802 FXUS63 KDMX 270400 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1100 PM CDT Sat Jul 26 2025 ...Updated for the 06z Aviation Discussion... .KEY MESSAGES... - Fog developing over eastern into much of central Iowa after midnight tonight through about 8 or 9am Sunday. The lowest visibilities will be over eastern into northern Iowa and dense fog is possible. - High heat and humidity Sunday creating heat index values of 100-110 degrees with a heat advisory over much of central Iowa midday to early evening. High heat indices may persist into Monday and perhaps Tuesday over portions of the state. - Storm chances Sunday night/early Monday over eastern into perhaps central Iowa and then again Monday night/early Tuesday over northern Iowa. Gusty winds are possible Sunday night/early Monday if storms move into Iowa. - Most widespread and highest storm chances later Tuesday night into Wednesday evening. Locally heavy rainfall is possible with severe weather potential evaluated in the coming days for this period. && .DISCUSSION... Issued at 254 PM CDT Sat Jul 26 2025 Plenty of cloud cover is lingering over Iowa this afternoon as a shortwave trough pushes eastward over top the Southeastern US ridge. Cloud cover should continue to dissipate through this afternoon into this evening. With the clear sky, light winds, and the sultry airmass due to the recent rainfall, fog is expected to develop over eastern into much of central Iowa. It is possible that the fog may be dense over eastern into northern Iowa so will be monitoring trends into this evening and overnight for this possibility. As the fog dissipates Sunday morning, temperatures and dewpoints will climb with the combination by early afternoon creating heat index values between 100 to 110 degrees across our forecast area. The highest heat index values will be over western into much of central Iowa and that is where a heat advisory has been issued (with western Iowa under an Extreme Heat Warning). The Southeastern US ridge is expected to expand westward and then northwestward Sunday into early this upcoming week. During this period, this will push the shortwave storm track farther north, but not far enough north to negate all storm potential over Iowa. Over time too, the shortwave track will slowly sink closer to Iowa. In addition, high heat and humidity will set up and prevail over much of the area with some uncertainty due to potential storm debris cloud cover on Monday into Tuesday. Shortwaves look to top the ridge over the Dakotas through Minnesota about every 24 hours or so, centered over the overnight hours largely. The first storm chances comes Sunday night into early Monday as the first shortwave moves across. Some models show a mesoscale convective system (MCS) moving eastward and scraping our northern counties; however, more models such as the 12z NAM, ECMWF, and HRRR show this diving southeastward into more of Iowa flowing around the 700mb 13C contour roughly. Looking at the bulk of the data, this latter solution seems the probable outcome at this point and lines up with the broadly drawn day 2 SPC marginal risk. The main severe hazard here would be wind gusts with the mesovortex generation potential low given the weaker 0-3km bulk shear values around or under 20 knots. High heat and humidity may persist into Monday, but uncertainty in what, if any, cloud cover may linger from an overnight MCS precludes any headlines here. For example, the NAM, which is one of the models showing the MCS diving into some portion of Iowa Sunday night, recovers the airmass over the state such that conditions are similar to Sunday if not a touch higher on temperatures on Monday. Thus, additional heat headlines are still on the table for Monday as we sort out what may or may not occur with the MCS. Another shortwave passes north Monday night with the global guidance showing the QPF staying largely over Minnesota and perhaps grazing far northern Iowa. At the same time, this shortwave will be closer to Iowa with a surface boundary nearing the state as well from the north. The exception to the QPF staying north is the ECMWF, which seems to be a bit more generous with allowing storms in the hot 700mb air compared to other models. There will be no shortage of instability, but the higher amounts of deep layer shear will reside over Minnesota into perhaps northern Iowa. Tuesday will see the surface boundary moving south over the state with widespread storm chances developing later Tuesday night into Wednesday evening as low level theta-e pulses push over the surface boundary. Again, while instability will not be in short supply, deep layer shear is marginal at best. The heavy rainfall parameter space will be more favorable with precipitable water values over 2 inches along the boundary, weak mean flow and good moisture transport into the boundary. So, will be monitoring the potential for heavy rainfall in this period. With the boundary pushing well south of the state Thursday and surface high pressure following behind over the region, this should foster drier conditions for late in the week. && .AVIATION /06Z TAFS THROUGH 06Z MONDAY/... Issued at 1059 PM CDT Sat Jul 26 2025 Fog is expected to develop around 08-10Z, first and primarily in northeastern Iowa with lowering confidence toward the west and south. At ALO, IFR visibilities and ceilings are likely for several hours and LIFR or lower conditions have a 50% probability toward sunrise. At the other terminals have advertised generally MVFR to IFR conditions due to lower confidence, but it is likely amendments will be needed overnight into Sunday morning based on short-term observational trends. The fog will burn off fairly quickly after sunrise Sunday, with VFR conditions thereafter for the remainder of the TAF period. && .DMX WATCHES/WARNINGS/ADVISORIES... Heat Advisory from noon to 8 PM CDT Sunday for IAZ004>007- 015>017-023>026-033>037-044>049-057>062-070>075-081>086- 092>097. && $$ DISCUSSION...Ansorge AVIATION...Lee