textproduct: Des Moines

KEY MESSAGES

- Severe storms develop between 6 and 7pm this evening. Large hail is the main concern the first few hours, then storms transition to more of a damaging wind gusts threat. Storm threat ends for our forecast area by around 2am Saturday.

- Additional severe storms and locally heavy rainfall chances exist at times from Saturday afternoon through Monday. With any weekend outdoor plans, monitor the forecast for the latest forecast updates and changes.

- Cooler and drier Tuesday through at least Wednesday.

SHORT TERM /THROUGH SUNDAY NIGHT/

Issued at 301 PM CDT Fri May 15 2026

It is a warm, mid-May day as temperatures have soared into the 80s under a sunny sky with a wind from the south or southeast. GOES-East visible satellite imagery shows stable clouds developing from around Sioux Falls into our northwestern forecast area at present, which are tied to the surface cold front that is sinking towards the state. Additionally, there is a patch of clouds over southeastern Nebraska, which have shown attempts at sustained convective initiation. However, these have only sporadically produced lightning with the forcing mechanism for these clouds likely tied to weak, low level warm air advection. These showers should continue to struggle and remain elevated per the 18z KOAX RAOB, which shows capping around 150 J/kg. As the cold front continues southeastward, forecast soundings show the capping weakening and it looks like isolated to scattered storms will develop between 6 and 7pm in our forecast area. There may also be a very narrow window for surface based storms, but that varies depending on model sounding. Storms that develop early will pose a risk of large and potentially significant hail given the forecast hodographs. A tornado or two cannot be ruled out initially, but high storm bases (LCLs around 2km) and weak 0-3km CAPE and storm relative helicity (SRH) point to this being a very low, but not zero, concern. The hazard threat will transition quickly from hail to damaging wind gusts concerns with gusts up to around 75 mph possible as storms move quickly to the east. Mesovortex generation may be possible, but the 0-3km bulk shear magnitude is just near the 30 knot threshold if storms can orient to the east-northeast. With fast storm motions and dry soils per NASA SPoRT soil moisture percentiles, the swaths of 1 to 2 inches of QPF shown in models will keep flash flooding risk isolated to a low end threat to an urban area if things line up just wrong. As these storms move to the east, there is some indication that the storms become out of balance with the outflow and wind pushing away from the storms, especially into eastern Iowa. However, damaging wind gust risk looks to persist through all of our forecast area.

All of the Friday night storm activity should be clear of our forecast area by sunrise Saturday with clearing skies through the morning as the surface boundary settles over far southern Iowa. Forecast soundings show a capping inversion that will inhibit storm development until at least mid if not late afternoon. As the cap holds, the environment south of the boundary will have dewpoints into the low to middle 60s, mixed layer instability upwards of 2000 J/kg, favorable deep layer shear of 30 knots for storm organization, and modest low and steep mid-level lapse rates. The question becomes whether there will be a forcing mechanism to initiate the storms with weak to modest low level theta-e advection and no strong surface convergence despite the surface front over southern Iowa. Yesterday's and today's runs of CAMs have shown this struggle of initiating storms. The less likely scenario at this time is that if storms do develop in the afternoon or early evening, they could be surface based as shown in the latest soundings with the highest risks being large hail and damaging wind gusts. As mentioned yesterday, the lack of stronger surface flow is limiting storm relative helicity values (SRH). Thus, the risk for tornadoes is conditional. Storms that do form in this lesser scenario may have slower storm speeds and that may lead to narrow paths of locally heavy rainfall that would pose only an isolated flash flood risk to an urban area. However, the more probable scenario is that little in the way of storms develop Saturday afternoon with activity developing on the nose of the lifting into Iowa low level jet Saturday evening. These scattered storms would have a hail and gusty winds risk. In addition, a mesoscale convective system (MCS) is forecast to develop over the High Plains and move into some portion of Iowa late Saturday evening into Saturday night/early Sunday morning. The main concern would be gusty winds as this arrives and locally heavy rainfall, though the intensity may be waning by the time it reaches the state.

Any lingering MCS activity will be pushing out of Iowa by mid- morning Sunday. Beyond this activity, much of Sunday during the daytime even into perhaps some part of the evening is looking to have limited storm chances as forecast soundings show stout capping remaining in place. However, am cautious to say no storm chances due to uncertainty in whatever outflow or differential heating boundaries may exist post-MCS with an atmosphere that may otherwise have plenty of instability. Sunday will otherwise be windy as south to southeast winds prevail at 20 to 30 mph with gusts up to 40 mph. Storm that develop west of Iowa on Sunday look to move into the state later Sunday night.

LONG TERM /MONDAY THROUGH THURSDAY/

Issued at 301 PM CDT Fri May 15 2026

Monday will dawn with storms either lingering over some portion of eastern Iowa as a result of storms that push across Iowa Sunday night. As these depart, the stronger synoptic scale forcing will be moving closer to central Iowa through the day along with a surface cold front. The forcing should be more than sufficient for storm development and severe storms are likely given the strong instability and shear parameter space. While four days away, the most likely timing will be from mid-afternoon into the evening hours before the front likely pushes the storms out of our forecast area by Tuesday morning. In addition to the severe storm concerns, there is a slight risk of excessive rainfall on Monday. Given successive rounds of rainfall, will have to see if certain areas receive repeat rainfall and become more susceptible to flash flooding. Of course, urban areas will be in play in any event if rainfall of 2 to 3 inches occurs in just an hour or two. This risk was highlighted in the 6z run of the experimental WPC Urban Rain Rate Dashboard for Des Moines with a 5% chance of reaching the 5 year return interval. Will need to keep an eye on rivers; however, many are near baseflows so there is capacity along with soils expected to take the first round to perhaps two rounds of rainfall. This is shown in the 5 day National Water Model (NWM) forced by the GFS that generally shows high annual exceedance probabilities (AEP), though tributaries in both the Thompson and Chariton basins shows 10% AEPs/higher flows.

With the cold front clearing the area by early Tuesday, cooler and drier conditions are then forecasted through Wednesday and perhaps Thursday as surface high pressure moves across the region.

AVIATION /18Z TAFS THROUGH 18Z SATURDAY/

Issued at 118 PM CDT Fri May 15 2026

VFR conditions will prevail this afternoon before isolated to scattered storms toward 0z and then move to the east-southeast. Generally have shown prevail -TSRA with impacts shown in PROB30s as timing is refined in impacts to a given terminal. For DSM, did add a PROB30 due to more uncertainty of whether storms will develop closer to this terminal or move in from farther away. While hard to specify at any terminal, storm wind gusts over 50 knots will be possible. These will be clear of all terminals by 9z with VFR conditions prevailing until the end of the period with light winds.

DMX WATCHES/WARNINGS/ADVISORIES

None.