textproduct: Des Moines

KEY MESSAGES

- Areas of fog this morning could become dense.

- Scattered thunderstorms tonight, mainly north and again Friday night, mainly over the south. A strong storm is possible with both periods.

SHORT TERM /THROUGH FRIDAY/

Issued at 257 AM CDT Thu Aug 20 2026

The upper level flow will continue to be driven by an upper level low over northeast Canada and by an intensifying sub-tropical high over the Four Corners region. The evolution of these two systems will lead to increasing northwest flow over Iowa through Saturday before the upper high ridging begins to move eastward. A couple of short wave impulses will be passing through this flow later today and tonight and again Friday night and each will bring a chance for showers and thunderstorms to the state. In the immediate term, surface high pressure centered over northern Iowa. Sky conditions across Iowa range from mostly clear over the majority of the state to scattered mid-level clouds over central to south central Iowa. Dense fog is the immediate challenge this morning. Areas of dense fog have developed over portions of northwest Missouri into eastern Nebraska in addition to a few localized locations over far southeast Iowa. These areas align fairly well where the more widespread precipitation has occurred over the past 24-48 hrs and it is these area with wet soils that are the first to have fog impacts. The Nighttime Microphysics satellite imagery is showing some river valley fog occurring in the forecast area near Taylor and Ringgold counties in the south and in the west central near the Nishnabotna valley. The southern valley fog has actually diminished some per the satellite trends and that may be in part to drier dew points from the high pressure system still advecting into the state. That is the challenge this morning is how widespread dense fog will become under mostly favorable radiational cooling conditions along with the lowering dew points in addition to several days of dry weather north. Currently no Dense Fog headlines for the area but will add if trends change.

The short wave impulse to impact the area later today is showing up on water vapor imagery over southern Manitoba. The main kinematic energy associated with that system will focus over Minnesota and Wisconsin. The main forcing processes will thermally and moisture driven as return flow develops into northwest Iowa as the high pressure departs and a weak surface boundary approaches from the north. The storms will be moving into an increasing strong elevated mixed layer (EML) farther south through Iowa. Storms may persist but will become increasing elevated above the EML which will decrease available instability. A strong storm could occur over the far north/northwest but for the reasons listed above, that potential will decrease further into Iowa.

LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/

Issued at 222 PM CDT Wed Aug 19 2026

The ridge-trough pattern across the CONUS will continue to amplify as we head into the weekend, with Iowa continuing to be positioned on the interface of the two, under enhanced NW flow. A similar pattern appears to be in place into early next week as well. There is still significant uncertainty timing of the frontal passage Friday, with the NAM even suggesting it holds off until Friday evening. NAM/GFS/ECMWF all suggest some storm chances Friday night, though more focused to our west under better instability. These storms would have a moderate amount of instability and bulk shear overlap and could perhaps pose a severe threat, hence the marginal across our area and slight risk to our west. GFS/ECMWF suggest the next shortwave (or waves) passing through the enhanced flow sometime Saturday night through Sunday, though timing still varies. With the passage of the cold front on Friday, and replacement with high pressure through the weekend, gulf moisture flow seems unlikely. However, evapotranspiration is still significant and may provide enough moisture for some marginal instability and low end storm chances. All things considered, the weekend seems drier overall, with a few marginal storm chances with uncertain timing. Temperatures remain relatively unchanged from this past week, with highs in the upper 70s and low 80s. More significant instability appears to return early next week, ahead of another cold frontal passage according to deterministic models. There is still uncertainty in timing of that front, and any further details given by deterministic models at this stage are certain to change.

AVIATION /12Z TAFS THROUGH 12Z FRIDAY/

Issued at 621 AM CDT Thu Aug 20 2026

Patchy fog near KMCW to begin the period will dissipate quickly. VFR conditions will then prevail through the day with potential for passing clouds in the 4-6 kft range. Light wind this morning will become southerly but remain less than 10 kts. A few showers or storms may occur over far northern Iowa tonight but chances remain quite low at all sites.

DMX WATCHES/WARNINGS/ADVISORIES

None.