textproduct: Des Moines

KEY MESSAGES

- Increasing late season heat and humidity this week, especially over southern Iowa the next few days where heat index values will peak around 100 degrees in the afternoons and evenings. A Heat Advisory remains in effect for portions of southern Iowa from this afternoon through Wednesday afternoon.

- Thunderstorms will be possible at times for the next several days, but generally coverage and frequency will be low. Only 20% chances are carried at times through Tuesday, with a higher chance across northern Iowa Tuesday night. There is some potential for severe weather, but it is generally limited.

SHORT TERM /THROUGH WEDNESDAY/

Issued at 241 AM CDT Mon Aug 31 2026

The forecast for the next few days remains largely unchanged and characterized by persistent heat and humidity, along with occasional thunderstorm chances. 500 MB analysis indicates the large southern U.S. heat dome is currently centered over Missouri, Arkansas, and Oklahoma. Meanwhile, a low has closed off over far southern Saskatchewan. Between these two systems, west southwesterly 500 MB flow is found across the Midwest and Iowa. This general synoptic pattern will not change appreciably through Wednesday, except that both the low and the heat dome will slide slowly eastward, such that by Wednesday afternoon they will be centered near the Ontario/Quebec border, and around Kentucky/Tennessee, respectively.

At the surface, a weak low pressure trough has developed along the High Plains in response to the approaching Saskatchewan 500 MB low. This surface trough currently stretches from around Western Kansas, up across Nebraska to near the South Dakota/Minnesota border. A few thunderstorms have developed near this trough in western Kansas overnight, but have generally faded as they have attempted to move northeast toward Nebraska and Iowa. Later today, the trough/wind shift will move eastward and sag into Iowa, becoming a nebulous stationary front draped somewhere across our service area. As it does so, there is some potential for convective initiation to occur in the frontal zone at times, likely somewhere from our southwestern, through central, to northeastern counties. A generally moist airmass will be in place and MUCAPE of 1500-2500 J/KG or so will support the potential for thunderstorm development. That being said, a warm layer aloft associated with our proximity to the 500 MB heat dome will have a suppressing effect, and there is also no large-scale forcing mechanism to support convective initiation. The resulting expectation is that a few thunderstorms may develop during the day, however, coverage will be very low and any potential for strong or severe storms would be highly conditional.

Tonight into Tuesday morning, the frontal zone will shift slowly northward in response to the heat dome to our southeast reasserting its influence. Once again showers and storms may develop overnight, however there remains a lack of large-scale forcing for ascent, and mid-level lapse rates will also become much less favorable during this time. There are some signs that the nocturnal low-level jet may redevelop which would aid initiation, but model depictions of the jet are far less impressive than on previous nights. Have thus maintained only slight chance (20%) POPs in a strip near the expected frontal location overnight, but local adjustments may be needed to POPs today and tonight based on short-term convective trends if and when any storms do develop.

After a likely dry day on Tuesday, a 500 MB shortwave trough will move over the Upper Midwest on Tuesday night and likely generate clusters of thunderstorms moving roughly from west to east along the surface frontal zone. By that time mid-level lapse rates will be steeper and instability will be even higher, supporting a somewhat larger severe weather potential. However, it remains to be seen how much these storms would impact northern Iowa versus Minnesota, as many model solutions have been trending farther and farther north with the position of the front.

Aside from generally low chances for thunderstorms and severe weather, the more widespread weather impact over the next few days will be oppressive heat and humidity. The onset of the heat wave was somewhat stunted on Sunday by lingering after-effects of morning convection. However, more substantial warming is expected today with the lower rain chances and continuing airmass modification, and will persist through much of this week. The primary uncertainty in temperature forecasts revolves around the position of the stationary front discussed above, and the nebulous precipitation/thunderstorm chances at times. Confidence in stifling heat is therefore highest in our southern counties, which will be more consistently south of the front and any convective impacts, and also in closer proximity to the heat dome and associated warm air advection. Have maintained daily highs in the low to mid-90s in this area for the next several days, and the Heat Advisory remains in effect to highlight cumulative heat health effects during this time.

LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/

Issued at 237 PM CDT Sun Aug 30 2026

The pattern doesn't change appreciably late this week with the center of the ridge over the Tennessee Valley, trough over the western US, and Iowa on the periphery of the shortwave track. This will keep the warmest conditions over southern Iowa with storm chances highest late at night diminishing in the morning hours over northern Iowa. Into next weekend, there is a signal the ridge axis amplifies over the Plains, which pushes storm chances north away from the state.

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

Issued at 629 AM CDT Mon Aug 31 2026

VFR conditions will prevail through the TAF period. There will be isolated showers and thunderstorms at times, however, even within the showers and storms any low ceilings or reduced visibility will be brief and highly localized. The probability of such impacts at any terminal at any given time are far too low for TAF inclusion. If any storms are imminent at a terminal then an amendment may be considered.

DMX WATCHES/WARNINGS/ADVISORIES

Heat Advisory from 1 PM this afternoon to 7 PM CDT Wednesday for IAZ059>062-072>075-082>086-093>097.