textproduct: Des Moines

KEY MESSAGES

- Band of snow passes over northwest into central and southeastern Iowa today into this evening with accumulation of 1-3 inches possible. On the southwest side over portions of western and southern Iowa, light ice accumulations are possible.

- Accumulating snow Saturday with very cold air Saturday into Sunday. Cold weather headlines look increasingly likely early Sunday.

- Moderating temperatures and trending dry early next week.

UPDATE

Issued at 1042 AM CST Thu Dec 11 2025

A few notable trends this morning have led to a change in the Winter Weather Advisory. Profiles are notably colder compared to output yesterday, that max wet bulb aloft remaining below freezing where the QPF axis will be. This means that there is a decrease in freezing rain, and a slight increase in snow. Strong isentropic ascent on the 295K surface (approaching 50kts) pack the moisture along a tight fgen band angled on a NW to SE diagonal. The fgen will be the primary source of forcing and will lead to a tight moisture gradient on the southwest side of the snowfall axis.

Looking at cross sections, as the fgen band ascends towards the DGZ, omega eclipses -20 microbars. The peak lift occurs just below the DGZ, favoring sub-climo SLRs. This is because snow formation will favor plates over dendrites. That being said, the DGZ just aloft will benefit from convective processes and thus an added boost to snowfall rates, evidenced by spreading/folding of the theta-e surfaces in cross sections. The snowfall rates peak in the mid afternoon, geographically Des Moines. So the highest rates will affect the beginning of rush hour. Given the southwest shift as well as the travel sensitivities in the populated portions of central Iowa, have expanded the Winter Weather Advisory southwest by a county length. Trimming of the northeast flank is possible this afternoon, which will be evaluated in the full forecast package.

DISCUSSION

Issued at 242 AM CST Thu Dec 11 2025

High pressure across the area tonight has kept things quiet and allowed for breaks in the cloud cover. Pull up GOES-east water vapor imagery and look of to the west and see our next approaching system reaching South Dakota early this morning. Precipitation will reach northwest Iowa late morning and expand to central and southeast Iowa through midday. This diagonal band bisecting the state from NW to SE will produce a band of accumulating precipitation across the area this afternoon into this evening. Most of the precipitation in the band is expected to fall as snow with soundings showing deep saturation through the dendritic growth zone paired with moderate lift in this deformation zone. As the event progresses, the area of best lift will shift lower in the atmosphere below the DGZ, resulting in lower accumulation rates. Meanwhile, warm air advection will be pushing into the state out of the southwest and into central Iowa. This will result in a warm layer aloft. Models have trended colder in the 00z and 06z runs, with less ice accumulation than previous runs as the warm nose aloft does not quite exceed 0z (or at least, not for as long or over as deep a layer). Have trimmed down the ice accumulations to reflect recent trends, however it is important to note that the potential for ice accumulation has not gone away completely and event minor ice can result in hazardous driving conditions. As the evening update aptly noted, afternoon insolation should help to mitigate slick spots early in the afternoon, however late afternoon and through the evening conditions will likely worsen as the sun goes down. Slack spots will likely make for hazardous travel during the evening commute. As the event tapers off late tonight, the HRRR has noted a brief period with lack of ice introduction resulting in freezing drizzle. This should be short lived and light, but would add to any earlier ice accumulations. It is also worth noting that the synoptics (GFS/Euro) have been further south with the band bisecting the state practically corner to corner from NW to SE, the CAMs favor a solution further northeast. With placement still uncertain, have issued a winter weather advisory for areas expecting to see 2-3 inches of snow. These are marginal totals for an advisory, but the the possibility of a wintry mix including ice accumulation and the event occurring through the evening commute, felt it would best convey potential impacts. Adjustments may yet be needed if the band shifts or if higher ice accumulations look possible.

There will be a lull in activity on Friday before the next wave in this wave train in arrives on Saturday. As a fellow forecaster noted, this should be the caboose in the wave train as the following several days look to remain dry and warming back towards seasonal temperatures for this time of year. Before that, the Saturday system. This embedded shortwave passes quickly across the area on Saturday the the deformation zone passing over Iowa. Conditions are cold enough that snow is the only precipitation type with 1-3 inches expected through the day. Cold air pushes into the area on Saturday with morning lows from the single digits above zero south to below zero north (and wind chill from 0 south to -20 north). Reinforcing cold air follows the Saturday snow and overnight lows thorugh Sunday morning will be well below zero. Sunday morning wind chills will be in the the -20 to -30s across most of Iowa. Cold Weather Advisories and Extreme Cold Warnings are expected this weekend. Dry conditions settle in Sunday through at least the first half of next week with temperatures moderating back into the 30s to low 40s.

AVIATION /18Z TAFS THROUGH 18Z FRIDAY/

Issued at 1136 AM CST Thu Dec 11 2025

Main change has been a southwestward shift in the snow band and a downtrend in ice potential. So changed the prevailing weather groups to -SN and removed FZRA. That being said, some FZRA could mix in, but accumulations will be minimal.

Added a Prob30 group for 1/2sm vsbys because snowfall rates will likely peak near the terminal in the Prob30 window. The uncertainty comes from how narrow the band will be, which at issuance time looks to just miss the terminal to the northeast. Will AMD to TEMPO if upstream obs warrant. Similar adjustments have been made to KOTM, but with 1SM.

IFR cigs will linger for much of the rest of the period. Easterly winds will shift to northwesterly through the night. Some freezing drizzle possible at 12z, but not enough factors supporting it to add to the TAF at this time.

DMX WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory until midnight CST tonight for IAZ004>006-015>017-023>027-033>039-046>050-060>062-074-075-086.