textproduct: Des Moines

KEY MESSAGES

- Much cooler from today through Friday, with daily highs in the mid-50s to mid-60s. Late tonight/early Wednesday low temperatures will range in the mid-40s southeast to mid/upper 30s north and west, where patchy frost will be possible.

- The forecast is dry from tonight through Wednesday, then rain chances will gradually increase Thursday and peak on Friday when showers will be likely (60-80% chance) with a few thunderstorms. Severe weather is unlikely.

- Temperatures will warm this weekend into early next week, with daily highs returning to the 70s and 80s. Rain chances will be generally lower during that time, but some showers or storms may still be possible at times.

SHORT TERM /THROUGH THURSDAY/

Issued at 130 PM CDT Tue May 19 2026

It has been decidedly cooler today, with thick stratus clouds and brisk northwest winds keeping temperatures in the upper 40s northwest to upper 50s southeast early this afternoon. Cold air advection will weaken this afternoon, but the clouds will still inhibit warming and temperatures will peak in the mid-50s northwest to lower 60s southeast today. Meanwhile, a large surface high pressure ridge is building across the Dakotas, and will move eastward into Minnesota and northern Iowa tonight. This will cause winds to steadily diminish later today and this evening, becoming much lighter and gradually turning from northwest to northeast by Wednesday morning.

The primary forecast question during this time pertains to the degree and timing of clearing that occurs, and how far temperatures fall overnight as a result. Forecast soundings depict a rapid drying of the cloud layer during the evening, roughly coincident and just after sunset, which is consistent with climatological expectations. High-resolution model output also supports this scenario, however, thus far today the model depiction of cloud cover evolution has been well below what is being observed in satellite imagery. That being said, over the last couple hours some slight clearing has been evident on satellite upstream, closer to the center of the high pressure ridge, and the expectation is that we will in fact gradually clear out this evening and early tonight. This will result in prime conditions for radiational cooling overnight into early Wednesday, especially in our northern and western areas where clearing will occur earlier, and where the high pressure area will build in sooner. Temperatures in these areas will fall into the 30s by sunrise Wednesday, however, frost formation is still uncertain due to two primary factors. One is that dewpoints are predicted to be in the mid-30s, so temperatures may not fall below the 35-37 degree range. The other is that while winds will be light, the core of the high pressure ridge will remain over Minnesota and South Dakota, and winds in our area may not go completely calm. Have included an hour or two of patchy frost in just a few spots in our northern counties, but have held off on a Frost Advisory for now due to these mitigating factors. Whether an advisory becomes warranted later today or not, there is some possibility of frost particularly in low-lying areas of northern Iowa and over toward the Nish valley.

From Wednesday into Thursday Iowa will lie beneath west southwesterly 500 MB steering flow, through which a couple of very subtle shortwave impulses will move. While these will provide some modest but broad forcing for ascent, initially any precipitation attempting to develop will be fighting low-level dry air/subsidence associated with the slowly departing surface high pressure system. Initially this will result in increasing mid/high level clouds on Wednesday, then by Wednesday night and Thursday we will see gradually increasing rain chances returning to the area, especially our southern counties farther from the influence of the departing high pressure. PoPs increase to 40-50% in that area by Thursday afternoon, however, a lack of instability will preclude thunderstorms or severe weather during this time. The increasing clouds and persistent easterly surface flow out of the high pressure area will support a continuation of cooler weather, however, with high temperatures both Wednesday and Thursday ranging in the upper 50s to mid-60s.

LONG TERM /THURSDAY NIGHT THROUGH MONDAY/

Issued at 130 PM CDT Tue May 19 2026

By Thursday evening a 500 MB trough will be moving over the U.S. Rockies, and will subsequently become negatively tilted as it emerges over the northern High Plains Thursday night and Friday. As this trough swings across the region, it will generate a broad swath of rain associated with surging moisture, increasing winds aloft, and modest theta-e advection. This swath will enter our southwestern counties Thursday night and move across our forecast area on Friday. Instability will be very limited with surface temperatures only in the 50s to lower 60s, but will still be sufficient for a few elevated thunderstorms at times though severe weather will be unlikely. The rain will move out to the northeast late Friday, after which one last lobe of vorticity clearing out the larger-scale trough will cross Iowa late Saturday. Moisture will be much more limited and it remains to be seen whether any additional showers or thunderstorms will occur in association with that feature. For now 70-80% POPs are carried across Iowa Thursday night into Friday, with only 10-20% POPs on Saturday.

In the first half of next week, from around Sunday to Wednesday, a large thermal trough will build over the central U.S., as a blocking pattern sets up with a large 500 MB low forming over the western U.S. Synoptically this would tend to support significantly warmer and generally dry weather for our region. However, there will initially be a week 500 MB trough/low over Texas as the larger ridge begins forming on Sunday, and long- range models are at odds with how that feature will interact with the ridge. The EC and GEM essentially wash out the low as it is overwhelmed by the larger thermal ridge, however, the GFS maintains it as a discrete feature that becomes trapped in the ridge and advects slowly northward over Iowa early next week. This leads to rapidly decreasing confidence in forecast details in the outer periods, however, even in the GFS solution generally warmer temperatures would still be favored, and it seems a safe bet that the cool weather of today and the next several days will be broken as we head into next week.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 1217 PM CDT Tue May 19 2026

Low stratus clouds are blanketing the area with MVFR ceilings this afternoon. These will slowly rise this afternoon and eventually clear out late today or early tonight, though timing of the clearing is uncertain. Have indicated these trends in the 18Z TAFs, but amendments are possible as the day progresses based on satellite and surface observations. Once ceilings break up and/or rise above FL030, VFR conditions are then forecast for the remainder of the TAF period.

DMX WATCHES/WARNINGS/ADVISORIES

None.