textproduct: Des Moines

KEY MESSAGES

- Patchy fog will occur early this morning, and could be dense in a few areas.

- Shower and storm chances return late tonight into Saturday morning, with a low risk of severe weather in the form of gusty winds. Additional storm chances are forecast at times into early next week.

- A warming trend is forecast this weekend and into next week. On Sunday high temperatures are predicted to range in the low to mid-90s across much of Iowa, with the Heat Index approaching 100 degrees in some areas.

SHORT TERM /THROUGH SATURDAY/

Issued at 228 AM CDT Fri Aug 28 2026

A large 500 MB high remains over the southwestern U.S., centered across Arizona and New Mexico, with a ridge extending up the High Plains into central Canada. Two large gyres are centered on either side of the ridge, one over the coast of British Columbia and the other over Hudson Bay. A ribbon of 500 MB vorticity associated with a subtle shortwave impulse is moving through the southwesterly steering flow over the U.S. Rockies, and will overtop the ridge over the High Plains tonight, flattening it and then crossing Iowa early Saturday morning. Meanwhile, at the surface, a high pressure ridge is currently draped across much of Minnesota and Iowa, resulting in nearly calm winds and clear skies across our service area. This is allowing for efficient radiational cooling, with some sites in northern Iowa already in the lower 50s as of this writing. Spotty, shallow fog is also developing given the near-zero dewpoint depressions, but so far any reported visibility below one mile has been quite sparse. High-resolution models do support the potential for expansion of more dense fog toward sunrise, particularly in favored areas such as river valleys and our northeastern counties that are directly within the center of the surface ridge. Have modestly expanded fog mention areas in the forecast accordingly, but not ramped up dense fog wording yet in deference to the very slow development so far, and in light of the sparseness of any dense fog on Friday morning under similar conditions. Nevertheless, will be closely monitoring observational trends early this morning in case dense fog does become any more widespread than currently expected.

During the day today, the surface ridge will begin to move away to our east, while a broad surface trough develops along the lee of the Rockies in response to the approaching shortwave impulse mentioned earlier. Around this evening, diurnal convective development is anticipated somewhere over the Dakotas, as maximum destabilization occurs within the joint region of the surface trough and the upper wave. These storms will then move east southeastward as the impulse overtops the 500 MB ridge, crossing northern Iowa early Saturday morning. Models are essentially unanimous in maintaining a seasonally strong 850 MB low-level jet late tonight that pivots from around the SD/MN/IA tri-state area over into north central Iowa after sunrise Saturday. There is also general consensus in a mesoscale environment characterized by 0-6 KM Bulk Shear of around 40-50 KT, as well as 1000-1400 J/KG of MUCAPE. Normally this environment would support MCS maintenance, such that if the storms approaching from South Dakota have sufficient organization of an associated cold pool, we would expect some possibility of gusty winds as the storms enter our forecast area. However, most CAM solutions wane the intensity of the storms as they enter our area, and are not reflective of a severe weather threat. This potential will be closely monitored today, as there is some potential for strong winds mainly in our northwestern counties as the storms move in early Saturday morning.

By Saturday afternoon any lingering showers/storms should move off to the east with the parent shortwave, after which the 500 MB ridge will begin to build back in overhead with a return of southerly surface flow. Forecast soundings indicate pretty good mixing potential, especially in our western counties, supporting modestly gusty winds and warmer temperatures. The official forecast currently calls for highs near 90 degrees in our southern and western counties accordingly, however, the fly in the ointment could be any lingering cold pool effects from morning storms, depending on how they evolve. As such, did not make any adjustments to the ongoing Saturday high temperature forecast.

LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/

Issued at 240 PM CDT Thu Aug 27 2026

Through the remainder of the weekend, the upper level ridge is expected to shift further eastward over the Central CONUS, but also become more squished south as a rather potent trough moves across the Pacific Northwest. The net result will be a return of even warmer and humid conditions through early next week as southwest flow persists, with various shortwaves suggested to ride the upper ridge, ultimately putting Iowa into a more active pattern for various chances of showers and thunderstorms. With Iowa well into the warm sector, capping looks likely at least on Sunday, keeping chances for precipitation rather low (20-30%) until around Monday evening/Tuesday when a cold front is suggested to push into Iowa. More specific details are expected to become more clear in the coming days for this time period. Temperatures will generally top out in the upper 80s through the 90s, with central and southern Iowa reaching near 100 degrees for heat indices Sunday and Monday.

AVIATION /06Z TAFS THROUGH 06Z SATURDAY/

Issued at 1036 PM CDT Thu Aug 27 2026

Anvil cirrus from Nebraska storms is moving over parts of Iowa. In addition, pockets of scattered stratus around 5 kft is moving over the state. Fog potential remains the primary challenge overnight and conditions are favorable for some river valley fog and in some other low spots. At this time, confidence remains low at any site however will continue to monitor. KFOD likely would have the greatest chance considering the proximity of the sensor to the Des Moines river. The light and variable winds overnight will become sustained from the south/southwest mainly at 10 kts or less. Additional diurnal cumulus development in the 5-7 kft range is possible.

DMX WATCHES/WARNINGS/ADVISORIES

None.