textproduct: Des Moines

KEY MESSAGES

- Showers and storms this morning. Isolated flash flooding risk would be in urban areas given present trends.

- Storms develop over eastern half of Iowa early this afternoon. Questions remain on if favorable conditions will overlap for any tornado potential. Otherwise, isolated storm wind gusts and locally heavy rainfall possible. Any flash flood risk will be focused on areas that received the highest rainfall from earlier today and urban areas.

- Increasing winds with gusts in excess of 35 to 40 mph spreading from northwest Iowa late this morning to eastern Iowa by sunset.

- Relatively cooler and quiet weather is then forecast Saturday through at least Monday.

SHORT TERM /THROUGH SATURDAY/

Issued at 220 AM CDT Fri Jul 31 2026

Early morning radar imagery shows several areas of showers and storms in and around the state. The first is associated with the warm air advection wing just east of I-35 at this early hour and these showers will continue to slowly push off to the east. An area of stronger storms fueled by the nocturnal low level jet is pushing southeastward through southwest Iowa and southeastern Nebraska moving into Missouri and Kansas. Finally, a shortwave trough is gradually deepening with surface cyclogenesis occurring over Siouxland and causing additional showers and storms in South Dakota, which may result in new storm development between it and the southeastward diving storms over the next several hours. The damaging wind threat looks very low and would be mainly concerned with the storms scraping our southwestern forecast area. Heavy rainfall potential still remains favorable given the parameter space with deep warm cloud depths and precipitable water (PWAT) values well over 2 inches. In fact, the 0z KOAX ROAB had a 2.4 inch PWAT value, which is not only a daily record, but looks to be their all- time highest on record! Looking at deterministic and ensemble data, there is no readily identifiable corridor for the heaviest rainfall in Iowa. The most favorable area when looking at the environmental data would be northern Iowa where the 850-300mb flow is relatively lighter at generally 15 to 20 knots compared to 20 to 30 knots over southern Iowa. Thus, storms in northern Iowa will be slower moving than those in southern Iowa. Rainfall amounts through midday today range between 1 and 3 inches with a few of the convective allowing models shooting between 3 and 4 inches. Looking at antecedent conditions, the soils are dry with the pre-rainfall soil moisture from NASA SPoRT showing 0-10cm and 0-40cm generally at or below the 30th percentile with parts of western Iowa at the second percentile. With 3 hour flash flood guidance (FFG) between 2.5 and 3.5 inches and 6 hour FFG between 3.5 and 4.5 inches and mature crops, flash flooding concerns will be isolated mainly to urban areas where there will be less infiltration compared to rural areas. Additional details are outlined in the hydrology section below.

By daybreak Friday, showers and storms will be mainly east of I-35 and pushing eastward as surface low pressure moves and slowly deepens as it enters the central part of the state. This should offer a break in the rain, though showers may linger or move back in over far northern Iowa late in the morning or afternoon. Attention by midday or early afternoon will be monitoring for possible stronger storm development to the east of the low primarily east of I-35 and probably south of I-80. There are many questions on what hazards may evolve with the first being tornado potential. Soundings at ALO and OTM point to various challenges to be able to call it a favorable tornado environment. At ALO, hodographs are curved and elongated with 0-1km storm relative helicity (SRH) over 200 m2/s2 from the RAP. However, there is no surface based instability/CAPE or 0-3km CAPE for tornadoes or funnels to be of much concern. Moving to OTM, 0-3km CAPE is generally above 75 J/kg with surface instability over 1000 J/kg. However, due to being farther from the surface low, low level winds are at best south-southwest before veering more to the southwest and west. Thus, the hodograph and SRH are not highly supportive of rotating storms, though funnel clouds may be possible given the 0-3km CAPE. Regardless of location, the one thing that is favorable everywhere will be the low lifting condensation level (LCL)/ceilings. Of course, these are forecast parameters so if more clearing can occur or the position of the low drifts more southward, this may change the risk. Thus, there may be a narrow spatial window where there is favorable CAPE and SRH for a tornado so mesoanalysis will be key to hone in on any area that may evolve. Otherwise, isolated damaging wind gusts may be possible as there is plenty of dry air working into the backside of these departing storms in the mid-levels. Finally, these storms will still be in the favorable heavy rainfall parameter space that an additional 1 to 2 inches is possible in narrow swaths. Any flash flood risk would seem to be relegated to areas that receive heavy rainfall earlier today or an urban area. The severe risk should end by early evening as the low pressure is east of the area. However, showers and isolated storms will linger over eastern Iowa as moisture is wrapped behind the low through much of Friday night.

As the surface low meanders east-southeastward across Iowa on Friday, surface winds will be increasing with a tight pressure gradient coupled with modest pressure height rises on the backside of the low. Forecast soundings show low level winds of 35 to 45 knots that will be in the shallow mixing layer. Thus, forecast sustained winds from the north will be at 20 to 30 mph with gusts in excess of 35 to 40 mph. A site or two may even reach wind advisory criteria gusts of 45 mph per forecast soundings, but given lack of consistent signal and location have held off on an advisory and will monitor trends and upstream observations. The timing of the increase in winds will be around midday from Emmet to Crawford counties, the I-35 counties around mid- afternoon, and the Highway 63 corridor toward sunset. Winds will stay breezy overnight tonight into much of Saturday, though will not be as strong as they will be in the aforementioned times. These winds along with a good amount of lingering cloud cover should help stave off fog development early Saturday. Any lingering showers will end by midday Saturday in our southeastern forecast area with the clouds also gradually decreasing as well. With the northerly flow, will see a return of high level wildfire smoke over the area with the clouds clearing out.

LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/

Issued at 252 PM CDT Thu Jul 30 2026

Drier conditions return on Saturday as an upper ridge starts to build back into the area. Temperatures remain cooler for the season and in the 70s Saturday to 80s into early next week making for another pleasant stretch. The next chances for precipitation start to move in midweek as upper troughing returns with several rounds possible as waves move through the overarching upper level pattern. Details will continue to be fine-tuned as we get closer.

AVIATION /12Z TAFS THROUGH 12Z SATURDAY/

Issued at 642 AM CDT Fri Jul 31 2026

MVFR and IFR restrictions will be widespread at most terminals through much of the period. One area of showers is departing and there should be a several hour window at all sites with dry conditions. Then, additional showers and storms are expected to become scattered early to mid this afternoon. The highest chance for thunderstorms looks to be east of I-35 so have confined any TSRA mention to ALO and OTM, but will refine at 18z. Otherwise, PROB30 or prevailing SHRA are shown in the official forecasts. Winds will increase from a northerly direction to between 15 and 25 knots with gusts to around 30 knots from late this morning and afternoon and remain breezy through the end of the period. Ceilings may improve near the end of the period at the northwestern terminals.

HYDROLOGY

Issued at 1144 AM CDT Fri Jul 31 2026

Rainfall for this event has generally underachieved vs. earlier forecasts thus far, thus hydrologic responses on area rivers and streams has been less than anticipated. The Cedar basin from southeast MN into northeast IA received the heaviest rainfall, and the Cedar River itself is expected to crest around action stage early next week in the Waterloo-Cedar Falls area. The upper-most portions of the Des Moines basin in southwest MN received some locally heavy rainfall which may lead to minor rises downstream into northwest IA next week, but is expected to remain below action stage.

Additional moderate to locally heavy rain is expected this afternoon and evening, mainly across the eastern third of IA. Greatest flood risk is ponding or possible low-end flash flooding mainly in towns and cities, mainly east of US 63, which would be primarily within the Cedar-Iowa basin. Going Cedar River forecasts for the Waterloo- Cedar Falls area incorporate the most likely QPF scenario through 12Z Saturday.

DMX WATCHES/WARNINGS/ADVISORIES

None.