textproduct: Des Moines

KEY MESSAGES

- Dense fog to continue through the early-to mid-morning hours.

- A break in the rain is expected this morning through evening before storms move back into the area tonight0 into Friday.

- Elevated streamflows are more common than not south of Highway 30 to the Missouri border and will be monitoring for additional rises through the weekend.

- Active pattern continues into this weekend with scattered showers and intermittent storms possible, more so Saturday than Sunday.

SHORT TERM /THROUGH FRIDAY NIGHT/

Issued at 355 AM CDT Thu Sep 17 2026

Dense fog has developed across much of the state leading to visibilities at or under a half mile, save for the far northern portions of the state. Additionally, showers and storms have developed which will lead to locally heavy rainfall through the morning until they taper off later this morning.

SPC mesoanalysis shows an area of deep moisture convergence covering most of the I-80 corridor which falls in line with where showers and storms have developed. This activity is being kicked off by the nocturnal Low Level Jet (LLJ) which has strengthened and is positioned over southern to central Iowa. As the morning progresses, showers and storms will continue to fire in the zone of moisture convergence and move with the LLJ, resulting in the potential for training storms. The good news here is that this activity should only last until 15z or so, before we lose the LLJ. Precipitable water values are right around 2 inches and warm cloud depths are in the neighborhood of 14kft. While the temporal window is rather short for heavy rainfall this round, the potential for training storms over areas that are already saturated could lead to localized flash flooding concerns through the morning, especially if shower and storm coverage fills in west of I-35. HREF probabilities hint at the axis of heaviest rain being generally east of I-35 through the morning which would be a best-case scenario, with probabilities of exceedance sitting around 45% for 3" or more of rainfall in the easternmost I-80 counties. While the probabilities are slightly higher than previous runs, the axis of heaviest rain is still consistent.

After showers and storms taper off, there is relief from the rainfall through much of today. With a moist airmass in place, clouds will blanket most of the state with the potential for minimal clearing in far southern Iowa as the ridge over the Lower Mississippi Valley begins to translate to the west and the northern fringe grazes the area. Temperatures will be higher than the past few days, pushing into the 70s to 80s with more southwesterly to southerly flow.

Showers and storms are expected to return to the area late tonight into Friday morning. This will become the next focus for heavy rainfall, with the heaviest axis focused across areas north of Highway 20. HREF probability of exceedance targets an area with around a 50% chance of getting more than 3" of rainfall with a the same general area of our northern counties having a 10% chance of 5" or more of rainfall. Deterministic guidance hints at a somewhat widespread 1-2" of rainfall across northern Iowa with CAMs hinting at stripes in the 3-5" range. As far as the setup for heavy rain goes, PWAT values are at or just above 2" with a robust push of Gulf moisture in both the low and mid levels. Another thing to note for Friday is the potential for severe weather, as a surface low will develop over South Dakota and push east in the afternoon. There's a push of around 2000 J/kg of CAPE with around 30kts of shear across the area. Assuming the air across northern Iowa becomes fairly worked over due to the showers and storms that persist throughout the day, severe convection is far less likely. Across portions of southern and central Iowa, however, conditions should be a bit more favorable given the shear and instability. There are some questions about a potential cap in the low levels which would inhibit coverage of severe storms. At the moment, it looks like severe wind gusts are the most likely threat.

After the severe window passes, Friday night continues the trend of heavy rain, again focused across the north as a strong LLJ points itself into northern Iowa with another strong push of mid-level moisture. NAEFS PWAT values are either hitting the 99th percentile value or maxing out on its climatology across the north, with PWAT values still in the neighborhood of 2".

LONG TERM /SATURDAY THROUGH WEDNESDAY/

Issued at 240 PM CDT Wed Sep 16 2026

The surface low mentioned above along with a slow moving front looks to slide southeastward into Iowa over the weekend as an upper level shortwave pushes over the central and northern Rockies onto the Plains this weekend. These features will bring the next round of showers and storms with the favorable heavy rainfall environment still over the region more so on Saturday than Sunday. While there could end up being a few stronger storms, more marginal lapse rates diminish this concern at this time. As the upper level shortwave moves more over the state or immediate vicinity, this will keep shower potential in place into early next week. The intensity, coverage, and duration should be less with time with - I hesitate to type - drier conditions towards Wednesday as the influence of surface high pressure moves into the area. This may not be long-lived, but a break would be welcome for those Iowans that have seen enough rain.

AVIATION /18Z TAFS THROUGH 18Z FRIDAY/

Issued at 1240 PM CDT Thu Sep 17 2026

There is high confidence in IFR/MVFR restrictions prevailing through the period at all terminals. An arc of showers and storms is forecsat to lift across the state after midnight tonight through the end of the period. Have continued PROB30s groups timing the window of highest thunderstorm potential. Further, in the moisture rich environment, fog may develop and linger through much of Friday morning as winds from the south or southeast increase.

HYDROLOGY

Issued at 240 PM CDT Tue Sep 15 2026

Continued rounds of overnight showers and storms will keep some flash flooding and river flooding concerns in the forecast for the next several days. Training thunderstorms may develop during the overnight tonight with isolated pockets of 2-4 inches possible across central to eastern Iowa tonight. Soil moisture is already in the 90th percentile across central portions of the state and a poorly placed band of thunderstorms could be enough to create renewed flash flooding concerns during the overnight, though the threat does not appear to be widespread.

The Raccoon River crested this morning and is currently coming down out of minor flood stage. Downstream at the intersection with the Des Moines River, the stage appears to have turned over already at action stage. Given the multi-day rain chances, concerns for additional crests for these sites and others in central and eastern Iowa may continue for several days. In the HEFS guidance, the 50% probability (most likely solution) shows many sites with perhaps a 2nd rise with overnight precip, but generally not exceeding the stage it was at today. However, the 30% probability shows rises to action or minor across many central Iowa basins (North, Middle, Raccoon, etc.). So, again, a poorly placed band of thunderstorms tonight could bring some locations to minor flood stage or higher, but the most likely outcome is continued recession.

DMX WATCHES/WARNINGS/ADVISORIES

None.