604
FXUS63 KDMX 200816
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
316 AM CDT Wed Aug 20 2025

.KEY MESSAGES...

- Areas of fog developing through this morning, with
  visibilities under a mile possible by sunrise.

- Dry for most today, with a chance (< 15%) for an isolated
  shower and a few rumbles of thunder central and south through
  mid-day.

- Patchy fog possible late Wednesday night into Thursday
  morning, with dry and mild conditions during the day.

- Next best window for storms will be on Friday with a ~30-40%
  chance of precipitation.

&&

.UPDATE...
Issued at 312 AM CDT Wed Aug 20 2025

Surface high pressure remains overhead with little in the way of
impactful weather over the next couple of days. Light winds and
clear skies have once again brought fog potential to the area this
morning, albeit not nearly as extensive as it was at this time
yesterday. We are seeing a a large area of patchy fog developing
over west central Iowa, then placed more sporadically in the rest of
the area. Fog is expected to continue developing as temperatures
cool through sunrise, although the exact locations of the worst
fog/lowest visibilities is still a bit uncertain. Most of the
00z guidance does show dense fog occurring somewhere in the
forecast area by sunrise, but the exact locations are seemingly
random from model to model. This has resulted in 00z HREF
probabilities showing a fairly widespread 40 to 60% chance for
fog under a quarter mile, slightly favoring western and northern
Iowa. These areas are as good of a prediction as any, as these
were the most affected locations with yesterdays fog and are
roughly coincident with the wettest soils from previous
rainfalls. Therefore, the main takeaway is at least patchy fog
development continuing through the morning, with the potential
for fog to become dense (1/4 mile or less), especially over
northern and western Iowa.

Fog dissipates after sunrise, giving way to mostly clear skies this
morning. Surface high pressure will continue to build to our north
with northerly flow advecting cool, dry air into the low levels. As
this occurs, very subtle convergence looks to be generated,
producing some light showers and even an isolated rumble of thunder
through mid-day and into the afternoon hours. The best chances for
these light showers/isolated thunder looks to be over the southern
half of the state, following a pocket of relatively cooler mid-level
air. Fortunately, warm air advection increasing in the mid-levels
and a very dry vertical profile should greatly limit convective
development, especially as the mid-level warm layer increases and
lower levels mix out through the late afternoon and evening.

Aside from the slightly cooler and drier low level air advecting in,
very little change is expected in the overall surface pattern today,
keeping the high pressure in place through tonight and into
Thursday. As a result, fog development will once again be possible
over much of the area late tonight into Thursday morning. The
Dene fog signal does weaken some compared to the last few
nights, likely due to soils slowly drying out with the dry
conditions. 00z HREF probabilities for visibilities of a quarter
mile or less only max out around 30 to 40% by Thursday morning
with most of the deterministic guidance only showing a very
light fog. Therefore, while fog potential does seem to continue
tomorrow morning, the severity of the fog is dwindling with
each night. Mild temperatures and dry conditions are expected
through the day Thursday. &&

.DISCUSSION...
Issued at 212 PM CDT Tue Aug 19 2025

High pressure has set in behind the departing cold front from
yesterday, which will hold mild and calm weather for today and
tomorrow. We`ll be able to mix out efficiently this afternoon thanks
to the dry air provided by the high. A subtle synoptic feature will
graze northeast Iowa this evening and profile lapse rates are steep
enough to give a chance of at least some showers; the depth of
saturation is too shallow for more upscale growth. Thanks to the
light winds, fog will again be possible in the morning hours,
especially in the northern half of the state. The LAV guidance
highlights this well with visibilities for MCW and ALO dropping
below 3 statute miles in the early morning.

The weather will remain relatively unchanged for much of the work
week. With the upper-level ridge parked in the High Plains, the
thermal ridge will just graze western Iowa, keeping highs in the
upper 80s to near 90 degrees. High pressure will sit over the
eastern half of the state, this being beneath northwest flow. Highs
in this portion of the state will be closer to the 80 degree mark,
with a least patchy fog in play through Thursday morning. More
convectively-driven showers will be possible Wednesday afternoon
around peak mixing.

The ridge will attempt to pivot over on Friday, but will be
interrupted by a deepening trough seen over the PNW today. This
trough will develop a closed low, increasing height falls and wind
speeds in the region. It will stack in southern Canada by Friday,
subjecting Iowa to the effects of its cold front. The consensus seen
right now by models is the front being somewhere in central Iowa
late Friday afternoon. F vector convergence is stronger in the
southern/southwest part of the CWA, making this the more likely spot
for convection. On the synoptic scale, the upper-level jet will drop
down into the state, increasing divergence and bulk shear. LLJ
support will be cut off after Thursday night due to high pressure in
the Central Plains, reducing the amount of low-level moisture and
concentrating the shear in the mid and upper levels. 0-6km bulk
shear vectors will start orthogonal to the front and over 25kts and
MUCAPE values will be around 1500 J/kg. There could be some
supercells initially, but due to the speed and evolution of the
front, they would quickly congeal into a line. Threats will start as
large hail and damaging winds, then become solely a wind threat. The
low-level shear is weak (<10kts) which lowers the confidence in any
tornado threat (due mostly by the lack of a LLJ). The CSU MLP has a
<15% outlook across the state, which works well given the
uncertainty of available moisture. Rainfall amounts will stay under
an inch.

Behind the front, a big pattern change is in store starting this
weekend. The upper level closed low is slow to leave the Great Lakes
region, locking Iowa in northwest/northern flow through at least the
middle of next week. Highs will be in the 70s and the main moisture
axis remains south of the state.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1045 PM CDT Tue Aug 19 2025

Fog remains the main aviation forecast challenge in the first
6-9 hrs of the period. Some light fog is beginning to develop
over south central Minnesota. The fog potential into Iowa
remains more uncertain but again still is on track to not be as
prevalent as the previous night. Some fog mention at most sites
with the focus over northern Iowa. Locally dense fog is possible
over much of the state though, especially in the river valleys
which leaves KDSM as the least likely chance for IFR or lower
conditions. Conditions will improve Wednesday morning with VFR
becoming more widespread. The wind will be light and variable
overnight then will remain below 12 kts from the north to
northeast diurnally Wednesday.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

UPDATE...Dodson
DISCUSSION...Jimenez
AVIATION...Donavon