textproduct: Des Moines

KEY MESSAGES

- Breezy with scattered, intermittent shower chances of 30 to 50% over western into central Iowa today. Embedded thunderstorms are also possible with this most likely in western and southern Iowa. Severe weather is not forecast.

- Locally heavy rainfall and isolated flash flooding possible late this afternoon into tonight over southern Iowa and northern Missouri.

- Chances for severe storms exist on Sunday in western Iowa, though potential is low at this time.

SHORT TERM /THROUGH TONIGHT/

Issued at 247 AM CDT Sat May 30 2026

While twenty four hours later the upper level pattern remains in an omega block, features have progressed to varying degrees this morning. The potent shortwave trough over southeastern Canada has moved over the northern Appalachians this morning. The plume of Gulf moisture on GOES-East upper level water vapor imagery has shifted farther northeast, though only about 100 miles or so in the 850 and 925mb RAP analysis, with the shortwave energy arriving over the state. An area of showers on the fringe of this low level moisture gradient is shifting into Minnesota this morning while another band of showers and storms has developed over southwestern Iowa into northwestern Missouri. These storms are in an airmass with higher instability and precipitable water (PWAT) values. However, there is a lack of shear so the only hazard will be lightning early this morning.

As these slowly lift northeastward into central Iowa through the morning, the instability looks to lag behind through early afternoon staying mostly southwest of Des Moines per latest HREF 1 hour probability of thunder. Even as these showers and the embedded storms lift northeastward, they will be scattered and fighting against drier air over eastern Iowa. In fact, parts of our eastern and northeastern forecast area, such as Mason City and Waterloo, are unlikely to see any rainfall until sometime Sunday. While instability may build over southwestern Iowa to between 1000-2000 J/kg, the deep layer shear is unremarkable so severe storms are not forecast. That said, the environment has some reminders of a funnel cloud setup with low cloud bases, 0-3km mixed layer CAPE of 75 to 150 J/kg per Creston RAP sounding, and weak shear. A lacking piece is no apparent wind shift or surface boundary.

The concern as we head through late this afternoon into tonight will be locally heavy rainfall and the potential for flash flooding. The airmass will be one of the juicer ones so far this year with precipitable water values well above normal and nearing the 90th percentile with seasonally deep warm cloud depths for efficient rain process. These combined with storm motions of around 10 to 15 mph point to localized areas of heavy rainfall. Initially, storms will have nebulous forcing, which is when the concern may be highest for flash flooding due to the slow storm motions. As stronger shortwave energy lifts over the region late this evening into the overnight, a band of storms is forecast to sweep through portions of Iowa that is more progressive. Rainfall bullseyes by Sunday morning of 3 to 5 inches are shown in the 0z HRRR, WRF-ARW, and regional Canadian over far south central Iowa, the 0z RRFS (FV3 core) over southwestern Iowa, the 0z GFS/FV3 over southwest Iowa into northwestern Missouri, and the 0z RRFS (MPAS core) over north central Missouri along with the 0z NAMNest, which has amounts over 10 inches in that area. The 0z HREF, which is influenced by the NAMNest, shows a similar footprint over north central into northwestern Missouri while the 0z RRFS ensemble is split between pockets in southern Iowa and northern Missouri. While much of Iowa has dry soils per NASA SPoRT 0-10cm and 10-40cm layers generally at the 20th percentile or lower, there remains a more susceptible area in far south central Iowa from rains around May 18 that is in the 30th to 70th percentile not to mention any areas that may be primed this morning over southwest Iowa. With these less dry soils lining up with modeled areas of heavy rainfall not to mention high rain rates, flash flooding is possible over southern Iowa tonight and is within the marginal risk of excessive rainfall. If confidence grows and narrows in on a more targeted area of heavy rainfall, a flood watch may be necessary.

LONG TERM /SUNDAY THROUGH FRIDAY/

Issued at 256 PM CDT Fri May 29 2026

Some consensus is coming into view on Sunday's synoptic pattern, though convective evolution is still highly variable across the mid- range CAMs. The western trough will finally pivot northeast over the high plains, with waves of energy perhaps far enough east to influence storms in Iowa Sunday. Lee cyclogenesis to the west and associated southerly flow will likely advect in a warm sector with upper 60s and 70s dewpoints. EPS/GEFS ensembles show Iowa on the edge of meaningful 500 mb flow in the exit region of the trough, meaning improved shear may be in place, though it certainly isn't a guarantee. Latest GEFS runs have 50% chance of supercell composites >5 in southern Iowa. SPC now has a marginal risk in western Iowa as a result of these trends, and some of the machine learning guidance suggests low end severe probabilities. Convective evolution in Nebraska/Kansas will be important for sorting out details for our storm chances Sunday, and CAMs show too much uncertainty to discuss at this time.

By early and mid week, ensembles show some agreement that the western trough will continue to lift out to the northeast, and central US ridging/eastern troughing will hold their positions. If the lifting trough can pull moisture across Iowa and overcome the dry airmass sitting under the eastern troughing/Great Lakes high pressure, scattered storm chances will continue through the mid- week.

AVIATION /12Z TAFS THROUGH 12Z SUNDAY/

Issued at 605 AM CDT Sat May 30 2026

While VFR conditions are expected at all terminals, we will be monitoring for restrictions due to showers moving into Central Iowa this morning. PROB30 groups have been issued at FOD, DSM and OTM due to showers arriving later on today. Winds will also increase out of the ESE at 10-15kts with gusts reaching up to 22kts later this afternoon. Clouds bases are expected to decrease around around 4,000ft as we head into the afternoon. Showers and storm chances may increase after midnight, with confidenceonly high enough to issue PROB30 at OTM at this time.

DMX WATCHES/WARNINGS/ADVISORIES

None.