textproduct: Des Moines

KEY MESSAGES

- Hot and humid conditions will continue through the week, especially over southern Iowa where heat index values will peak around 100 degrees each day. A Heat Advisory remains in effect for portions of southern Iowa through Wednesday afternoon.

- Thunderstorms will be possible at times for the next several days. Any storms today will be very isolated and rain chances are 20% or less. Tonight thunderstorms will be more likely in northern Iowa, close to the Minnesota border, where severe weather will be possible after dark.

SHORT TERM /THROUGH WEDNESDAY NIGHT/

Issued at 207 AM CDT Tue Sep 1 2026

A large heat dome is still smothering the southern/southeastern U.S., while a low pressure system is moving over parts of Saskatchewan and western Ontario early this morning. Between these systems, Iowa lies beneath modest zonal flow, while out to the west a 500 MB shortwave trough is moving across the Colorado and Wyoming Rocky Mountain region. At the surface, a weak low pressure trough is draped across western Kansas and Nebraska, while high pressure is centered over the Dakotas. A nebulous, washed out stationary boundary lies across Iowa roughly from around Omaha up to Dubuque, though it is very difficult to locate precisely given the total absence of surface flow or convergence. Radar imagery depicts several tiny showers and a couple of brief thunderstorms developing near the boundary, but they are small, short-lived, and of little impact. Also of note is that patchy fog has developed in our northeastern counties overnight, where winds are calm and moisture is pooling along and just north of the aforementioned boundary. This is resulting in bouncing visibilities with a couple of sites dropping below a mile at times. However, any dense fog observations have been sporadic and brief, and webcams indicate the fog is generally shallow and not restricting visibility much at eye level. Will hold off on an advisory for now, but monitor observations this morning in case one should become necessary.

As the low over Canada moves eastward today, in its wake the heat dome to our southeast will reassert itself. This will bring mid-level ridging across our area during the day and also allow the surface low over the Plains to move northward, reaching central Nebraska by this evening. The nebulous boundary overhead will also move northward in response, reaching northern Iowa by early tonight. Isolated showers and thunderstorms will likely continue to bubble near the boundary this morning and move northward with it, but then largely fade out during the day as the ridging aloft sets in. This evening, however, the 500 MB shortwave trough currently moving across the Rockies will emerge over the High Plains and interact with the surface low, generating more vigorous thunderstorms over parts of Nebraska and South Dakota. These storms will then track roughly eastward along the boundary and pushed by the advancing shortwave, moving along or near the Iowa/Minnesota border after dark tonight. A nocturnal 850 MB jet will intensify after sunset with its exit region pivoting over far northern Iowa, and also bringing very warm 850 MB temps around 23-25C up toward northern Iowa. This should help the thunderstorms remain in the north, and perhaps even send the bulk of them slightly east northeastward into southern Minnesota. Any southward expansion of thunderstorms should also be hampered by the weak mid-level lapse rates stemming from the elevated warm layer advecting in. That being said, wherever the northern edge of these warm 850 MB temps and wherever the surface boundary reaches, that region will be conducive to severe weather especially if the thunderstorm clusters can develop an organized cold pool. MUCAPEs of 1500-2500 J/KG and 0-6 KM Bulk Shear around 50-60 KT support strong updrafts and a threat of damaging straight-line winds, and SPC has updated the Severe Weather Outlook to include a Slight Risk for southern Minnesota and northern Iowa, roughly north of Highway 20 in our service area.

The 500 MB shortwave and thunderstorm system moving by on Tuesday night will pass east/northeast of our area by early Wednesday morning, but will likely leave the old frontal boundary dangling nearby through Wednesday, where it may once again generate a few showers and thunderstorms though these will once again be suppressed, similar to today, by another brief period of mid-level ridging and a general lack of large scale forcing for ascent. Aside from thunderstorm concerns over the next couple of days, oppressive heat will also continue given our proximity to the large southeastern heat dome as well as mostly sunny skies and southerly winds south of the nebulous frontal boundary. The ongoing Heat Advisory thus continues unchanged and remains in effect through the day Wednesday. It may eventually need to be extended further toward the end of the week, however, with increasing time there is increasing uncertainty regarding the potential for additional thunderstorms inhibiting warming at times, especially around Thursday.

LONG TERM /THURSDAY THROUGH MONDAY/

Issued at 242 PM CDT Mon Aug 31 2026

Into the end of the work week, the ridge over the southeastern CONUS flattens out somewhat, but still generally keeps its influence over Iowa, which will allow for the continuation of rather uncomfortable conditions with the heat and humidity. Forecast highs in mid 90s and heat indices around 100-105 degrees over much of southern Iowa are expected, which could lead to an extension in current heat headlines into Friday if forecast trends continue to lean that way, so stay tuned for updates. Additional shortwaves traveling through the western periphery of this ridge will continue to bring continued periods of showers and thunderstorms, particularly during the overnight and morning Thursday and Friday. Location of the associated boundary(s) and the influence of the cap overhead are still generally uncertain in this range, and even more so through the weekend considering how earlier activity in the week plays out, so will need to monitor trends over the coming days. For now, have left at least low PoP mentions over portions of the forecast area through the period given this overall potential.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 1230 PM CDT Tue Sep 1 2026

VFR conditions to prevail the rest of the evening. A few showers and storms are present at 1pm near KMCW but should clear out by 19Z. As we head into the overnight hours, a nocturnal LLJ will get going across central Iowa providing some LLWS at all sties from 06z until 11z. Storms will be possible overnight across northern Iowa, so PROB30 groups have been introduced at KMCW from 06z to 10z.

DMX WATCHES/WARNINGS/ADVISORIES

Heat Advisory until 7 PM CDT Wednesday for IAZ059>062-072>075- 082>086-093>097.