757 FXUS63 KDMX 232343 AFDDMXArea Forecast Discussion National Weather Service Des Moines IA 643 PM CDT Tue Sep 23 2025
...Updated for the 00z Aviation Discussion...
.KEY MESSAGES...
- Showers, and an isolated thunderstorm, will continue across southern Iowa through early tonight. No severe weather is expected.
- Patchy fog possible again tonight, low /20% chance/ of dense fog.
- Dry and seasonal conditions return from Thursday through the upcoming weekend. Pleasant but warm highs in the 70s/low 80s with modest humidity.
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.DISCUSSION... Issued at 258 PM CDT Tue Sep 23 2025
Upper air analysis and water vapor imagery this afternoon is depicting several circulations over the upper Midwest, an upper low over Lk Superior and second elongated area of low pressure from Iowa wwd to the Rockies. The primary sfc low is located over OK, with a baggy sfc trough extending newd across nrn Missouri into IL.
The latest short term guidance continues to depict modest QG upper forcing /PVA/ along with a upper level deformation zone persisting across nrn MO and srn IA through the evening and early overnight hours. This combined with residual low level moisture /H8 dewpoints of 8C to 10C/ and weak moisture convergence will result in a continuation of showers and maybe an isolated thunderstorm, mainly along and south of I-80. Mid-level lapse rates remain generally poor /moist adiabatic/ so instability is minimal /MUCAPE < 500 J/kg/ with feeble deep layer shear less than 20 kts. A consensus of the CAM output suggests precip activity will continue through the evening hours before waning by midnight, with partial clearing thereafter. If skies do clear, patchy fog will again be possible per the typical autumn fog season. Low confidence on fog coverage and vsby restrictions attm, due to around 5 kts of sfc flow and uncertainty in the extent of clearing.
Wednesday...
After any lingering fog, high pressure is expected to build into the region from the west. By this point, the upper low should have shifted far enough east to place us in a mostly subsident upper flow regime. The one caveat is that model soundings do show moisture lingering in the PBL. Thus, as heating commences mid- morning, we`ll likely see a decent amount of cumulus/stratocumulus. A majority of the 23/12Z deterministic models show that the cloud depths should be shallow enough to limit any precipitation. However, its possible a few higher based very light showers or sprinkles develop during the afternoon as noted by SimZ in a few of usually robust CAMs. At this point, we`ll leave any explicit mention of showers out of the forecast for now. With a northerly low level flow, high temps on Wednesday will stay in the 70s.
Extended Forecast...Thursday into the Weekend.
Surface and upper ridging will develop across the central CONUS by late week. Expect partly to mostly clear conditions will temperatures nudging back upwards by Friday into weekend. Expect highs in the upper 70s to lower 80s on Thursday, with widespread 80s on Friday into the weekend and early next week. This would be about 5 to 10 degrees above seasonal averages. Dewpoint values should remain modest in the 50s, so humidity won`t be a major issue.
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.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/... Issued at 638 PM CDT Tue Sep 23 2025
VFR conditions in place to start the period with broad mid to high level cloudiness across Iowa. Scattered cumulus under this cloud cover is beginning to diminish though there remain some BKN cigs over parts of central and southwest IA with bases around 3.5 kft. Fog remains possible overnight, specifically over northern Iowa where some clearing is likely. Redevelopment of cumulus is expected from late morning and into the afternoon with some MVFR cigs possible with diurnal rising of bases into the afternoon. Light wind will persist through Wednesday.
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.DMX WATCHES/WARNINGS/ADVISORIES... None.
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DISCUSSION...Fowle AVIATION...Donavon
NWS DMX Office Area Forecast Discussion