textproduct: Des Moines

KEY MESSAGES

- Increasing late season heat and humidity today and through much of this week, especially over southern Iowa where heat index values will range around 100 degrees at times.

- Most of the week will be dry, but peppered with occasional chances for isolated or scattered thunderstorms. Confidence in the details of timing and location of any such storms is unusually low for the next several days, but for the most part the best chances will be in the overnight to morning hours each day.

SHORT TERM /THROUGH MONDAY/

Issued at 158 AM CDT Sun Aug 30 2026

A large heat dome that has sweltered over the southwestern U.S. for many days now has finally begun to shift eastward, and is centered roughly over Oklahoma at this time. A 500 MB trough/low is swinging along the U.S./Canada border near Washington/Idaho/Montana and aiding in the eastward slide of the dome. Between these two systems, broad west southwesterly steering flow has overspread the High Plains and flattened the 500 MB ridge over the Midwest and Iowa. Later today through Monday (and beyond), the heat dome will slide further east/northeast toward Missouri and the Ohio/Missouri/Mississippi river confluences region, bringing it in closer proximity to Iowa. However, the aforementioned 500 MB trough will also move eastward and deepen as a stacked cyclone during this time, reaching southwestern Saskatchewan by Monday evening. At the surface, a broad low pressure trough extends from the lee of the Rockies east northeastward across Nebraska, southeastern South Dakota, and northwestern Iowa at this time. Over the next couple of days, this trough will be nearly stationary but will wobble around the region and Iowa, squeezed between the low pressure system well to our north and the high pressure approaching the to south. To the south of the trough/boundary, persistent south to southwesterly low-level flow along with daytime mixing down of warm air aloft will support multiple days of heat and humidity. Around and north of the trough/boundary, however, warming will be more tempered especially if showers and storms develop within the zone. In southern Iowa temperatures will peak in the 90s each day with heat index values peaking near 100 degrees in the afternoons and evenings.

The details of any convective evolution are difficult to pin down in this pattern. This morning, an 850 MB low-level jet is parked right across Iowa, stretching from southwest to northeast. Within the core of the jet radar imagery is quiet and there is no discernible forcing mechanism for convection. However, on the fringes of the jet we have seen development both over far eastern Iowa, and out west across central Nebraska. The latter cluster of storms is tracking east northeastward along the surface trough, and should persist into our forecast area later this morning and afternoon. However, deep-layer shear is very weak and the storms will encounter diminishing instability with eastward extent, so severe weather is unlikely and a general weakening trend should occur as the morning progresses. The question then becomes the extent to which any showers and storms will be able to persist this afternoon and evening. Forecast soundings illustrate a stout warm/dry layer above the surface that should suppress any redevelopment, however, there will still be some skinny elevated instability and although the 850 MB jet will weaken after sunrise, bubbling convection may still linger later in the day as indicated by the majority of CAM solutions. As such, have maintained 30-50% rain chances this morning, but tapered them off to 20% later today and ended them before midnight. On Monday the precipitation forecast is even more uncertain. The large trough/low over southern Canada will be moving by but most of the associated cyclonic flow aloft will remain well to our north, and the influence of the heat dome to our south will again help to suppress convection. Even so, the presence of the surface boundary/trough may support some isolated thunderstorms during the day, and have maintained a strip of 20% POPs near its expected location. In this case, deep-layer shear will be more favorable by Monday afternoon/evening, and instability above the warm layer will be stronger, so if any storms are able to develop then there may be some potential for severe weather. Given the conditional nature of the threat however, only the low POPs are maintained and SPC's Marginal Risk outlook is appropriate.

LONG TERM /MONDAY NIGHT THROUGH SATURDAY/

Issued at 216 PM CDT Sat Aug 29 2026

The mid-level ridge will be drifting and lifting northeastward becoming centered over the St. Louis area by Tuesday. The broad trough in the western US will persist keeping our low level flow from the southwest and the storm track passing over northern portions of the state with a surface quasi-stationary boundary in place in that vicinity as well. The sensible weather effects will be drier and warmer conditions over southern Iowa with opportunities for storm chances over northern Iowa with a timing peaking likely overnight into the early morning hours. This is not a forecast to be very specific with fine tuning of the details such as timing and location in the coming days. What seems more confident is the late- season heat with potential for several days of record warm low temperatures possibly in jeopardy, pending any convective development farther south in Iowa and cloud debris contamination. While heat index values never breach the 105 degree advisory criteria, there will be multiple days of values peaking in the 100- 103 degree range over southern Iowa paired with low temperatures in the upper 70s through at least Wednesday. Experimental heat risk is in the extreme category (level 4 of max of 4) over many portions of southern Iowa during this period, which aligns with the magnitude of the uncommon late season heat and high low temperatures. Those with vulnerabilities to heat will want to take precautions through this upcoming week.

AVIATION /06Z TAFS THROUGH 06Z MONDAY/

Issued at 1047 PM CDT Sat Aug 29 2026

VFR conditions will prevail through the TAF period. Winds have diminished for the most part this evening with the low level jet increasing. This will result in a period of LLWS across all sites through sunrise. Then Sunday morning scattered thunderstorms will move across the area, lasting into the afternoon. KALO and KDSM are the most likely areas to see precipitation and prob30 groups are included. Scattered convection is possible at KFOD/KMCW/KOTM however timing is shorter and less certain, so have kept from TAFs at this time.

DMX WATCHES/WARNINGS/ADVISORIES

None.