Marshalltown, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

127
FXUS63 KDMX 130350
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1050 PM CDT Tue Aug 12 2025

 ...Updated for the 06z Aviation Discussion...

.KEY MESSAGES...

-  Sfc trough/cool front will trigger isolated showers and
   tstms through early evening. Severe weather is not expected.

-  High pressure will result in benign weather Wednesday into Thursday,
   with just a small /20%/ chance of showers/tstms far north
   Wed night and Thursday.

-  Warmer and more humid Friday into the weekend with a couple
   periods of evening and nocturnal showers/thunderstorms.

&&

.DISCUSSION...
Issued at 228 PM CDT Tue Aug 12 2025

This Afternoon into Tonight...

Upper air analysis and water vapor imagery this afternoon is
depicting a seasonally strong upper trough north of MN, with an MCV
spinning across nrn MO. At the sfc, temps at 2pm have warmed into
the upper 70s and lower 80s, with Tds in the mid 60s northwest, to
the lower 70s central and south. A sfc boundary/wind shift
extends from southeast MN, swwd across n-central IA, and into
central NE. Convergence along this feature, combined with modest
instability /MLCAPE ~1000 J/kg/ is expected to trigger isolated
to widely scattered showers and thunderstorms from late this
afternoon into the early evening hours. Deep layer shear is
lacking, with 0-6km shear < 25kts, thus any updrafts/storms are
expected to struggle to organize, with pulsey type storms the
most likely storm mode. While the threat for severe weather is
very low, cannot completely rule out a stronger wind gust due to
drier air in mid-levels, especially as any cores collapse.
Storms should weaken pretty quickly after sunset, with loss of
diurnal heating.

Tonight, skies should gradually clear as sfc high pressure moves
into the region with temps cooling into the mid 50s to lower
60s, perhaps an opportunity to open those windows!  However,
with residual low level moisture and light winds, areas of fog
are expected develop once again. A few guidance tools /MOS and
HREF/ are showing the possibility for at least locally dense
fog, especially in the more fog prone areas such as river
valleys. As we know, fog forecasting is fickle, so at this point
will just highlight patchy fog in the forecast. Low confidence
on any headlines being needed, but will pass concerns on to
future shifts.

Wednesday through Thursday...

Sfc high pressure will reside over the region on Wednesday, with sfc
dewpoints a bit lower in the 60s.  Not exactly "delightful" on the
muggy meter, but a little better than the full on tropical values
we`ve seen much of July/August. Besides a few diurnal cumulus,
skies should remain sunny/mostly sunny with highs in the lower
80s. At this point, the HRRRSmoke is not depicting a major
amount of smoke to infiltrate the region, but will keep
monitoring this trend. Overall, should be rather pleasant late
summer day.

By Wednesday night and Thursday, the sfc high begins to retreat off
to the east, which will gradually turn winds back to the
south/southeast.  At upper levels a compact shortwave will move
across SD into MN, with a potent LLJ developing on Wednesday over
the central Plains. The various global models and their ensemble
counterparts are showing about a 20% chance of showers/storms over
portions of northern/northwestern IA on Wednesday night into
Thursday. Storm should diminish with ewd extent as they run into the
high and encounter drier air.


Extended Forecast...Friday into the Weekend.

Well...were not done with the summer pattern just yet as the
12.12Z guidance continues to show the upper ridge re-
establishing over the Corn Belt, with a series of low amplitude
shortwaves traversing the top of the ridge. With a southerly low
level flow becoming re-established expect temperatures to rise
again with 850mb temps rising into +20-25C range this weekend
into early next week. Highs on Friday through Monday will range
from upper 80s to low 90s, with sfc dewpoints surging back into
the sweaty 70-76F range. Heat indices will correspondingly rise
back to near 100F on Sat/Sun. There will be periodic chances
for mainly evening and nocturnal thunderstorms from Friday into
the weekend as the atmosphere becomes much more unstable. There
will likely be issues with capping /as H7 temps approach 12C/
so the LLJ will likely play a key role again. We`ll refine the
severe wx threat in the upcoming days.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1045 PM CDT Tue Aug 12 2025

VFR conditions continue early this morning, although some sites
over the area are beginning to report visibility reductions with
fog over southern and central Iowa. The expectation is for this
fog to expand through the morning and impact at least KDSM, KOTM
and KFOD. KMCW and KALO are certainly possibilities, but
confidence is lower at these locations. There may also be
pockets of dense fog as well, but have kept TAF reductions to
MVFR for now.

Fog gives way to clear skies tomorrow morning with light winds
generally out of the north and minimal aviation concerns.

&&

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

&&

$$

DISCUSSION...Donavon/Fowle
AVIATION...Dodson