textproduct: Des Moines

KEY MESSAGES

- Areas of dense fog likely tonight. Dense Fog Advisory in effect for about the northern and eastern half of our area overnight into Monday morning.

- Gray and soggy Monday, with fog giving way to light rain and thickening clouds.

- Much colder from around Tuesday night through the end of the week, with blustery winds Tuesday night through Wednesday. Precipitation chances return around next weekend, with a potential for accumulating snowfall but details of timing and magnitude still unclear.

DISCUSSION

Issued at 233 PM CST Sun Nov 23 2025

It has been another mild and sunny day, with light south breezes and temperatures soaring well into the 50s once again. Unfortunately, today will be the last hurrah for such pleasantness for quite some time.

A 500 MB low is moving east northeastward across Colorado this afternoon, and will meander eastward across Kansas on Monday. Meanwhile, an energetic shortwave trough will come ashore over the Pacific Northwest coast later tonight and move quickly across the northern U.S. Rockies on Monday. At the surface, a low pressure center has developed in eastern Colorado and a broad trough will extend northeastward from it tonight, acting as a weak warm front developing over Iowa and nearing the IA/MN border by Monday. Within broad but modest south southwesterly flow ahead of this system, forecast soundings indicate substantial warm air and moisture advection overnight, as well as clouds associated with the approaching trough spreading in late. Rising surface dewpoints, weak but organized SSW low-level flow, increasing clouds, and warming aloft will all support steady or slowly rising temperatures overnight and have maintained that trend in the ongoing forecast. Meanwhile, fog and stratus that developed across parts of Iowa last night dissipated after sunrise this morning, however, a large fog bank has persisted across central Missouri and is beginning to creep northward this afternoon per visible satellite imagery. This fog bank should advect northward and expand across Iowa this evening and overnight. With skies clear across our forecast area initially, once the sun sets temperatures should fall pretty quickly, even with winds not calm, then as higher dewpoints surge northward conditions will be quite conducive for fog expansion. High-res guidance is all over it with the NAM, HRRR, RAP, and HREF all indicating areas of dense fog across at least portions of our area tonight. Confidence is highest in our northern and eastern counties, where clouds and light rain from the approaching system will arrive later, and in the north where the effective warm front mentioned earlier will likely set up later tonight into Monday morning. HREF probabilities of <1/4 mile visibility are 60-80% in that area and forecast soundings almost unanimously indicate a fully saturated layer from the surface up to an inversion around 1000-2000 ft above the surface. Given the set-up, model guidance, and climatology, have chosen to be aggressive in issuing a Dense Fog Advisory for generally the northern and eastern half of our service area, beginning late this evening in the southeast and at Midnight in the north and northeast.

On Monday and Monday night, the trough approaching from the west/southwest will slide eastward across Iowa and Missouri, making for a gray and soggy day as areas of light rain or drizzle move through and fog is slow to dissipate beneath thickening clouds. Rainfall amounts will be light and no impacts from that are expected, but it will make things dreary. On Monday night, even as this system begins to move away to our east, the previously discussed and more robust 500 MB shortwave will emerge from the northern U.S. Rockies and dig southeastward into the Dakotas, then close off and deepen further over Minnesota and Wisconsin on Tuesday-Tuesday night. Snow is forecast to develop in the deformation zone on the back side of the deepening cyclone, but will primarily track across Minnesota with light snow just scraping far northern Iowa. At this time we are forecasting negligible accumulations right along the border and nothing farther south into our area, however this will bear watching to ensure no southward shift in the forecast which could have some impact on our northern counties. Of greater consequence to most of Iowa is that this second system will produce blustery northwest winds and send temperatures plummeting from late Tuesday onward. Highs on Wednesday are forecast to be 15-20 degrees below Tuesday levels, and by sunrise Thursday readings will fall into the mid-teens to lower 20s across our entire forecast area. The result will be the first real taste of winter air for most of Iowa from Tuesday night onward.

The cyclone over the Upper Midwest will move away to our east from Wednesday onward, but leave brisk northwesterly steering flow overhead supporting a continuation of cold but dry weather for a couple of days. Around Friday morning or so another energetic 500 MB trough will come ashore over the Pacific Northwest coast, and quickly carve out a deep western U.S. trough by this weekend. This will bring our flow aloft around to southwesterly, promoting increasing moisture and ejecting ribbons of vorticity out of the larger trough up through the Midwest and a region of broad forcing for ascent. This leads to increasing precipitation chances for several days at the end of next week into early next week, and forecast surface temperatures generally support snow as the dominant precipitation time for much of that period, but with rain or a mix certainly being possible at times. At this range however, the details of when and where regions of enhanced lift will allow for any impactful precipitation amounts, and to what extent those opportunities could manifest as accumulating snowfall, are still quite uncertain. Anyone with travel plans around next weekend should monitor the forecast updates in the coming days as we gradually begin to refine these details and determine any potential for meaningful snowfall that could impact post-Thanksgiving travel plans.

AVIATION /18Z TAFS THROUGH 18Z MONDAY/

Issued at 1118 AM CST Sun Nov 23 2025

VFR conditions are forecast through the next six hours with clear skies and light winds. However, tonight fog and stratus will spread from Missouri northward across Iowa, resulting in IFR or lower conditions for most of the period from late evening/tonight through the end of the 18Z TAF period. Have indicated this with broad timing of trends, but expect to refine timing and magnitude of visibility/ceiling reductions in later issuances and amendments will be likely tonight based on short- term observational trends.

DMX WATCHES/WARNINGS/ADVISORIES

Dense Fog Advisory from midnight tonight to 10 AM CST Monday for IAZ004>007-015>017-023>028-035>039-047>050.

Dense Fog Advisory from 9 PM this evening to 10 AM CST Monday for IAZ061-062-074-075-085-086-096-097.