textproduct: Des Moines

KEY MESSAGES

- Fog is likely tonight into Tuesday in areas with snow on the ground. There is some potential for dense and/or freezing fog, at least in patches.

- Milder temperatures are forecast Tuesday and Wednesday. Additional fog is possible Tuesday night into Wednesday morning.

- A band of light rain will move through the area late Wednesday night into Thursday morning, followed by cooler temperatures and gusty winds on Thursday. Some icing could occur on roads.

DISCUSSION

Issued at 216 PM CST Mon Dec 15 2025

Weak ridging has built over the region today, with modest west southwesterly surface breezes across most of Iowa. This has helped temperatures warm into the 30s and melting of the snow pack has just begun. Some thin high cirrus clouds are streaming overhead, but otherwise mostly clear skies are anticipated tonight and winds will become lighter, still from the southwest but closer to 5 KT overnight. With dewpoints in the 20s currently being advected into the area and snow melt adding near-surface moisture, this is a solid setup for fog development after sunset tonight and into Tuesday morning in portions of our area with snow coverage. However, there are some flies in the metaphorical ointment. The first is the aforementioned cirrus clouds, but these should be intermittent and remain high and thin, so any mitigation of radiational cooling will be minor. The second and more inscrutable impact is the degree to which snow melting actually occurs today. Visible satellite imagery indicates any erosion along the southern edge of the snow pack has been very slow to develop, and over most of the snow area air temperatures are still only in the upper 20s even at 2 PM. This may limit the degree of water vapor introduction in the near-surface layer. The third is uncertainty over winds will really go calm (or nearly calm) or remain organized though light. Given these competing factors, have expanded coverage of fog in the forecast and messaging of its potential impacts, but do not have sufficient confidence in widespread, dense fog occurring, or ice accretion occurring on roadways, to issue an advisory at this time. This potential will be closely monitored this evening and tonight.

On Tuesday southwesterly winds will pick up again and temperatures will rise even higher, peaking in the upper 30s to around 40 over the snow pack, and well into the 40s in our southwestern counties where the ground is bare. This will promote additional, and likely more substantial, snow melt and support potential fog development again Tuesday night into Wednesday morning. It does appear a weak surface high pressure area will move through around sunrise Wednesday which may cause winds to go more calm and enhance fog potential. This will be further assess tomorrow, once we have some data from the first round of fog tonight. Wednesday will then be very similar to Tuesday, once again mild but becoming breezier in the afternoon, before the next weather system approaches Wednesday night.

By Wednesday evening an energetic 500 MB trough/low will be moving eastward along the Canada/Montana border, and will subsequently deepen and dive southeastward Wednesday night, crossing Iowa and neighboring states on Thursday. This will push a surface cool front across the region, and along the front low-level convergence beneath broad forcing for ascent will support a wide band of rain moving across our forecast area late Wednesday night into Thursday morning. Seasonally limited moisture content and the progressive nature of the rain band will keep amounts light, generally around a tenth of an inch, and no impacts are anticipated initially. However, behind the front temperatures will fall during the day on Thursday and gusty winds will surge in, enhanced by a tight surface pressure gradient and good cold air advection/low-level mixing, and the winds and falling temperatures could freeze overnight rain on roadways, leading to slick travel conditions. In addition, as the core of the 500 MB trough/low moves overhead around Thursday afternoon, the associated forcing may generate some snow showers. Right now the probability of accumulating precipitation is low, but if any snow does reach the surface it could compound road condition concerns. All in all Thursday will be blustery, damp in the morning, and travel considerations will need to be monitored in the coming days.

From Friday onward the steering flow becomes roughly zonal and the synoptic pattern supports generally dry and quiet conditions, with alternating weak surface ridges and troughs keeping temperatures moderate. The seven day forecast is dry after Thursday as a result, with daily high temperatures varying from the upper 20s to the lower 40s.

AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 1033 PM CST Mon Dec 15 2025

Patchy fog, some locally dense, is expected to become more widespread over northern into north central Iowa this morning. So far though, the current fog is quite isolated so have made minor adjustments to the forecast, with IFR and more isolated LIFR conditions still possible at times this morning. Freezing fog is still possible as well at the northern terminals, with any fog development expected to fade by late morning. Winds out of the southwest through the afternoon will gradually shift northwesterly into the evening behind a dry frontal passage, with isolated LLWS possible at the northern terminals around 21-00z.

DMX WATCHES/WARNINGS/ADVISORIES

None.