textproduct: Des Moines

KEY MESSAGES

- Dry and pleasant during the daytime today!

- Thunderstorms return overnight into Tuesday. A few strong to severe storms are possible along with locally heavy rainfall.

SHORT TERM /THROUGH WEDNESDAY/

Issued at 308 PM CDT Mon Aug 17 2026

High pressure continues to move across the area today keeping conditions pleasant and dry with temperatures in the upper 70s to low 80s as of 2 PM. Dew points have also been lower than recent days, mainly in the upper 50s to 60s and leading to more reasonable humidity levels.

Diurnal cumulus which has developed this afternoon will dissipate after sunset but cloud cover will gradually build in to follow as several rounds of shortwave energy swing through at least portions of the area overnight into Tuesday, the strongest of which appears to be early Tuesday morning. Another wave then looks to move across later Tuesday afternoon-evening.

Theta-e advection really ramps up again tonight with moisture gradually returning into Tuesday morning behind the departing high pressure. Thunderstorms are expected to develop upstream in SD/MN along a cold front that will move southeast with time into Tuesday morning, but doesn't really reach northern Iowa until closer to midday Tuesday. Although some CAMs beg to differ, these storms are largely expected to dissipate or at least weaken as they near northern Iowa and will also encounter drier air. As noted in the previous discussion, an isolated strong wind gust can't completely be ruled out but with the LLJ pointing to the south of northern Iowa, the weakening and dissipating trend for showers/storms looks to win out.

Another area of storms to watch that may be of greater impact are the storms that develop near the pre-fontal trough in Nebraska. CAMs suggest several rounds of storm clusters could be possible but what remains uncertain is the longevity of storms as they enter western Iowa. Although forcing overall moves east with time, moisture return is better into Nebraska than into Iowa by around 09Z and doesn't really become more favorable until closer to 12Z. The LLJ strength is also in question, not nearly as strong as in some cases last week and also largely pointed into Nebraska then weakening as it turns into Iowa through the morning hours. The latest runs of the HRRR may be starting to capture this weakening trend. This morning's solutions of the HRRR brought a wind threat into Iowa during the overnight hours, but the 18Z largely dissipates all rounds overnight and has the main thunderstorm window across southwest to southern Iowa in the 6-9a timeframe Tuesday morning. The other lingering question is the largely scattered nature of the storms which could ultimately make upscale growth and thus sustenance difficult, as well as questions of how the warm mid- level temperatures over Nebraska may factor into storm strength/intensity prior to reaching the IA/NE border.

What happens overnight into Tuesday morning will largely dictate what is able to develop Tuesday afternoon and evening. The warm front looks to remain south of Iowa so it would depend on any lingering outflow boundaries from early day convection and what kind of atmosphere recovery we are able to see. As noted in the previous discussion, if we do get some clearing/recovery there may be some funnel cloud potential this afternoon, as well as the potential for a few stronger storms. However, this redevelopment and associated risks are starting to look farther south into more of MO/KS/NE, as has been reflected by the southward shift of the Day 2 SPC slight risk (level 2 of 5) now only impacting the southwest portions of our forecast area.

The other thing to note will be in the increasing moisture into the area. Precipitable water (PWAT) values start to return to 1.5 to even 2" in southwest Iowa, which could lead to some heavy rainfall potential, and rain rates of up to 1-2" per hour. The QPF signal in Iowa is highest in southwest Iowa, largely out of our area with most seeing less than 1" and highly isolated locations seeing 1-2", localized higher amounts. Refer to the hydrology discussion below for more information on flood potential.

LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/

Issued at 308 PM CDT Mon Aug 17 2026

Additional precipitation chances are in the forecast on Wednesday as shortwave energy continues to move through the otherwise northwesterly upper flow. Drier conditions are then forecast on Thursday as another high pressure moves across before returning again Thursday night into Friday with another push of energy. This seems to be a common pattern into the weekend with about every other day dry- precipitation chances but with overall lower confidence in the placement and timing at this range. Temperatures remain pleasant this week in the the 70s to 80s.

AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 1054 PM CDT Mon Aug 17 2026

VFR conditions continue late tonight/early this morning. Thunderstorm activity is still expected to move into Iowa early Tuesday morning, albeit diminishing as it arrives. Confidence is decreasing in thunder chances at more eastern sites (KMCW, KALO, KOTM) but still possible at more western sites (KDSM and KFOD). Scattered shower and storm chances linger at times through the day Tuesday, but low confidence in persistent impacts at any given site so have kept as PROB30s for the time being.

HYDROLOGY

Issued at 308 PM CDT Mon Aug 17 2026

Additional flooding concerns exist regarding the potentially heavy rain tonight into Tuesday. At this time, concerns involve flash flooding more than river flooding.

Anomalously high rainfall over the past week across much of the CWA led to increased soil moisture in many areas, with significant increases in some locations. That said, the past 1-2 days of dry or relatively low rain amounts across much of the CWA allowed soil infiltration capacity to recover modestly. Highest 0-10 cm (4 in) soil saturation values mainly extend from along the I-80 corridor south to US Hwy 34, with a secondary maximum extending from the Des Moines to Fort Dodge areas. All other things being equal, these two regions would be most susceptible to flash flood impacts from the incoming rain. Based on current QPF amounts and placement, the most likely scenario is ponding of water especially in urban areas, with some lower end flash flooding possible especially in the aforementioned areas with elevated soil saturation.

Based on current QPF amounts and placement, the river flood risk is minimal. Isolated to scattered moderate to significant within-bank rises are the most likely scenario. Our QPF ensemble hydrographs and HEFS suggest higher-end rain amounts may result in some locations rising to between action stage and flood stage, especially between I- 80 and US Hwy 34.

DMX WATCHES/WARNINGS/ADVISORIES

None.