textproduct: Des Moines

KEY MESSAGES

- Showers and storms over northern Iowa diminish this morning.

- Flash flooding possible tonight with highest risk south of Highway 30 and west of I-35. Isolated rain totals could exceed 5 inches in these areas. A few severe storms are also possible with hail and perhaps a tornado or two late this afternoon or evening.

- A lull in rain and storms Friday, but storms return late in the afternoon into the night. Renewed concerns for flash flooding and severe storms.

- After a break in storms and rainfall on Saturday, chances return to the state later Sunday into Monday.

SHORT TERM /THROUGH FRIDAY NIGHT/

Issued at 355 AM CDT Thu Jun 4 2026

Regional radar shows showers and thunderstorms over far northern Iowa and points north on the edge of an 850mb dewpoint gradient and tied to the low level jet. This activity is expected to sputter and diminish to a large degree over northern Iowa this morning; however, have maintained low PoPs around 20% to cover any spotty activity that may try to gin up in the dissipating low level jet and thermal lift. As we move into the afternoon hours, dewpoints will push into the 60s across central Iowa under a sky filled with clouds and some breaks of sunshine. Upstream of Iowa, a shortwave trough, perhaps of MCV origins, will be lifting towards western Iowa reaching there by late this afternoon. By then, mixed layer (ML) CAPE values will be approaching 1500 J/kg in southern Iowa and with the shortwave's forcing and thermal lift, will see an increase in scattered storm coverage. There are both hydrologic/flash flood and severe storm concerns as these develop in the afternoon into tonight. A common theme of late has been the weak deep layer shear at or less than 25 knots, which should limit storm organization. Initial severe storm concern will be on hail that should be limited in size by a water laden atmosphere. As we move into the evening hours, the low level jet will point into southern and eastern Iowa and ramp up with 850mb speeds approaching 40 knots. This will increase the low level curvature and area underneath the hodograph so that storm relative helicity values (SRH) increase to near 100 m2/s2 in the 0-500m and a bit above in the 0-1km layer. Not surprising, LCLs are low in this environment and the 0-3km CAPE will be over 200 J/kg so cannot rule out a tornado or two this evening over southwestern quadrant of Iowa.

While the severe risk will wane by late evening, the flash flooding concerns will be growing. Forecast soundings show precipitable water values peaking around 1.8 inches, which is well above the 90th percentile (1.4") and could exceed the daily max. In addition, there are very deep warm cloud depths nearing 4000m leading to efficient, warm cloud rain processes. While there is no apparent surface boundary, the 850-300mb flow around 25 to 30 knots should favor rainfall that could repeat over the same area. Rainfall amounts by sunrise Friday could be 2 to 4 inches with bullseye higher amounts approaching 6 inches over southern into portions of central Iowa with lesser amounts in northern Iowa. NASA SPoRT soil percentiles are largely unchanged from this time yesterday at the 30th percentile or lower in the 0-10cm rapid response and 0-40cm river response layers. However, there are localized pockets in Adair, Greene, and Taylor Counties that could be more susceptible to rainfall response due to recent rainfall over the past 5 days or so. Further, rainfall rates may exceed infiltration rates and this is even more true over urban areas that have more impervious surfaces. The experimental WPC urban rain rate dashboard for Des Moines has shown increasing probabilities for 2"/3 hours over the last 24 hours and is now up to 15-20%. A flood watch for flash flooding was coordinated amongst Omaha and Kansas City for this growing flash flood risk late this afternoon into tonight, which roughly lines up with a portion of the day 1 slight risk of excessive rainfall from WPC.

Storms will be pushing east of I-35 in the first several hours of daylight Friday morning. There is growing consensus amongst models that after this pushes out Friday morning, there will be little in the way of additional storms until late Friday afternoon if not Friday evening. There are exceptions, like the RAP and GFS, but PoPs for now have favored a lower percentage. There should be some degree of cloud clearing with temperatures rising into the 80s and dewpoints in the 60s. Beneath a low level capping inversion, strong instability will grow with MLCAPEs up to around 4000 J/kg in parts of the state by late in the afternoon, but once again deep layer shear will be a limiting factor perhaps reaching 30 knots. Storms may wait until evening to develop either along the weak convergence of a west to east surface boundary over Iowa if not more likely, per convective allowing models, Nebraska closer to the origin of the low level jet and then move into Iowa later in the evening/night. Either way, severe storms with large hail given the large CAPE profile and steep mid-level lapse rates and damaging wind gusts given the dry mid-level air with downdraft CAPE values nearing 1000 J/kg may be possible in portions of Iowa in the evening. Of equal and perhaps more concern, depending on how much and where rain falls tonight/Thursday night, will be flash flooding into Friday night. Precipitable water values and warm cloud depths will still be quite favorable for heavy rainfall. In addition, the 850-300mb layer wind is oriented closer to the orientation of the surface boundary, which may lead to rainfall rounds anchoring to the boundary. Soils will be primed in some areas by tonight/Thursday night's rainfall and with another 1 to 3 inches possible in parts of southern or central Iowa Friday night, the flash flood threat will need careful monitoring into the overnight hours.

River flooding potential continues to look unlikely with the 5 and 10 day experimental Hydrologic Ensemble Forecasting Service (HEFS) showing a little more than a half dozen action stage forecasts. However, HEFS is forced by the GEFS rainfall, which often tends to undo the rainfall. When looking at a singular run of the GFS, there may be a bit more of a signal for higher flowers. For example, the 3/0z and 3/18z runs of the 3 day GFS forced National Water Model have shown lower annual exceedance probabilities, which relates to higher, more impactful flows. While this has not been over central Iowa and has moved around spatially from the Cedar Rapids area to east of the Quad Cites, it does suggest there could end up being higher flows somewhere in the region with exact placement dependent on future rainfall magnitude and location.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

Issued at 257 PM CDT Wed Jun 3 2026

Showers and thunderstorms are expected to continue into Saturday morning, mainly over southern Iowa in relation to where the frontal boundary is progged to generally stall for the day. While most areas look to be dry for much of the day, additional showers and storms look to redevelop over parts of southern Iowa in relation to the lingering aforementioned front by the evening. Confidence however on exact placement is on the low end at this time, as models such as the GFS push this boundary near the IA/MO border into northern Missouri by Saturday evening. Certainly some placement differences to keep an eye on over the coming days. The pattern Sunday and beyond looks to remain on the more active side as additional wave pass through the area, along with slightly warmer temperatures overhead. More specific details however on timing and location of showers and storms will be better known in the coming days as models better capture these features.

AVIATION /06Z TAFS THROUGH 06Z FRIDAY/

Issued at 1052 PM CDT Wed Jun 3 2026

Showers and thunderstorms in northwest Iowa tonight will diminish as they head southeastward, but there is a low probability (30%) of TSRA at FOD and MCW by around sunrise. Otherwise, expect more widespread SHRA/TSRA spreading across the area Thursday afternoon through evening, with a higher probability of reduced visibility and possibly ceilings at all terminals during that time. Have used extended PROB30 groups to convey this in the 06Z TAFs, but expect refinement in future issuances.

DMX WATCHES/WARNINGS/ADVISORIES

None.