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