855
FXUS63 KDMX 190758
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
258 AM CDT Tue Aug 19 2025

.KEY MESSAGES...

- Areas of dense fog will produce visibilities down to a 1/4
  mile or less this morning, especially over northern and
  western Iowa.

- Fog potential again late tonight into Wednesday morning.

- Cooler and mostly dry through Thursday with highs in the low
  to mid 80s. Even cooler by this weekend.

&&

.UPDATE...
Issued at 252 AM CDT Tue Aug 19 2025

Surface high pressure has filled in behind the departing boundary to
our south, bringing clear skies and light winds this morning. This
has allowed surface temperatures to cool off quickly. Dewpoints
remain relatively high over the area, pushing near surface air to
saturation and generating fog. Surface observations and road cameras
showed the dense fog starting as fairly shallow and transient but
visibilities around 1/2 mile or less are becoming more widespread
over northern and western Iowa. As of this writing, a dense fog
advisory is in effect for counties along and north of Highway 18.
There will likely be some expansion of this advisory as light winds
and resultant fog develops southward. While not dense at this time,
patchy shallow fog is present over central and southern Iowa as
well, especially in low lying areas where the cooler, moist air
pools. With multiple hours of cooling left this morning, it`s likely
we will see continued patchy to dense fog development over much of
the area. Impacts to the morning commute are expected, especially
for those out prior to sunrise. Use caution if out traveling
this morning, as even patchy fog can lead to sudden drops in
visibility.

High pressure remains overhead today and tomorrow, keeping
significant precipitation chances at bay. That said, some guidance
has been kicking out isolated showers over northeastern Iowa
overnight tonight into Wednesday morning. With dry air and
subsidence beneath the high pressure, it`s unlikely these will be
anything more than a light shower, if they materialize at all. There
will also be a small amount of instability present in the shallow
saturated layer, so can`t rule out a rumble of thunder, but
thunderstorm chances are very low and impacts will be minimal.

With high pressure still overhead tonight, there will be another
chance for at least patchy fog overnight into Wednesday morning. The
highest likelihood will again be over northern and north central
Iowa, but fog potential will exist over the entire forecast area by
Wednesday morning. &&

.DISCUSSION...
Issued at 228 PM CDT Mon Aug 18 2025

Widespread showers and thunderstorms across the area this morning
dissipated quickly by midday. Cloud cover has also dissipated in
many locations, and in areas along and south of the boundary this
has made for another muggy afternoon. Temperatures have warmed into
the mid to upper 80s with dewpoints in the 70s. Meanwhile, central
to northern Iowa has remained in the low to mid 70s through the
afternoon.

Early this afternoon cumulus has grown along the boundary draped
across east central in south central Iowa. Expect this to
become more robust in the next few hours with diurnal heating.
This will be the impetus for convective redevelopment through
the evening. By 2 pm a few isolated showers were already
beginning to develop on radar. Conditions across southeast Iowa
where the boundary will traverse are nearly uncapped with
MLCAPE values of 3000+ J/kg. Despite the deeply unstable
conditions, shear is virtually nonexistent in this area which
will inhibit more organized storm development. Expect that
storms will likely develop quickly and then collapse, with these
collapsing core producing gusty winds as the primary convective
hazard. This will be confined to southeast portions of the area
as the boundary continues to sink south and east this afternoon
and evening.

After convection clears the area this evening expect a quiet
evening. By Tuesday morning patchy fog develop is possible across
the area, especially across northern Iowa. The past several mornings
have seen isolated fog development north of the boundary in cooler
air and this is likely to continue on Tuesday morning. Temperatures
Tuesday through the end of the week with be in the low to mid 80s
with dewpoints in the 60s, feel more comfortable than this past
weekend. A quick moving cool front Friday will bring another
chance for precipitation and cooler temperatures further with
highs in the 70s by the weekend.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1049 PM CDT Mon Aug 18 2025

Fog remains the main focus for the overnight and into early
Tuesday morning. Fog and there is stratus over parts of
southeast Minnesota and into Wisconsin. Expect the fog to expand
into northern Iowa early in the period with the main impacts at
KMCW/KFOD/KALO with periods of IFR to possibly LIFR vsbys/cigs.
Conditions will gradually improve Tuesday morning with VFR
becoming more widespread in the afternoon. There are low chances
that fog reaches KDSM with a better chance of minor vsby
reductions at KOTM. Light winds through much of the period.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 9 AM CDT this morning for IAZ004>007-
016-017.

&&

$$

UPDATE...Dodson
DISCUSSION...Hagenhoff
AVIATION...Donavon