textproduct: Des Moines

KEY MESSAGES

- Areas of fog developing tonight.

- Storms return to the state after midnight tonight through midday Friday. A few wind gusts to 45 mph are possible.

- Chance for a few strong to severe storms late in the afternoon and evening, but this is conditional on enough surface heating to overcome the low level cap. Locally heavy rainfall possible north central into northeastern Iowa Friday night/early Saturday.

- Shower and thunderstorm activity becomes less intense, less widespread over the Sunday to early Tuesday period.

SHORT TERM /THROUGH SATURDAY NIGHT/

Issued at 150 PM CDT Thu Sep 17 2026

Showers and storms from last night pushed off to the east rather quickly this morning as the low level jet veered into Illinois. There has been little in the way of structured forcing today with the center of the mid-level high pressure over the middle Mississippi River Valley. Unfortunately, cross sections show a narrow band of low level moisture, which has kept sunshine limited, though there have been some cracks in the stratus this afternoon. Farther to the west, GOES-East upper level water vapor imagery shows an area of moisture lifting north-northeastward from Arizona and New Mexico into the central and northern Rockies. This moisture is associated with shortwave energy, which will rotate around the mid- level high into the Dakotas tonight. Strong theta-e advection will occur as the low level jet ramps up this evening, initially focused over Nebraska, but pushing towards Iowa after midnight. This will bring a warm air advection wing of showers and storms pushing eastward from western Iowa after midnight clearing much of central Iowa by mid to late morning Friday. There may be some vigor to this activity with updraft helicity streaks with wind gusts up to 40 mph possible. Further, these showers and storms will once again be in a favorable environment for efficient rainfall production with deep warm cloud depths and unusually high precipitable water values for this time of year. The mitigating factors to the flash flooding threat will be the progressive nature of this activity along with capacity in the soil moisture over northern Iowa, which National Water Center soil moisture analysis has near 60% with the NASA SPoRT soil percentiles around the 10th to 30th percentile in the upper 4 and 16 inches of soil. In addition to the storm potential overnight, fog may once again develop in the moisture laden airmass.

As the morning activity departs, attention will be on low pressure developing over western Kansas with a trough axis/extension of the surface low reflection near Sioux City by evening. This will result in breezy winds from the south or southeast across all of the state while south of the boundary a warmer day with highs possibly reaching into the middle 80s. However, there looks to be residual moisture and cloud cover that may limit this and also plays a role into the main severe weather inhibiting factor - low level capping. The low level moisture will be more common across the state in the morning hours, but mixing in the boundary layer south of the warm front should allow for some sunshine over southern into perhaps central Iowa. Northern Iowa looks to remain cloudy unless the front lifts beyond that point. For now, a majority of forecast soundings point to the cap holding during the daytime while mixed layer CAPE values grow up to around 2500 J/kg. If more clearing occurs and allows for greater surface heating and mixing out of the inhibition in the warm sector, then the environment would be supportive of surface based storms with curved low level hodographs for rotating storms/low tornado potential. However, this scenario is quite conditional, but remains a possibility at this time. As we move into Friday evening, storms may try to initiate along the boundary, especially as the low level jet points into Iowa and increases to around 40 knots. By this time, lapse rates will be rather meager, which should limit the large hail concern. Gusty winds seem to be the most likely severe hazard with what should be elevated storms. Locally heavy rainfall will be possible Friday night as well, especially later into the night with favorable parameter space for efficient rainfall. Rainfall amounts over parts of north central and northeastern Iowa will average 1 to 3 inches overnight Friday, though locally higher amounts upwards of 4 to 6 inches will also be possible.

As we move into Saturday, the ridge will be in the process of being pushed away from the region as an upper level trough moves toward Iowa. This will help to push the surface low and boundary southeastward across Iowa through the day into the night, albeit slowly. Coverage of showers and storms looks highest late in the day into the night, but at this time chances remain through the day. The environment will remain primed for locally heavy rainfall and there could also be strong storms later in the afternoon or evening. However, inhibition remains stout and points to elevated storms. Further, deep layer shear is marginal and lapse rates remain weak, but a few strong wind gusts could be possible over southern and eastern portions of the state ahead of the boundary.

LONG TERM /SUNDAY THROUGH THURSDAY/

Issued at 150 PM CDT Thu Sep 17 2026

The upper level trough will move into the region late Sunday into Monday departing on Tuesday. This will mean that Sunday and Monday will have more clouds than sunshine as well as showery conditions with occasional thunderstorms. Rainfall totals should be on the lighter side with 20% chance or less of 1" in 24 hours. While soils will have reduced capacity, the anticipation is that lower rates and amounts will lessen the risk of any short term, flash flooding concerns as well as much impact of on area waterways. With the upper level trough moving away, drier and brighter conditions may return as soon as later Tuesday into Wednesday. However, the pattern looks to remain active with the next trough approaching by late Wednesday or Thursday with some level of rain and storm chances.

AVIATION /00Z TAFS THROUGH 00Z SATURDAY/

Issued at 652 PM CDT Thu Sep 17 2026

Near widespread MVFR to low VFR conditions are ongoing across the state early this evening with showers, and a few storms starting to reenter the area in the west. Best chances for impacts are in northern Iowa (KFOD/KMCW/KALO) and later tonight/overnight but a few showers may be able to reach KDSM/KOTM before then. Confidence is lower at those two sites so kept with prob30 groups. In all cases, timing adjustments may still be needed. Conditions degrade back towards low MVFR to IFR through the overnight with fog once again possible to even likely. Winds will be a bit stronger which could be the helping factor with the fog, but conditions otherwise remain favorable with all the lingering moisture. Improvements are expected by later morning into midday Friday, though shower/storm chances remain in the north and then renew later in the afternoon to evening. Through Friday daytime, winds out of the southeast will increase and become breezy with gusts to 15 to 25 knots.

HYDROLOGY

Issued at 150 PM CDT Thu Sep 17 2026

Focus of concern regarding potential flooding over the next several days will shift north, mainly north of US Hwy 30. Moderate to heavy rainfall totals are expected with the highest probability near the IA-MN border and east of I-35. Hydrologic Ensemble Forecast System, National Water Model and QPF ensemble hydrographs suggest the most likely scenario will include moderate to significant within-bank rises, along with up to a few river forecast points seeing minor flooding. If actual amounts overachieve vs. QPF then more pervasive minor flooding would be expected along with some moderate flooding. In the extreme, more pervasive moderate to major river flooding would occur, however its probability is quite low (around 10%). One reason for the relatively modest expected responses is more storage capacity in the ground across that part of the state.

Flash flooding will also be a concern, however the best chances will be in urban areas, where ponding to low-end flash flooding is the most likely scenario.

DMX WATCHES/WARNINGS/ADVISORIES

None.