textproduct: Des Moines

KEY MESSAGES

- A prolonged period of hot and humid conditions will continue through the coming week and potentially the Fourth of July weekend. An Extreme Heat Warning is in effect for the entire area.

- Thunderstorms will be possible tonight and again Tuesday night, mainly over the northwestern half or so of Iowa. Some storms could be strong to severe, with damaging winds and hail the primary threats. However, confidence in the occurrence and placement of any storms is relatively low at this time.

SHORT TERM /THROUGH TUESDAY/

Issued at 131 AM CDT Mon Jun 29 2026

A much anticipated and well-advertised 500 MB blocking pattern has become established across the Continental U.S., with a large trough in the west and a "heat dome" of high pressure in the south and southeast. Iowa remains beneath southwesterly steering flow between these two systems, and while we are more under the influence of the high and thus seeing much warmer and more humid conditions, we are far enough on its periphery to be potentially subjected to convective effects from shortwaves ejecting out of the western trough. For today this will not be an issue and with a relatively tight surface pressure gradient promoting gusty southerly breezes, a hot and muggy day is in store across our service area. With temperatures peaking in the low to mid-90s and sweltering mid-70s dewpoints, Heat Index values will peak around 104-108 degrees area-wide. Not only will this be quite unpleasant for anyone venturing outdoors, it will also be hazardous for those not taking appropriate precautions, especially in vulnerable populations. With the heat expected to persist into Tuesday and at least to some extent beyond, an Extreme Heat Warning remains in effect for the entire area.

By this evening a positively tilted shortwave will be rounding the southeastern quadrant of the western U.S. 500 MB trough over the Dakotas. The southeastern periphery of this shortwave will move from northeastern Nebraska across southeastern South Dakota, far northwestern Iowa, and into Minnesota overnight and will likely force convective initiation with clusters of thunderstorms affecting parts of the region. However, there is general agreement that the nocturnal 850 MB southwesterly jet will be located across eastern Kansas and roughly central Iowa overnight, well displaced from the forcing associated with the 500 MB impulse. Central Iowa, closer to the influence of the southeastern U.S. heat dome, will maintain a good EML overnight as evidenced by forecast soundings, which should inhibit any convection. However, there may be a metaphorical Venn diagram overlap in northwestern Iowa and adjacent states where the forcing on the periphery of the 500 MB shortwave, strong instability, and the left exit region of the LLJ create a window of opportunity for thunderstorms which could be severe. With strong winds just off the deck (40-50 KT at 3000 FT at Estherville) and ample CAPE (2000-4000 J/KG depending on which model one looks at), any storms that are able to develop may be capable of damaging winds and hail. That being said, there are so many uncertainties in the location of the LLJ, location/magnitude of the mid-level lift, and strength of the cap, that confidence in the convective forecast for tonight is quite low. CAM solutions illustrate this as they vary widely both between models and from run-to-run between no thunderstorms at all, thunderstorms well northwest of our forecast area, and only occasionally depict thunderstorms in our northwestern counties. That being said, there is enough environmental support for some strong storm potential that SPC is justified in maintaining a Slight Risk severe outlook for our far northwest.

On Tuesday, in wake of the Monday night 500 MB impulse, weak ridging will slide across the High Plains to our northwest and decrease the pressure gradient in our area. This will result in somewhat lighter south southwest breezes during the day. While this will decrease warm air advection, the airmass in place will already be sufficiently warm and humid that combined with good insolation, temperatures/dewpoints/heat index values will be similar to those of Monday. The proverbial fly in the ointment here will be whether there are any lingering effects from overnight convection, i.e., a remnant cold pool or clouds that may inhibit warming (most likely in our northwestern counties). With heat index values still expected to peak around 105 degrees in our south and east, will maintain the Extreme Heat Warning through Tuesday evening for now and if convective effects do reach the northwest tonight and are expected to lower the heat impacts on Tuesday, then some portion of that area could potentially see the Heat Warning cancelled tonight. However, if any storms tonight remain well northwest of our area then it is likely the warning will continue right on through Tuesday and possibly beyond (see Long Term discussion below).

LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/

Issued at 312 PM CDT Sun Jun 28 2026

There remains the signal for another period of showers and storms as another wave within the larger scale trough lifts near Iowa Tuesday evening into Wednesday, along with an increasing low level jet overhead. Showers and storms are suggested to develop across portions of northern and western Iowa per Euro, while the GFS and NAM are on the drier side as any activity is further north into Minnesota closer to the warm front. These details are more on the uncertain side at this time, so will need to monitor this over the next few days. A Marginal Risk covers this area per SPC, with hail and gusty winds the main hazards where storms do develop and grow upscale. Otherwise, the main story will continue to be the long stretch of extreme heat as the upper ridge moves very little through the mid to late week period, including the 4th of July holiday. General highs are expected through the 90s each day, with heat indices above 100 degrees for much of the area, while overnight lows generally ''bottom out'' in the mid to upper 70s, providing little relief from the heat. Additional heat headlines are certainly possible in the coming days, but will need to monitor potential limiting factor such as overnight storm activity prior to each day, which models indicate are certainly possible over parts of Iowa. Specific details regarding coverage and intensity of these periods of active weather will be better known in the coming days.

AVIATION /06Z TAFS THROUGH 06Z TUESDAY/

Issued at 1048 PM CDT Sun Jun 28 2026

Removed gusts in the short term due to lack of observations. Stratus has formed over the northeast part of the state, affecting KMCW and KALO the most. Currently expecting these to remain VFR. LLWS common overnight. VFR conditions with gusty winds during the day with gusts over 30kts common after 18z, easing winds after 00z. Have reflected chances for stratus with a FEW deck in the north in this same timeframe.

DMX WATCHES/WARNINGS/ADVISORIES

Extreme Heat Warning until 9 PM CDT Tuesday for IAZ004-005-015- 023>025-033>037-044>050-057>062-070>075-081>086-092>097.

Extreme Heat Warning from noon today to 9 PM CDT Tuesday for IAZ006-007-016-017-026>028-038-039.