textproduct: Des Moines

KEY MESSAGES

- Line of storms moving from west to east through the area this evening. Damaging winds and embedded tornadoes are possible, especially in the northern half of the state. Locally heavy rainfall also possible.

- Additional, potentionally widespread severe weather expected again on Monday. All severe hazards are possible, including tornadoes, large hail, strong winds and heavy rainfall. - Multiple rounds of efficient rainfall tonight and Monday night may lead to additional rises on rivers, particularly in southern Iowa. Flash flooding also possible in urban areas.

SHORT TERM /THROUGH TUESDAY/

Issued at 342 PM CDT Sun May 17 2026

A surface boundary currently sits draped from west to east over Iowa early this afternoon. In areas north of this boundary, cloud cover has generally limited mixing and weaker winds aloft have prevented gusty conditions thus far today. Farther south, breezy winds have started to pick up and will continue to increase as the stronger winds aloft push northward into the area. This boundary is expected to lift northward through the afternoon and evening hours, bringing these gustier conditions farther north in the state. Wind gusts up to 40 mph are possible, although should generally be around 30 to 35 mph.

Later this evening, we are looking at yet another round of severe thunderstorm potential. Warm moist air will advect northward with the boundary through the afternoon, presenting a highly unstable air mass to an approaching cold front from the west. Fortunately, a warm layer aloft has generally kept a lid on the atmosphere today and prevented any convection along the front so far. As this boundary lifts northward, a few scattered showers and storms may develop along it, but the capping generally appears to keep this activity at bay. The main show will be with the frontal passage later this evening, producing a roughly north to south oriented line of storms moving from west to east through the area. Instability ahead of these storms will be around 2000 to 3000+ J/kg, with a fair amount of shear maximized along the warm front that will then be across northern Iowa. Given this storm mode, the primary concern for our forecast area will be damaging winds, as modeled reflectivity structures imply strong cold pools being produced along this line through the northern half of the state. HRRR and RRFS wind gusty output support this potential for strong winds 60 to 75+ mph winds, especially as storms initially arrive in northwestern Iowa this evening. In addition, 30 to 40 kts of 0-3 km shear and 150+ J/kg of 0-3 km CAPE ahead of the line of storms could support tornadoes embedded within the line of storms, especially with any segments bowing to the northeast. This will be most favorable as storms initially arrive in the area and for the part of the line closest to the warm front in the northern half of the state. Waning shear and instability will cause the strong cold pools to fall out of balance and become outflow dominant through the night, which should diminish the tornado risk as the line passes over our area. However, gusty outflow winds will still be possible into the early morning hours.

While storms won't be as severe farther south in the area, slower storm motions as the front begins to stall in Iowa may lead to prolonged periods of heavy rainfall through Monday morning. Should this rainfall linger over areas that already saw multiple inches of rainfall, particularly far southern Iowa, we could be looking at the potential for flash flooding and/or additional river rises. See hydro discussion below for more information.

Overnight activity should generally wrap up by mid-morning on Monday, but our days of perpetual thunderstorm chances will not yet be finished. The atmosphere will work to recover ahead of an approaching 500 mb wave which is slated to produce storms across the area today. The synoptic support on Monday will be far more robust, leading to a highly dynamic scenario. That said, leftover mesoscale features from tonight will still play a role in how storms play out Monday. Of course, atmospheric recovery will be important, but this seems fairly likely by the time storms are expected to start in the late afternoon/early evening. The other factor will be where the cold front ends up on Monday. Right now, this is depicted to be over western Iowa, but very well could move depending on the strength of the MCS tonight. Regardless, of these features, it still looks likely we will see strong storms on Monday. Assuming the atmosphere recovers, some models indicate 4000+ J/kg of surface based instability and 40 to 50 kts of deep layer shear developing ahead of the cold front over central Iowa Monday afternoon. As the front progresses eastward, storms will fire along it, taking advantage of the loaded environment in Iowa. 300+ m2/s2 of 0-3 km SRH will be available to storms with large looping hodographs. This environment will be conducive for all severe modes, including damaging winds, large hail and tornadoes. SPC has an enhanced risk (level 3 of 5) for severe weather over much of southeastern into central Iowa, with a 10% chance for tornadoes some of which could be strong (reasonable max up to EF-2). A moderate risk (level 4 of 5) is displaced just to our southwest where the best tornadic potential resides Monday afternoon. Finally, widespread storms across the area with plenty of moisture available from the low level gulf moisture stream will bring another round of heavy rainfall on Monday. This activity should be progressive enough to avoid long residence times over a particular area, but could produce efficient rainfall leading to more water being added to rivers in Iowa.

It is worth mentioning, however, that both HRRR and RRFS output from this morning have not been recovering the atmosphere as well on Monday, leading to more convective inhibition than is being indicated by the coarser models. This lack of surface based instability would reduce our risk for tornadoes across the area, should that scenario play out. Storms would still occur with plenty of MUCAPE around, but they would be more of the elevated flavor. This degree of destabilization will certainly be something worth watching as we get through tonights storms and especially into tomorrow morning.

LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/

Issued at 342 PM CDT Sun May 17 2026

A much needed break from the active weather stretch begins on Tuesday as a high pressure nudges into the region later Tuesday into Wednesday. Mid-level winds behind the departing system Monday night also turn out of the north to northwest ushering in cooler temperatures with highs in the 50s to 60s on Tuesday through Thursday and lows Wednesday and Thursday mornings in the upper 30s to 40s. Winds on Tuesday will remain elevated through much of the daytime with a tight pressure gradient still in place with sustained winds out of the northwest 15-25 mph, and gusts to near 30 mph in the northwest. Showers and thunderstorms then return to the forecast from later Thursday into Friday as an upper trough crosses the Central Plains with a developing surface low reaching towards the area into Friday. Additional details to come in the next several days.

AVIATION /18Z TAFS THROUGH 18Z MONDAY/

Issued at 1241 PM CDT Sun May 17 2026

MVFR conditions are impacting much of the area today, as clouds around 1.5kft to 3kft span much of Iowa. These will likely linger into the afternoon, although daytime heating should help lift the ceilings, especially south. Winds have stayed lighter over northern Iowa where more cloud cover has been present, but are quite breezy farther south near KDSM and KOTM. Winds will remain breezy (south) or increase (north) through the afternoon and into the evening. Scattered showers are possible near KFOD, KMCW, and KOTM later this afternoon, before a roughly north to south oriented line of stronger storms moves from west to east across Iowa around 02z and beyond, impacting all sites. Strong winds and lowered ceilings may accompany these storms overnight, before moving out into the early morning. LLWS may also develop overnight, primarily at KDSM and KOTM.

HYDROLOGY

Issued at 342 PM CDT Sun May 17 2026

The next two rounds of rainfall (this evening and Monday evening) will have ample moisture accessible to them thanks to a healthy gulf moisture stream up through the central U.S. Precipitable water values both tonight and Monday night will be nearing 1.6" to 1.7" (90th percentile for May 17th in Iowa is 1.27" and max is 1.6"). This suggests that anomalously moist air will be in place over Iowa as these storms move through, leading to efficient rainfall with both rounds. Fortunately, storm motions appear fairly progressive for both events, which should help prevent prolonged residence times. Rainfall amounts of 1 to 2" are possible with some of the heavier storms. That said, there is some concern that as the cold front/outflow boundary stalls over Iowa late tonight into Monday morning, LLJ induced showers and storms may persist over southern Iowa/northern Missouri for some time into the morning. This could lead to some locally higher rainfall amounts of 3" or more in these areas.

This is notable, as southern Iowa picked up the most rainfall yesterday as well, and a poorly placed swath of additional rain could exacerbate conditions along the Thompson and East Nishnabotna rivers. These rivers are typically a bit faster at routing water out, but full recovery is unlikely by tonight and rain could lead to additional rises. Likewise, soils in these areas are likely already saturated and crops are not to the point where they can help absorb water, leading to some flash flooding concerns as well. This is all contingent on if another swath of heavy rain lines up in these areas, but certainly something to keep an eye on tonight and again Monday night. Elsewhere in the state, urban areas will likely be most susceptible as soils in rural areas still remain on the drier side. However, with two potentially widespread rainfalls tonight and tomorrow, will be keeping an eye on where the heaviest rains set up. Should the same area be hit twice, ponding or flash flooding could develop. The weather prediction center has a marginal risk for excessive rainfall today and a slight risk for excessive rainfall tomorrow, likely due to the two back to back rainfalls.

Finally, in terms of river flooding, the perpetual rounds of storms should end by Tuesday, but could see rivers responding through the week, again depending on where the heaviest amounts fall. This will be something better assessed in the coming days.

DMX WATCHES/WARNINGS/ADVISORIES

None.