183
FXUS63 KDMX 210354
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1054 PM CDT Wed Aug 20 2025

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

.KEY MESSAGES...

- Light and sparse showers (~15%) in the south this evening.

- Patchy fog possible Thursday morning, especially in northern
  Iowa (60% chance).

- Marginal Risk for severe weather Friday. Damaging winds and
  isolated severe hail both possible.

&&

.DISCUSSION...
Issued at 158 PM CDT Wed Aug 20 2025

A weak boundary can be seen best in visible satellite imagery this
afternoon with pop-up showers developing across southern and eastern
Iowa. Showers are comparatively more present near the MS River due
to being closer in proximity to the synoptic shortwave from
yesterday afternoon`s activity. Dew points will mix into the 60s
this afternoon and the low-level lapse rates will be high, perhaps
even superadiabatic. The front will be swept up by the easterly flow
around high pressure, advected into south-central Iowa this
afternoon. Rainfall amounts will be minimal (<0.10") but there is a
chance for funnel cloud development. This is due to the instability
mentioned above as well as the weak flow and vorticity from the
frontal circulation. LCLs will continue to rise and cloud bases will
top off at around 4kft, high enough that confidence is low for
having land spouts. Highs this afternoon will be the warmest for the
foreseeable future with temperatures in the mid 80s south.

High pressure will again make fog possible tonight, but again even
less dense than the previous morning. There could be some
visibilities under 3 statute miles in northern Iowa, but it
will be shallow and patchy in the southern half. Easterly flow
will hold highs closer to the 80 degree mark for all but the far
west. Dry air will continue to funnel in to that easterly flow
as well. Meanwhile, a closed low will impact the Northern Plains
Thursday evening and overnight. Supercells will congeal into an
MCS and succumb to the weak shear and lapse rates near Iowa.
The better kinematics come in just behind the cold front
sometime Friday evening. Almost all of the shear is sourced from
the intensifying upper-level jet, creating straight hodographs
orthogonal to the front. For solutions, the NAM- based
simulations can be ruled out for now as it hangs on to too much
moisture in the low-levels; the blocking high pressure makes
these dew points seem too high. The GFS overmixes and has steep
lapse rates, making more intense storms than expected. Coverage
seen in the 12z RRFS makes the most sense as the more intense
activity resides with main moisture discontinuity in Nebraska.
For what moisture does reach Iowa, there could be some splitting
supercells early-on thanks to the straight hodographs. The main
concern would be damaging winds with DCAPE values in the low
1000s. Marginally severe hail will be possible, but limited with
mid-level lapse rates <7 C/km. Discrete activity will last only
as long as surface-based potential is there, congealing into a
broken line with a wind threat in the evening.

Northwest to northerly flow comes in behind the front and stays in
the region for the rest of the forecast and beyond, keeping us dry
and on the cool side for late August. The coldest morning looks to
be on Monday, the same time when high pressure intrudes on the
Dakotas and reinforces northerly flow at the surface. Lows in
northern Iowa could be closer to 40 degrees. Highs in the extended
are all in the 70s.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1046 PM CDT Wed Aug 20 2025

Little change in thinking. VFR conditions will continue through
09Z-ish at all terminals. After 09Z, patchy fog is expected to
develop once again. There are mixed signals regarding fog
formation, so at this point kept a persistence forecast and
maintained MVFR fog in for a few hours at most terminals in
that 10-13Z period. Like previous nights, IFR or even LIFR
conditions are possible for a time, most likely at KFOD. A few
models are also hinting at the possibility of pockets of stratus
developing, so will watch for that potential overnight as well.

&&

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

&&

$$

DISCUSSION...Jimenez
AVIATION...Fowle