textproduct: Des Moines

KEY MESSAGES

- Freezing fog this morning, mainly north and west. The fog may be dense at times.

- Flurries or a dusting of light snow late tonight into Wednesday, then much colder temperatures late Wednesday through Thursday. Lows well below zero Thursday morning, possibly -15 or below in a few locations.

- More active weather pattern from around the end of the week into early next week. Additional snowfall will be possible at times, but details are unclear at this range.

DISCUSSION

Issued at 209 AM CST Tue Dec 2 2025

A large surface high pressure area centered over the southern High Plains of Oklahoma and Texas has extended up across Iowa overnight, providing light southwest winds. A stubborn band of low stratus clouds remains from Monday's system and is currently draped from Wisconsin and eastern Minnesota down across eastern Iowa and Missouri. However, it is slowly moving eastward as the Monday system departs in that direction and the aforementioned ridge builds in from the west. Behind the stratus, over western and northwestern Iowa, temperatures are falling rapidly over the fresh snow pack and fog has begun to develop. Most high- resolution short-term model guidance blows the fog up between now and sunrise, which is at least to some extent consistent with recent observational trends. Have expanded wording of freezing fog this morning, especially across our northern counties, and will watch closely to see if the fog either becomes widespread dense, which seems less likely, or actually deposits ice onto surfaces which may be more of a concern. This will be the primary forecast problem of the morning, as otherwise the weather is quiet and will remain so into tonight.

By this evening a broad 500 MB trough will be rounding over the Dakotas, with a weak accompanying surface trough trailing from around the Minnesota arrowhead southwestward into South Dakota. Behind that, a large Arctic high pressure area will be surging southeastward from southern Canada into the north central U.S., essentially building in right behind the weak surface trough. As the trough moves through our forecast area late tonight and Wednesday morning, the low-level convergence along it beneath broad but modest forcing for ascent aloft should generate clouds and light snow. However, forecast soundings indicate a prevalence of dry air and any notable frontogenetical forcing is quite transient, so flurries or a dusting of snow appear to be in store and have maintained low (20-30%) POPs accordingly with no snow accumulations advertised. Given the timing of the trough passage, it is likely that surface winds in our area will remain south southwesterly for much of tonight with thickening clouds overhead, leading to steady or even slowly rising temperatures overnight. This will then come crashing to a halt behind the trough, as the much colder airmass associated with the Arctic high surges in, resulting in falling temperatures through the day on Wednesday. Have accounted for this with non-climatological diurnal temperature trends in the forecast, and it will be a rude awakening as the cold air pushes in on Wednesday. The large high pressure area will then move directly over Iowa late Wednesday night into Thursday morning, leading to ideal conditions for radiational cooling on the deep snow pack in place across much of our service area. This will push Thursday morning lows down to around 10 below zero or colder over much of our area, and despite light winds the Wind Chill may scrape 20 below at times, mainly across the north.

The cold high pressure area will begin to move away to the east late Thursday, and thereafter a northwesterly 500 MB flow regime will set up from the end of this week through the weekend and into early next week. Within this pattern, multiple "clipper system" impulses will move quickly overhead in rapid succession, resulting in quickly alternating low-level ridge/trough passages and multiple opportunities for additional precipitation during this active period. Temperatures generally support snowfall, and while the systems will be moving through quickly there will be multiple opportunities for some snowfall accumulations between around Friday and next Monday or Tuesday. However, at this range the details of when, where, and to what depth any such snowfall may occur are unclear. This will bear watching in the coming days as travel impacts may occur at times around this weekend and early next week.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 1141 AM CST Tue Dec 2 2025

Fog and stratus have been stubborn to clear out of KMCW and KALO, but both should see restrictions lift within the next hour or two. MVFR cigs return along a front tonight. Terminals could see a mix of light snow and freezing rain, but the duration would be limited. Accumulation appears limited to none at this time. Left precipitation chances out of most sites due to lack of impacts anticipated, but will continue to evaluate in later issuances.

DMX WATCHES/WARNINGS/ADVISORIES

None.