textproduct: Des Moines

KEY MESSAGES

- Severe thunderstorms late this afternoon into this evening with large hail, damaging wind gust, and a few tornadoes possible. Heavy rainfall with the potential for flash flooding becomes an increasing concern overnight, particularly in cities and towns.

- Heavy rain and storm chances continue through at least the middle of the week and possibly the end of the week, leading to both flash flooding and river flooding concerns.

SHORT TERM /THROUGH WEDNESDAY/

Issued at 355 PM CDT Mon Sep 14 2026

The main weather theme over the period will be an active, wet pattern into this next weekend. Of course, there will be intervals of dry conditions, but rounds of rainfall will lead to increasing threats of flash flooding over varying parts of the state. All of the rainfall will also lead to rises on many if not most of our area waterways. In some places, this may push the rivers above flood stage. In addition, there is the risk of severe storms late this afternoon into this evening before the event transitions to heavy rainfall and flash flooding potential.

At present, the warm air advection wing of showers and thunderstorms has lifted northeastward through much of central Iowa early this afternoon. One of the forecast questions in previous days has been the amount of clearing and potential destabilization. GOES-East visible satellite imagery shows clearing in southwestern Iowa, which should migrate into central Iowa. To the north over northern Iowa, cloud cover may linger along with showers and this may create a differential heating boundary in addition to the synoptic cold front that will move in late today into tonight. Forecast soundings show low level warm layer capping, which is expected to inhibit any surface based storms. However, if storms are able to develop and overcome this inhibition through daytime boundary layer mixing, a few tornadoes would be possible given the strong kinematic environment. If surface based storms do not form, elevated storms are likely with strong MUCAPE values and mid-level lapse rates around 6C/km. As alluded to, there is no lack of kinematic forcing and shear with the right entrance region of a jet streak over the region. The storms being elevated does take some edge off of the impressive deep layer shear and storm relative helicity (SRH) values; however, the bulk shear and SRH values are still very favorable for severe storms, including elevated supercells, at least initially, until they merge into a line of storms. These storms would pose a risk of damaging wind gusts and hail with the hail risk waning as the storms merge into a line. As this occurs, the 0-3km shear values over 30 knots are in place to support a mesovortex potential, especially if a line of storms can orient itself north to south and thus more orthogonal to the shear vector. This remains to be seen whether anything to the surface could materialize.

While the severe risk will wane by late evening, the heavy rainfall and flash flooding potential will be going up through the evening. There will be strong moisture transport into the region on the nose of the low level jet upwards of 50 knots. This will bring precipitable water values (PWATS) up to around 2 inches with warm cloud depths topping 4000m - both unusually high for mid-September. The 850-300mb around 40 knots, this is a bit fast, but it is oriented fairly parallel to the boundary, which would result in repeat rounds of rain (aka training storms). Deterministic has 2 to 4 inches with ensemble localized probability matched mean (LPMM) from the HREF and REFS of 3 to 5 inches with localized bullseye amounts of 7" in southwest to northeast oriented swaths from somewhere in southwest Iowa into central Iowa. These are areas that saw significant rainfall middle of last week with additional rainfall across much of the area today. NASA SPoRT soil moisture percentiles show reduced capacity in those areas and a heightened risk of flash flood potential. Thus, the flash flood watch continues to look well placed for tonight into Tuesday morning with a slight expansion on the eastern side, which will be discussed more below.

The surface boundary is forecast to slip south of the state by midday Tuesday, but this does not mean the end of heavy rainfall, just a change in location. The elevated 850mb boundary never leaves the state and is forecast to lift back northward Tuesday night into Wednesday. So, while the highest PWATs may be pushed to around the Missouri border Tuesday night, they will move back northward, especially into the day Wednesday. Thus, the risk of heavy rainfall and flash flooding potential in play is highest from roughly I-80 southward through Tuesday night into Wednesday morning. With the risk for another round of heavy rainfall, have extended the flash flood watch through through midday Wednesday. There may be some adjustment in area, but for now have kept the watch as simple as possible. There does remain uncertainty in the amount of coverage of rain and storms on Wednesday, but the environment that will be in place will be favorable for heavy rainfall.

LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/

Issued at 209 PM CDT Mon Sep 14 2026

On a broad scale, Iowa will reside in generally fast flow around mid- level high pressure over the Deep South/Southeastern US, which will migrate slowly back to the west by week's end. In this fast flow, there will be a few shortwave troughs that pass through along with fragmented vorticity pieces, which will interface with a surface and elevated boundary draped across the region if not the state at times. The pattern is thus conducive to additional rounds of rainfall on what will be increasingly saturated soil and thus increased risk of flash flooding with each successive round of rain. This rainfall will route into waterways and lead to increasing river levels with additional details in the hydrology discussion below.

AVIATION /18Z TAFS THROUGH 18Z TUESDAY/

Issued at 1255 PM CDT Mon Sep 14 2026

Aviation concerns in this period are multi-fold with storm timing and impacts as well as low level wind shear. Showers and storms are moving to the northeast from FOD to GGI and will continue to lift in that fashion with brief bouts of IFR/MVFR restrictions possible as these pass over MCW/ALO in the next few hours. MVFR ceilings over northern Iowa are forecast to largely miss terminals so VFR conditions are expected after the storms this afternoon. However, additional storms are forecast into tonight sliding southeastward with time, but lingering the longest at DSM and OTM. Low level winds will also increase and create a period of wind shear. Precipitation will end over northern Iowa (e.g. FOD/MCW) by daybreak Tuesday and likely ALO by midday Tuesday.

HYDROLOGY

Issued at 210 PM CDT Mon Sep 14 2026

Flash flood and river flood concerns continue this week due to forecast multiple rounds of moderate to heavy rain. Flash flooding is more of a concern in the short term (later today into Tue morning). Short term National Water Model rapid onset flooding (ROF) guidance continues suggesting the greatest risk of flash flooding mainly between US Hwy 30 and the MO border, or roughly the same area included in the current flood watch. Most likely scenario will be ponding and low end flash flooding, however if QPF overachieves then pockets of more significant flash flooding may occur especially over urban areas. Since general storm motion is anticipated to be from W- E or SW-NE, small stream basins oriented in those directions would be most susceptible to flash flood impacts especially with training thunderstorm activity. Additional heavy rain will prolong flash flood concerns beyond Tue morning.

Longer term, river flood concerns exist too. The NBM-forced National Water Model (NWM) with 10 days of QPF shows significant within-bank rises on many streams across roughly the southeast half of the CWA, with some minor flooding possible. The GEFS-driven Hydrologic Ensemble Forecasting Service (HEFS) basically agrees, with 50% exceedance probabilities showing several streams reaching action stage or minor flood stage in central IA. The 30% exceedance probabilities show more pervasive minor flooding with a few locations at moderate flooding. The 10% level shows moderate to major river flooding across much of the southeast half of the CWA. Variations in QPF location and amounts as well as mesoscale features after each successive round will greatly dictate what happens, but the river flooding component is worth monitoring closely.

DMX WATCHES/WARNINGS/ADVISORIES

Flood Watch through Wednesday afternoon for IAZ025>028-034>039- 045>050-057>062-070>074-081>084-092>095.