678
FXUS63 KDMX 291126
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
626 AM CDT Fri Aug 29 2025

 ...Updated for the 12z Aviation Discussion...

.KEY MESSAGES...

- Isolated showers and a few weak storms (~20% chance) possible
  over northern/eastern Iowa this afternoon and evening.

- Additional rain and storm chances (20-50%) at times this
  weekend, highest west. Severe weather is not anticipated.

- Cold front to sweep through roughly around Tuesday night next
  week, ushering in cooler weather in the latter half of the
  week.

&&

.UPDATE...
Issued at 348 AM CDT Fri Aug 29 2025

Fairly quiet conditions early this morning as high pressure
continues to influence much the region, which is currently centered
over the Upper Peninsula of Michigan. Surface observations depict
calm winds over much of Iowa, with mostly clear skies that have
allowed for some isolated areas of patchy to dense fog to develop
over portions of western and northern Iowa, with Forest City
reporting 1 mile visibilities for a short time, while Mason City has
been back and forth between 1/4 to 1 mile visibilities over the past
few hours so far this morning. Given that conditions aren`t expected
to change much, outside of temperatures falling a bit more into the
upper 50s across the state while dew points read similar, the
expectation is that fog should not expand much more but could still
become dense at times over parts of western to northern Iowa until
just after sunrise before dissipating. South/southwest flow in the
low levels over western to central Iowa will warm temperatures into
the low 80s this afternoon, while the flow pattern over the
northeast/eastern part of Iowa will be slightly cooler in the mid to
upper 70s given flow out of the west/northwest. A weak boundary over
the northeast, with some increase in moisture in the mid to lower
levels will result in the chance (<20%) for isolated showers over
that area in the afternoon to evening. This will be driven by
diurnal processes, with heating in the area allowing for a ribbon of
around 500 J/kg or slightly higher of instability to set up in
this area, though shear looks quite limited around 20 knots.
Given mostly dry air in the low levels, shower/storm development
looks overall limited, but if any develop, any activity looks
fairly weak, ending near sunset or sooner.

Additional chances for rain return at times this weekend, which will
primarily be driven by a shortwave tracking southeastward across the
Central Plains, with a developing surface low over South Dakota late
Friday into Saturday, dropping into Missouri into early next week.
An increase in moisture into western Iowa is expected to gradually
bring precipitation chances into that area by Saturday afternoon,
but may initially struggle to reach the ground as the dry air
influence from the departing high pressure leaving the Upper Midwest
gradually moves eastward. Differences in the track of the system and
overall coverage of precipitation remain evident, with the NAM
depicting a rather robust low pressure system with more scattered
activity west into portions of north and central Iowa, especially
into Sunday, while the GFS and Euro depict a weaker system that
keeps more limited coverage of showers mainly west and north. In
terms of storm potential, the primary axis of better moisture and
instability remains west out of Iowa, so overall concern for any
strong to severe storms is low, especially considering that shear
remains rather weak as well. Cannot rule out a few isolated weak
storms west and a few storms at times Saturday, becoming more
scattered Sunday.

&&

.DISCUSSION...
Issued at 123 PM CDT Thu Aug 28 2025

A surface high pressure ridge is draped across the region today,
resulting in light and variable winds and mostly sunny skies
throughout Iowa. A nebulous, mostly washed out boundary sagged
into our service area overnight and is providing a focus
mechanism for some cumulus development this afternoon from
around Pocahontas down to Grinnell. A few model solutions
attempt to generate light showers within this cumulus field,
however, forecast soundings show very limited moisture so will
hold any rain mention out of the forecast through tonight. Of
note, however, is that diurnal heating will promote seasonally
strong low-level lapse rates and 0-3 km AGL CAPE values in
excess of 100 J/kg near the boundary across northern Iowa this
afternoon and evening. This may promote some weak landspout-type
funnel clouds in that area, though with the shallow nature of
the clouds and high LCLs, any brief touchdowns appear unlikely.

From Friday through Sunday the surface ridge will slowly retreat
eastward, while a weak mid/upper level low pressure center
meanders from South Dakota southward or southeastward toward
Nebraska and Iowa. This results in a gradual return of clouds
and rain chances to our forecast area, but with ill-defined
focus mechanisms and uncertainty in where the mid/upper low will
track as it becomes trapped in a larger scale ridge, there is
low confidence in the timing and location of any resulting POPs.
By Sunday the GFS is moving the mid/upper low across Iowa, while
the GEM and EC wander it southward into Nebraska and have
relatively drier solutions in our area. In any event, severe
weather is not expected but obviously any showers or storms may
have an effect on outdoor plans over the holiday weekend.

Looking ahead into next week, a deep gyre will sink from south
central Canada down into the Great Lakes region around Tuesday
night or so, bringing a pronounced surface cool front sweeping
through with it. There will likely be a return of rain chances
in association with this system, then once the front pushes
through we will see cooler temperatures as we remain beneath
brisk northwesterly steering flow for the remainder of the week.
Long-range temperature guidance points to daily highs only in
the 60s to lower 70s during that time, so not a major blast for
early September but still noteworthy after the warmth of much of
August.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 626 AM CDT Fri Aug 29 2025

Patchy dense fog continues to impact KFOD and KMCW this morning,
though should clear in the next few hours. Until then, low
visibilities are expected to continue at times, leading to
LIFR/IFR conditions. Otherwise, outside of mainly high clouds
passing through, VFR conditions are largely expected. There is a
low chance (<20%) for a few isolated showers and storms that
could impact KMCW and KALO this afternoon to evening, though
given low confidence have not specifically included and will
monitor conditions closely. Winds will remain light primarily
out of the south/southeast. Another potential for fog is
signaled into Saturday morning over northeast Iowa, though given
this is several hours out and is low confidence, have not
mentioned directly at this time and will monitor trends through
the day.

&&

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

&&

$$

UPDATE...Bury
DISCUSSION...Lee
AVIATION...Bury