a minor shortwave bringing a brief chance of precipitation to northern portions of the area over the weekend.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 1225 PM CDT Tue Sep 29 2026

Showers are moving across much of central Iowa with this TAF issuance. Showers will continue to work across the area through the afternoon and into overnight; however, northern terminals (KFOD and KMCW) will see a break in showers through the overnight but KALO, KDSM and KOTM will remain under persistent shower activity through the rest of the TAF window. While terminals are presently VFR, showers and low CIGS/VSBY will bring category reduction this afternoon and continuing through the remainder of the TAF period. Categorical low-end MVFR to IFR can be expected. Lastly, winds are forecast to mainly southerly with a swing to the southwest Wednesday morning.

HYDROLOGY

Issued at 306 PM CDT Tue Sep 29 2026

Potentially significant hydrologic event will affect our CWA over the next several days, with flash flooding, areal flooding and river flooding possible. Shifts in locations and amounts of heaviest rainfall may significantly affect stream responses especially creeks and smaller streams.

Greatest threat of heaviest rainfall is presently across the southeast two-thirds of CWA per latest meteorological model trends, however possible later wobbles to the north may bring our northwest counties into play again, thus we have continued the flood watch in those areas. Elsewhere, the river basins of most concern presently include the middle/lower Raccoon basin, Des Moines River basin below Saylorville Reservoir including tributaries like the North, Middle and South Rivers, and the South Skunk River basin especially downstream of the Ames area. Of those three, the Raccoon is of most concern due to recent moderate to heavy rain across portions of that basin, leading to increased soil moisture, which will allow more runoff to occur more quickly.

Latest Hydrologic Ensemble Forecast System (HEFS) and QPF ensemble hydrographs, which include longer QPF time durations than our deterministic river forecasts, suggest the most likely scenario in many locations will significant within-bank rises to around bankfull, with some isolated minor river flooding. One outlier is the lower Raccoon basin where some moderate flooding is possible. If rainfall amounts overachieve then more widespread minor to moderate river flooding would be possible. Although unlikely, higher end rainfall amounts would lead to more widespread moderate to major river flooding. Refer to our HEFS or QPF ensemble hydrograph forecasts for more details.

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.