textproduct: Des Moines

KEY MESSAGES

- Quiet weather from today into Thursday, with very low rain chances and generally pleasant conditions.

- Showers and thunderstorms then return to the area with low (20-30%) rain chances in the west Thursday afternoon/evening, then near a near 100% chance of rain Thursday night. There is a low chance of severe weather in western Iowa during the day, but a more widespread potential for heavy rain Thursday night into Friday morning.

- Additional rain and thunderstorms are possible on Friday, with a low chance of severe weather. Relatively cooler and quiet weather is then forecast Saturday through at least Monday.

SHORT TERM /THROUGH FRIDAY/

Issued at 121 PM CDT Wed Jul 29 2026

A large deep-layer ridge is in place across the southern and western U.S., which is fairly typical for late summer. 500 MB analysis shows the ridge/high centered near New Mexico, with light northwesterly flow over most of the Midwest and Iowa, on the northeastern periphery of the ridge. At the surface, a large high pressure area is centered over Lake Michigan today, with light southeasterly flow across Iowa. Some showers and thunderstorms have persisted from eastern Nebraska down into western Missouri, but have been shunted southwest of our forecast area by the influence of the surface high. Over the last couple of hours these storms have come very near the southwestern corner of our area around Taylor County, but have remained just outside and have already begun to slowly weaken as forcing wanes. This trend is expected to continue, but a few storms may manage to pop just inside our counties during peak heating this afternoon and have carried slight chance (20%) PoPs there accordingly.

Tonight, a mid-level shortwave trough, or more precisely a closely associated series of small impulses within a broad trough, will overtop the southwestern U.S. ridge and emerge over the High Plains. This will induce additional shower and thunderstorm development over the Dakotas, Nebraska, and Kansas overnight. This trough will then curve slowly southeastward and across Iowa, with the initial smaller impulse moving through our area around Thursday afternoon and evening, and the larger/deepening wave coming through Thursday night into Friday. As the initial impulse moves into the area it will be fading, and also fighting against some near-surface and mid-level dry air as it attempts to moisten the column and produce any showers. At this time it appears the dry air and fading forcing will win out, limiting any daytime convection to perhaps our far western counties. The majority of model solutions support this, with even the CAMs only showing a few specks of precipitation any farther east. Therefore, have maintained low (20-30%) PoPs only in our western fringe counties, with a dry forecast elsewhere through 00Z Friday (i.e., Thursday evening). That being said, forecast soundings do indicate that in those western counties, for example at Carroll, the mid-level dry air may be overcome by late in the day Thursday while the near-surface profile resembles the classic "inverted-V", but with limited dry air present. Given modest instability and deep- layer shear, this may support a narrow window of opportunity for isolated strong wind gusts, which is appropriately outlined by a Marginal Risk severe weather outlook from SPC in our far west.

The more widespread and consequential convection/rain event will come Thursday night into Friday, as the more primary mid-level trough moves overhead. Broad, seasonally strong forcing for ascent will accompany this feature, inducing surface cyclogenesis somewhere around far eastern South Dakota late Thursday night, with a trough stretching south southwest toward Kansas. Scattered thunderstorms will develop initially around Thursday afternoon across parts of South Dakota and Nebraska, then eventually congeal into a line or broken swath of storms Thursday evening and early Thursday night as the surface low/trough develops, then surge eastward across our forecast area overnight. It appears the storms will be outracing the supportive upper-level forcing and instability as they progress eastward and will experience a weakening trend as a result. MUCAPE values will generally be less than 1000 J/kg across central Iowa, with some areas only around 100-300 J/kg, and CAMs are largely picking up on this and depicting a steady weakening of overall echo intensity with eastward extent. Given a nearly saturated column and limited instability severe weather is not anticipated in our area Thursday night, however, there is definitely a heavy rain threat as discussed in detail in the HYDROLOGY section below.

The forecast for Friday is somewhat more complicated and hinges on low-confidence details. This primarily stems from the passage of the surface low discussed above, which is expected to move across Iowa from northwest to southeast during the day. Ahead of the low, the swath/line of showers and storms from Thursday night will exit our service area to the east on Friday morning. Meanwhile, a second swath of rainfall will develop with a deformation zone on the backside of the low. Between these two areas a relative dry sector may develop and is hinted at by some model solutions, particularly the NAM which depicts a very narrow swath of elevated instability and SBCAPE around 1500-2500 J/kg, 0-3kmAGL CAPE of ~300 J/kg, and an NST of 3-5 in a small area just ahead of and into the core of the approaching low. In the presence of strong low- level directional shear and low LCLs, this could indicate some possibility of tornadoes with any storms that develop during the day Friday as the system moves through. That being said, any such potential will be heavily dependent on the degree of clearing and low-level destabilization that can occur, and other solutions are less foreboding with forecast soundings showing a saturated column above the surface and much less available CAPE. Thus while the potential for severe weather remains low on Friday, there is some possibility depending on after-effects of the Thursday night storms, the degree of resultant clearing (or lack thereof), and the track and timing of the surface low.

LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/

Issued at 156 PM CDT Tue Jul 28 2026

By Friday night the low pressure system discussed in the SHORT TERM discussion above will be departing to the east/southeast, with some showers and perhaps a few thunderstorms lingering into the night before likewise departing. By Saturday the southwestern U.S. deep-layer ridge will have been shunted westward by the trough deepening over the central U.S., being centered somewhere around UT/AZ/NV by Saturday afternoon. This will result in cooler northerly mid-level steering flow over Iowa. Meanwhile, a surface ridge will build down the Upper Midwest in wake of Friday's low pressure system, bringing modest northerly breezes to Iowa by Saturday, then variable winds later in the weekend. While Friday temperatures will thus be generally cool due to rain and cloud cover, this weekend will present a more enjoyable "cool" weather spell with clearing skies, generally light winds, and highs in the low to mid 80s. This will be a far cry from the intense heat experienced across the region last weekend!

Next Monday will be largely the same as the weekend, with highs around the low to mid-80s, mostly sunny skies, and light winds as the surface ridge begins to move away to the east late in the day. Another surface trough/front will then approach from the Dakotas on Monday night and Tuesday, then stall out somewhere across the region around the middle of next week. Just after this, a broad 500 MB trough will round the base of a large gyre over Canada, passing through the Upper Midwest and providing some modest forcing for ascent sometime between Tuesday night and Thursday or so. This is a classic set-up for multiple rounds of showers and thunderstorms along the stalled frontal zone, however, it remains to be seen where that will occur or how the timing and details will shake out. At any rate, after a dry period from Friday night or Saturday through at least Monday, some chances for rain will return around the middle of next week.

AVIATION /06Z TAFS THROUGH 06Z FRIDAY/

Issued at 1042 PM CDT Wed Jul 29 2026

VFR conditions are expected through the 00z TAF period. A windshift from southeasterlies to southerlies is expected during the morning hours and some VFR smoke may linger at times during the day tomorrow. Showers and storms build in from the west at the end of the period, with enough confidence to introduce PROB30 groups at KDSM/KFOD. Occurrence is highly confident, timing is less confident and may need to be adjusted earlier or later in future issuances.

HYDROLOGY

Issued at 121 PM CDT Wed Jul 29 2026

Potential for heavy rainfall Thursday night into Friday morning as thunderstorms move across the area. NAM solutions depict very deep warm cloud depths around 5 KM and PWAT of 2.4-2.5" supporting very efficient rainfall processes. However, most other model solutions are a bit less extreme with warm cloud depths of 3-4 KM and PWAT of 2.0-2.1", which still support some heavy rain threat but not quite as alarming. In addition, there is broad consensus that the main swath of showers/storms moving through the area Thursday night will be progressive, limiting overall rainfall amounts and precluding any widespread Flood Watch at this time. That being said, with efficient rates likely, any deviant motion or backbuilding storms could lead to localized heavy rainfall amounts.

Risk of flash flooding (initially) and subsequent river flooding returns to our CWA late in the work week due to aforementioned heavy rain potential. For the flash flood risk, the most likely QPF scenario would mainly confine flash flood concerns to urban areas, where ponding or lower-end flash flooding may occur. Crops in the rural areas are well into their growth which tends to significantly increase infiltration capacity. Three potential gotchas are (1) slightly elevated soil moisture percentiles from central into northeast IA, which would increase the risk of runoff in rural areas; (2) intense rainfall rates may increase the risk of increased runoff in rural areas; (3) rainfall that overachieves vs. QPF would increase the flash flood risk even in rural areas. Rural rain amounts that exceed ~3-4 inches over a few hours would especially increase the risk.

River flood risk depends significantly on rainfall amounts, location and timing. Presently the most likely scenario is moderate to significant within-bank rises on many streams with a few river forecast points possibly reaching action stage, but remaining below flood stage. Rainfall that overachieves vs. QPF would result in higher stages, with possibly up to a few river forecast points reaching minor flooding.

DMX WATCHES/WARNINGS/ADVISORIES

None.