textproduct: Des Moines

KEY MESSAGES

- Enhanced Risk (level 3 of 5) of severe storms this evening into tonight. Tornadoes, damaging winds, and large hail are also possible as a line of storms moves west to east across the state. The highest probability of severe weather will be from southwestern into central and south central Iowa.

- Moderate Risk (level 3 of 4) of flash flooding in extreme southwest Iowa this evening into tonight, with a Slight Risk (level 2 of 4) across the remainder of our forecast area. Flood Watches are in effect from extreme southwest Iowa into adjacent parts of Nebraska and Missouri.

- Breezy late tonight into Tuesday, with cooler and drier weather through Wednesday. Lows in the 30s north and west Wednesday morning.

- Rain chances return in the latter part of the week, especially around Thursday night and Friday. Warmer weather forecast next week.

SHORT TERM /THROUGH TUESDAY/

Issued at 122 PM CDT Mon May 18 2026

A deep 500 MB trough is digging across the Rockies and western High Plains this afternoon, with a pronounced vorticity maximum rounding the base and kicking out over eastern Colorado as a negatively tilted shortwave that will move northeastward over NE/IA/SD/MN this evening into early tonight. At the surface, an elongated inverted trough stretches from an attendant surface low developing near the TX panhandle northeastward over Kansas and Iowa, roughly from around Red Oak to Cresco at this time. West of the trough low stratus clouds have socked in the region over northwestern Iowa and adjacent states, with limited insolation reinforcing the developing baroclinic zone/effective cold front, while east of the trough skies are partly cloudy with cirrus also streaming overhead, resulting in limited insolation but combined with south southwesterly flow to promote modest diurnal warming.

As the 500 MB trough approaches the region this afternoon, instability will increase to extreme levels over northeastern Kansas where more clearing has occurred, and explosive convection is anticipated in that area. The storms will then propagate and expand northeastward ahead of the trough. Forecast soundings in our area, especially central to southwestern areas, depict very strong instability with CAPE values of 3000-4000 J/KG by late afternoon/early evening, along with strong shear as even 0-500 meter SRH values exceed 200 m2/s2. This environment will support supercells with all severe weather hazards possible including large hail, damaging winds, and tornadoes. However, satellite imagery indicates low-level cloud cover is steady to increasing across much of our area and even into northwest Missouri, and it remains to be seen how much clearing will occur. Short-range mesoanalysis reflects this potential for limited insolation with 0-3 KM CAPE values less than 50 J/KG through the afternoon. This would limit near-surface stretching and may somewhat mitigate tornado potential in much of our area. That being said, if any clearing does reach our area then 0-3 KM CAPE and surface stretching would increase rapidly, supporting a heightened tornado threat. At this time it appears this potential is highest in our southwestern counties and gradually diminishing to the north and east, however, these trends will be closely monitored as the day/evening progresses. Even if tornadoes are limited in our area, elevated supercells progressing into southern and central Iowa this evening would still be capable of producing large hail and damaging wind gusts, and the SPC has maintained an Enhanced severe weather risk across much of our area accordingly. Once the initial round of thunderstorms, forced by the 500 MB trough, exits our eastern/southeastern counties around midnight or so, some elevated showers/thunderstorms will linger overnight but be weaker with more limited forcing and instability, and a rapidly decreasing severe weather threat later tonight.

The aforementioned cold front will be exiting our southeastern counties by early Tuesday morning, with any lingering elevated showers or thunderstorms also moving off during the morning. Behind the boundary, northwest winds will surge and bring notably cooler weather across the area. Daytime highs Tuesday will be about 15 degrees lower than today. Meanwhile, gusty northwest winds spreading in behind the front late tonight will continue through Tuesday morning as moderate cold air advection, surface pressure rises, and a relatively tight pressure gradient work through Iowa. Even then, wind potential is limited by the shallow nature of the mixed layer, with forecast soundings indicating top of the ML winds around 25-30 KT during that time. Winds will then slowly diminish during the afternoon as the tighter pressure gradient moves eastward and cold air advection moderates.

LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/

Issued at 1250 PM CDT Mon May 18 2026

By Tuesday evening a large surface high pressure area will be building down the northern High Plains and on Tuesday night will settle across Iowa and Minnesota. This will lead to rapidly diminishing winds around sunset Tuesday and much cooler temperatures Tuesday night as winds go light or calm and skies clear out, especially near the Minnesota border closer to the center of the high and where chances for high cirrus clouds are lowest. The result is forecast low temperatures early Wednesday morning in the mid-to- upper 30s in our northern and western counties, but any frost potential will be dependent on skies remaining clear and winds becoming calm. This possibility will be better assessed tonight and tomorrow.

From Wednesday through the end of this week a weak 500 MB low will meander across southwestern Canada, with cyclonic flow extending down across the U.S. Rockies and near Iowa. A series of shortwave impulses moving through the flow will result in a return of rain chances for our forecast area. Initially, on Wednesday, the proximity of the slowly departing surface high pressure area and associated dry air will inhibit any precipitation chances. By Thursday atmospheric moisture and instability will be gradually increasing however, and around Thursday night and Friday the most pronounced 500 MB trough will move overhead and bring higher rain chances (50-70%). In wake of that trough, next weekend, weak subsidence may follow along with weak surface riding, supporting mostly dry weather and much lower rain chances (10-20%). Also of note is a consistent signal for some degree of thermal ridging around Sunday into early next week, especially from the EC and GEM, lending confidence to warmer temperatures at the end of the current 7-day forecast. Climate Prediction Center outlooks also support a likelihood of warmer than normal temperatures heading into next week.

AVIATION /18Z TAFS THROUGH 18Z TUESDAY/

Issued at 1245 PM CDT Mon May 18 2026

Low stratus clouds are producing IFR ceilings across much of northwest IA and MVFR ceilings into central IA early this afternoon. These will eventually spread eastward this evening and tonight ahead of an approaching cold front. Scattered SHRA/TSRA also expected this evening and overnight, with TSRA most likely between about 00Z and 06Z and SHRA more likely thereafter. Have attempted to time convective trends and maintained targeted TEMPO groups for TSRA, but expect updates/amendments later today. Widespread low ceilings around FL007-012 then expected later tonight, with winds turning to NW and increasing behind the front.

HYDROLOGY

Issued at 122 PM CDT Mon May 18 2026

Trends in flash flood tools suggest an increased risk of flash flooding from activity this evening into tonight. NWM rapid onset flooding (ROF) probabilities suggest the greatest risk will be across our southwest CWA. Soil moisture in that area is elevated due to heavy rainfall over the past few days. Heavy rainfall tonight may lead to flash flooding due to decreased soil capacity, or sheer rainfall intensity, or both. Main concerns will be in towns, however lower areas including road dips near creeks and small streams may see some issues as well. In addition, any flooding will occur mainly during the nighttime hours, which adds to its potential danger. For these reasons, we issued a flood watch for some of our southern counties through Tuesday morning.

Longer term, widespread river flooding is a minimal concern this week. Some moderate to significant within-bank rises are possible, however, especially in southern Iowa due to the combination of recent and potential future heavy rainfall.

DMX WATCHES/WARNINGS/ADVISORIES

Flood Watch from 5 PM CDT this afternoon through Tuesday morning for IAZ092>095.