textproduct: Des Moines

KEY MESSAGES

- Showers and storms are ongoing, mainly in northern Iowa this morning with the expectation that they taper off closer to sunrise.

- Strong to severe storms are possible this afternoon provided the surface heats enough to overcome the cap. Heavy rainfall is possible across north central and northeastern Iowa.

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

SHORT TERM /THROUGH SATURDAY NIGHT/

Issued at 357 AM CDT Fri Sep 18 2026

Showers and storms have begun to develop across northwestern and northern Iowa as the Low Level Jet (LLJ) has nosed into the state. This will be the focus for the next several hours for not just the heavy rain potential, but the severe weather threat for later today. The LLJ will diminish across Iowa through the morning which should lead to a lull in showers and storms across the area around mid- to late-morning. Later today, a 500mb shortwave and surface low will make its way across Siouxland. The surface low will have an associated cool front which, will serve as a forcing mechanism for showers and storms later on. Of course, as the case has been over the past several days, moisture is nothing short of abundant. Both low- and mid-level moisture transport are very strong, and anomalously high PWAT values of 2 to 2.25 inches can be found across much of Iowa. There are plenty of signals for not just heavy rain (more to come on that), but also a window for severe weather.

For the severe weather threat, the abundant moisture leads to plenty of MLCAPE, in the neighborhood of 2500 J/kg. Shear is okay, around 25-30kts, and the aforementioned forcing point to a setup for severe weather to at least some degree. There are a few lingering uncertainties, however. The first has to do with cloud cover after the ongoing showers and storms clear. This will mainly inhibit surface-based storms from firing in the north, as model soundings depict a stout capping inversion in the low levels. Farther south, capping doesn't look as strong, but there are still some signals that a cap could be in place. If the cap is able to erode and storms become surface based, the potential exists for a tornado or two thanks to southeasterly surface winds veering to the southwest around 900mb. This leads to 0-1km storm relative helicity values that are fairly high, around 200-350 m2/s2. Otherwise, if the cap holds strong and convection is elevated, the wind field is fairly uniform in terms of direction and the severe potential would be limited to an isolated strong wind gust or two.

In addition to severe weather, after the wet pattern that has plagued the state for the past several days, it should be no surprise that this activity carries the threat for heavy rain. While the axis of the heaviest rainfall looks to be outside of our forecast area, heavy rainfall is still expected with any storms that develop. Going back to the LLJ, it will begin to pivot and point more into Iowa and southern Minnesota as the low pushes east. While the LLJ is not as strong as models suggested yesterday, it is parallel to the surface boundary which would allow for training storms. High pressure pancaking the Lower Mississippi Valley and the Tennessee Valley leads to abundant moisture being channeled into Iowa (hence the aforementioned high PWATs). The same high means that monsoonal moisture over Arizona and New Mexico is being channeled into the area in the mid-levels. The abundant moisture and potential for training storms means that the axis of heavy rain comes down to how far north the boundary makes it. Guidance suggests the boundary makes it north of Iowa, hence the highest rainfall totals being progged outside of the area. Furthermore, guidance has brought things farther to the north, but this will still need to be monitored through the day. Given the current limited spatial coverage for excessive rainfall amounts in the area and low confidence in excessive rainfall in any one spot across the Des Moines county warning area, we are holding off on a Flood Watch for now and will reassess through the day.

Rainfall is expected to continue through much of the night into Saturday morning before tapering off around sunrise, as the LLJ continues to pivot across the state. That being said, showers and storms will continue to linger in spots around the state before becoming more widespread in the afternoon and into the evening, this time across southern Iowa. Once again, this brings the potential for severe weather and heavy rainfall as little changes in the way of moisture transport, and the biggest change being the position of the LLJ relative to the state. PWAT values are still expected to remain around 2 inches with MLCAPE of around 2000J/kg, although shear looks to be a potential limiting factor (30kts on the high end). The potential for a strong gust of wind and potentially large hail is there, but uncertainty remains due to the potential for capping to remain over the region.

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 /18Z TAFS THROUGH 18Z SATURDAY/

Issued at 1236 PM CDT Fri Sep 18 2026

Widespread stratus and fog/mist is resulting in poor flight conditions as low as LIFR this afternoon. While these have been slow to improve, steadily improving visibilities and ceiling heights are still expected through this afternoon. Some breaks in the clouds have developed near KALO and KMCW but low clouds and patchy fog/mist generally fill back in and/or stick around through the evening and overnight, especially at northern sites.

In addition to the visibilities and ceilings, we also expect scattered showers and thunderstorms to move through the area tonight. Have timed TAFs out to the most likely periods, but confidence is low in exact timing and evolution, so amendments are likely.

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.