textproduct: Des Moines

KEY MESSAGES

- Isolated to scattered showers/weak storms expected today and Monday, with the best chances(50-70%) south and west. Rainfall totals at or above 1" possible mainly in western and southern Iowa by Monday evening.

- Fall-like air mass settles in for the middle/end of the week, with high temperatures not making it out of the 60s and low temperatures dropping into the 40s - a few areas in northern Iowa may even drop into the upper 30s!

UPDATE

Issued at 355 AM CDT Sun Aug 31 2025

Fog has been rather spotty over northern Iowa this morning, which has been up and down in terms of dense/patchy coverage. Not expecting much further development as stratus covers much of Iowa, limiting further fog development, but may see some visibilities continue to go up and down over northern Iowa given occasional periods of dense fog that may impact some locations.

A few areas of showers and even some lightning have been another primary focus this morning, mainly associated with a weakly forced boundary just south of Pocahontas through Ames, which has been moving slowly west/northwest this morning. Localized rainfall amounts in some spots are close to an inch given the slow moving nature of this activity, with showers and weak storms likely lingering through the morning before fading. Given weak shear and low instability profiles, not anticipating much more than some occasional lightning. Otherwise, weak returns of very light showers is noted mainly over far southwest Iowa. The shortwave responsible for these showers is over eastern Nebraska this morning, with appreciable mid-level moisture that extends into the southwest half of the state. Further east, the surface high pressure that has been over the Upper Midwest over the past few days has remained relatively stationary, as the mid-level trough ahead of it continues to slowly move over the eastern CONUS. As the shortwave gradually moves south/southeastward today, the moisture axis is expected to move over much of southern and western Iowa, which will increase rain chances over those respective areas into this afternoon and evening. The boundary again looks to develop showers and weak storms during peak heating, though limited instability and shear should keep any severe weather concerns quite low, with lightning the main concern and another non-zero funnel cloud potential. As the stratiform rain across southern/western Iowa in relation to the shortwave looks to become more likely later this afternoon to evening, rainfall amounts under an inch are expected, though locally higher amounts are possible with activity near the boundary as slow motions are still anticipated. Areas further east and north are expected to remain dry given drier air over the area from the easterly/southeasterly flow. Clouds overhead will result in cooler temperatures compared to yesterday, with values in the low to mid 70s.

The moisture axis with the shortwave dropping further south and east will continue to graze the southwestern portion of Iowa Monday, with rain chances highest in the morning before gradually decreasing into the late afternoon to evening as models generally indicate. Similar parameter space would suggest little concern for strong/severe storms but cannot rule out lightning at times with any activity. Conditions look to generally turn dry for much of Tuesday as drier air moves in briefly, before the deepening trough dropping into the Midwest from southern Canada tracks a cold front across the state, bringing the next chance for showers/storms and cooler/breezy conditions that follow. Guidance varies a bit on details such as timing and intensity with the GFS/Euro bringing rain chances into northern Iowa by Tuesday evening, while the NAM stays dry until Wednesday, so will have to monitor and make some adjustments on PoP trends over the next few days.

DISCUSSION

Issued at 221 PM CDT Sat Aug 30 2025

Weakly sheared pattern dominates much of the CONUS, with one ridge of high pressure drifting southeast from the Canadian Prairie Provinces towards the Great Lakes; while a second is entrenched across the western U.S. In between these two features, resides a nearly stationary elongated area of shortwave energy. This feature will be the culprit for bringing an end to the recent run of sunshine, ushering in clouds and rain chances that will likely stick around through the first half of next week.

Satellite pix depict pockets of agitated cumulus already developing across the CWA along with residual subtle differential heating boundaries. KDMX 88D not showing anything at this point - however expect that to change over the next few hours as convective temps are met, sparking popcorn convection across portions of central Iowa. Primary hazard will be lighting with this activity, however sufficient 0-3km CAPE and surface vorticity coupled with boundaries in the vicinity and weak shear, will once again see an increased potential for weak funnels this afternoon/early evening. If funnels do manages to develop would be weak and not expected to touch the ground. No severe weather is expected. Convection will wane along this band with loss of daytime heating, just in time for the next push of moisture associated with the aforementioned shortwave energy to work in from the west tonight.

The pattern doesn't change much Sunday-Monday with this slow- to-move blocking pattern in place across the region. However, the shortwave will be slightly closer on both days and as such expect a better eastward push of moisture, thus increasing shower/thunderstorm chances across western and central Iowa. Widespread clouds will hold temperatures down as well, with highs mostly only reaching into the low 70s both days.

The blocking pattern finally breaks down for Tuesday kicking the shortwave energy out of the region, allowing what will be a very fall-like air mass, with origins in the arctic to dive into the state. So expect high temperatures only reaching the 60s Wednesday and Thursday, with overnight lows dropping generally in the 40s (perhaps even some 30s across northern Iowa!).

AVIATION /12Z TAFS THROUGH 12Z MONDAY/

Issued at 638 AM CDT Sun Aug 31 2025

Showers and weak storms are expected at times mainly impacting the KFOD, KDSM and KOTM terminals, with higher chances into the afternoon and evening today. Exact timing and extent remains more uncertain so lower probabilities remain. Otherwise, mid to low level clouds are expected to keep lower ceilings over most sites, especially this morning over KMCW. Winds will generally be out of the east to southeast.

DMX WATCHES/WARNINGS/ADVISORIES

None.