textproduct: Des Moines

KEY MESSAGES

- Hot and humid conditions persist through Independence Day. If outdoor for a prolonged period, take frequent hydration breaks in the shade or A/C if possible.

- Storm chances highest over western into northern Iowa each late afternoon and night from this evening through at least Friday.

- Severe storm risks exists this evening/tonight and each late afternoon into some portion of the night Wednesday and Thursday. Locally heavy rainfall risk is highest in urban areas from today through Saturday.

SHORT TERM /THROUGH THURSDAY NIGHT/

Issued at 211 PM CDT Tue Jun 30 2026

An amplified flow pattern is shown early this afternoon in the GOES- East upper level water vapor imagery with a ridge centered over the Tennessee River Valley with a mean trough over the western CONUS with embedded shortwave riding through its flow. One of these shortwaves is wounded up over Lake Winnipeg with another in the Great Basin. For Iowa, it is another hot, humid, and breezy day under a partly to mostly sunny sky as the expansive high pressure remains centered over Tennessee. There were some showers and storms over mainly north central Iowa this morning on the periphery of the high's influence, but those have moved off to the north. Additional storms during the daytime hours are unlikely as these will be squelched by 13C or higher 700mb temperatures over all but northwestern Iowa. High temperatures this afternoon were trimmed back a few degrees after our forecast highs yesterday overperformed on average by about 2.5 degrees. Still, this doesn't take much of an edge off of the conditions with temperatures peaking in the lower half of the 90s and heat index values between 95 and 105 degrees. The Excessive Heat Warning looks well in place through this evening.

As we head into the evening, forcing will be increasing along an elevated boundary draped from east-northeast to west-southwest, near if not north of I-80. As theta-e advection along with a 850mb low level jet begins to increase into Iowa, expect there will be attempts at storms by mid-evening. These may not sustain themselves initially until the stronger forcing arrives around or after midnight. These storms will have no lack of instability with over 3000 J/kg of most unstable CAPE, but deep layer shear will be marginal up to 30 knots. Forecast soundings over northern Iowa show a large amount of dry air aloft contributing to downdraft CAPE values in excess of 1000 J/kg. This dry air can be entrained into thunderstorm downdrafts creating damaging wind gusts and this looks to be the primary concern. The mid-level lapse rates are around 7C/km, but given the lack of deep shear large hail looks to be a secondary concern.

These storms should be shifting out of the state shortly after sunrise Wednesday with the rest of the daytime hours expected to be somewhat similar to today/Tuesday. Hot and humid conditions will persist with the Excessive Heat Warning prevailing, though heat index values will be a bit lower over western Iowa where the warning will expire this Tuesday evening. Storm potential will exist once again very late Wednesday afternoon or Wednesday evening over northern Iowa or southern Minnesota along a warm front or stationary boundary. Forecast soundings show a high instability environment with deep layer shear between 30 and 35 knots pointing to supercell potential. Downdraft CAPE values will still be high raising concern for damaging straight line winds with steep lapse rates contributing to fast parcel accelerations and large hail risk. If storms stay surface based, low level storm relative helicity (SRH) values are sufficient for a tornado potential, especially if a storm can anchor and ride along the boundary increasing its relative SRH. Locally heavy rainfall will be possible as well given the parameters, though nothing stands out of great concern at the moment beyond an isolated, likely urban area flash flood threat. This activity should be moving away from our forecast area overnight as soon as midnight or as late at 6am.

A change on Thursday may be more cloud cover over a larger portion of the state as the center of the high begins to slowly move towards the Appalachians. Northern into western Iowa still looks to be where the storm potential will once again be higher late in the afternoon into the evening hours. With less clouds and storm potential along with the highest heat index values south of I-80, have extended the Excessive Heat Warning another day in many of our southern Iowa counties. While the values may not be as impressive as earlier this week, the successive days of around of above 100 degrees leads to this decision. Will have to monitor clouds and storm potential in case adjustments are needed.

LONG TERM /FRIDAY THROUGH TUESDAY/

Issued at 211 PM CDT Tue Jun 30 2026

The center of the ridge will shift farther to the Middle Atlantic Coast by the start of the Fourth of July weekend with the flow becoming more zonal across the country. The airmass looks to remain sultry through Saturday, which may necessitate additional heat headlines, especially given the many outdoor activities and celebrations around Independence Day. There also looks to be more late afternoon and overnight storm chances Friday and Saturday with the latter potentially aided by a shortwave trough riding through the flow into Iowa Saturday night. However, specifics on any severe concerns, location, and storm timing are far too fuzzy at this time to detail. Precipitable water values and warm cloud depths will be seasonal to seasonally high with storms motions that look to be their slowest Friday night through Saturday night. This parameter space points to the potential for locally heavy rainfall. Flash flooding concerns are noted by WPC with a marginal risk of excessive rainfall each day from today/Tuesday through Saturday. This concern, at least at this point in time, would be highest over any urbanized area as NASA SPoRT soil moisture percentiles are at the 30th percentile or lower over northern Iowa through much of the western half of the state. However, there could be corridors, especially in northern and perhaps western Iowa, that are primed with rainfall before this weekend.

The zonal pattern will evolve into a ridge developing over the western US by late in the weekend into early next week as the shortwave trough moves away from Iowa. It will turn less hot Sunday into Monday along with lowering chances for storms behind the shortwave trough.

AVIATION /00Z TAFS THROUGH 00Z THURSDAY/

Issued at 700 PM CDT Tue Jun 30 2026

VFR conditions will prevail for the remainder of Tuesday evening. Storm chances return to northern Iowa around 06Z, bringing medium confidence for direct terminal impacts at both KFOD and KMCW; PROB30 groups have been continued to reflect this timing. Additionally, strengthening low-level winds will introduce LLWS concerns at all terminals overnight until sunrise Wednesday. Diurnal winds and cumulus fields will increase mid- to-late Wednesday morning with the return of daytime heating.

DMX WATCHES/WARNINGS/ADVISORIES

Extreme Heat Warning until 9 PM CDT this evening for IAZ004>006-015-023-024-033-034-044-045-057-070. Extreme Heat Warning until 10 PM CDT Wednesday for IAZ007-016- 017-025>028-035>039-046>050-058-071-081. Extreme Heat Warning until 10 PM CDT Thursday for IAZ059>062- 072>075-082>086-092>097.