textproduct: Des Moines

KEY MESSAGES

- Quiet today and tonight

- Thunderstorm chances return tomorrow, mainly in northern Iowa with isolated storms possible farther south. Wind, hail, and locally heavy rainfall are the main concerns.

- Additional shower and thunderstorm chances into Thursday and Friday, with more limited severe weather chances.

UPDATE

Issued at 207 AM CDT Wed Jun 11 2025

The short-term forecast through tonight has not changed appreciably since the yesterday afternoon discussion, but the Thursday daytime period has come into somewhat better focus. Around sunrise Thursday convection should be ongoing near/just north of the IA/MN border as discussed below, while another slug of moisture coincident with a weak 500 mb low drifts north northeastward across Missouri. A surface high pressure ridge will squeeze in between these two systems right across most of Iowa, and should provide a relatively dry and warm day for much of our service area, excepting the far north. Accordingly, have bumped temperatures up by a few degrees and lowered sky cover a bit. Also kept the southern half of the area dry through the morning, but maintained low (20-30%) POPs there the afternoon as some CAMs are depicting isolated convection coincident with peak heating and the approach of the weak Missouri low scraping by our southeastern corner. Shear fields continue to look fairly weak for the most part on Thursday, but more storms developing near the eastern South Dakota/Nebraska border will drift into our northwestern counties late Thursday night and could pose a marginal wind threat given the degree of instability present. This is well covered by the SPC Day 2 outlook.

DISCUSSION

Issued at 249 PM CDT Tue Jun 10 2025

The upper level pattern becomes more zonal overnight tonight into Tuesday as the ridge to our west breaks down ahead of multiple shortwaves/troughing developing to the north and west. At the same time, a closed upper low will develop over the southern CONUS. Warm, moist air streaming north ahead of the closed low to the south will meet the troughing to the north, generating a warm/stationary front draped from west to east and roughly between Highway 20 and Highway 18. This front will be the focal point for showers and thunderstorms as mid- and upper- level forcing passes along the boundary through Wednesday afternoon and evening.

There will be no lack of moisture transport on Wednesday as the gulf moisture from the south phases with a fairly healthy pacific moisture stream, pushing PWAT values to around 1.5-1.6", with locally higher values nearing 2". Normal PWAT values for this time of the year are right around 1", so not significantly higher than normal but around 125% to 150% of normal, suggesting efficient rainfall is possible. In addition to the moisture, temperatures will warm into the upper 80s to low 90s tomorrow, leading to a very unstable airmass along and south of the boundary. MUCAPE values of 2500 to 3000 J/kg will be common, and model soundings suggest this being primarily surface based south of the boundary through the afternoon. Any displaced parcels will allow for rigorous updrafts capable of producing hail and heavy rainfall. Similarly, the warm temperatures with dewpoints in the 60s to near 70 suggest a well mixed boundary layer in the afternoon tomorrow. Soundings show the typical inverted-v sounding with downdraft CAPE values over 1000 J/kg. These suggest the potential for strong downdraft winds as heavy rain falls and evaporates, as well as efficient cold pool production which could lead to additional wind concerns as they propagate outward.

While the thermodynamic portfolio is robust tomorrow, relatively weak flow will keep 0-6 km shear values below 30 kts in the warm sector, preventing balance between the cold pool and environment and limiting widespread organized convection. The environment will still be conducive for severe hail and strong winds as updrafts initially go up, but the lack of shear will limit the longevity of severe storms and the residence time needed for larger hail stone production. Wind shear does improve along and north of the boundary, making this area slightly more favorable for more organized convection, especially if storms can tap into the vorticity along the surface boundary. The mean wind will be oriented along the boundary, although storms may tend to drift more southeasterly as additional updraft development occurs along outflow from the initial convection.

The severe weather chances will be most likely in the late afternoon and evening, with diminishing likelihood for severe impacts overnight, especially with a meager low level jet. There will still be a steady stream of moisture through the area with showers and storms continuing along and north of the Iowa/Minnesota border overnight. This should give some relief from heavy rainfall along the surface front in northern Iowa, but will want to watch how the front evolves and how far south it progresses in the evening, as this could bring overnight precipitation chances further south as well, which would further exacerbate any heavy rain impacts. For more information on potential impacts due to heavy rain, see the hydro discussion below.

While Wednesday evening certainly has the best chances for severe weather, shower and thunderstorm chances persist into Thursday and Friday as the upper low continues to pull moisture up from the south and another lobe of forcing passes through the zonal flow to our north. The warm front will drift back north with the second wave, taking the better wind fields with it and leaving an unstable but weakly forced environment in it's wake. This will lead to more isolated severe weather chances in northwest Iowa, with more widespread showers and sub-severe storms. Recent guidance now has the better rainfall amounts rooted to the better forcing to our north in Minnesota, with more marginal amounts overhead. That said, Thursdays storms will be somewhat dependent on how things evolve on Wednesday, so will want to keep an eye on model and observational trends through tomorrow, as a southerly trend could cause more heavy rainfall to fall in the same places they occur on Wednesday.

AVIATION /06Z TAFS THROUGH 06Z THURSDAY/

Issued at 1113 PM CDT Tue Jun 10 2025

Mainly south/southwest wind overnight that will be mostly less than 12 kts. The wind will become more erratic wind direction with a boundary moving into northern Iowa with east to northeast flow on the north side of the boundary and south elsewhere. KOTM and KDSM will have little to no impacts during the period outside of localized low level wind shear that is possible all sites tonight. KMCW/KFOD/KALO with have chances for showers and thunderstorms from late afternoon and into the evening. This activity may be accompanied by MVFR cigs as well.

HYDROLOGY

Issued at 249 PM CDT Tue Jun 10 2025

Heavy rainfall over the next few days may lead to flooding concerns including flash flooding, urban flooding and river flooding. Location, intensity, duration and amount of QPF will be key in the ultimate flood risk. With the initial active weather setup potentially affecting subsequent active weather setups, there is more uncertainty than normal regarding overall QPF.

Latest model/NBM QPF and National Water Model (NWM) output suggests the most likely scenario would be some isolated flash flooding / scattered ponding of water especially in urban areas in conjunction with the initial active weather late Wednesday into Wednesday night across roughly the northern half of the CWA especially north of U.S. Highway 20. Location and evolution of subsequent active weather and QPF is more uncertain, however present thinking is that the rainfall would lead to additional isolated flash flooding / scattered ponding especially in urban areas early Thursday morning onward. In terms of river flooding, scattered significant within-bank rises are expected. Some locations are expected to approach or exceed the the bankfull to action stage range. Isolated minor river flooding is possible as well. The timing of the river responses would be mainly from Thursday afternoon onward into the weekend.

Since the above information is based on conditions and QPF as of the forecast discussion time, any deviations in QPF going forward may affect the anticipated flooding as well. This event will bear watching especially from a hydrologic perspective over the next few days.

DMX WATCHES/WARNINGS/ADVISORIES

None.