650
FXUS63 KDMX 211114
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
614 AM CDT Thu Aug 21 2025

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

.KEY MESSAGES...

- Patchy, shallow fog possible this morning then dissipating
  after sunrise.

- Dry today into tonight.

- Shower and thunderstorm chances return Friday, mainly in the
  afternoon and evening. A few strong storms are possible with
  strong winds and small hail being the main concerns.

&&

.UPDATE...
Issued at 333 AM CDT Thu Aug 21 2025

Another night light winds under high pressure has allowed for very
isolated fog to develop tonight, but any visibility reductions have
been fleeting. This lesser coverage was to be expected, as dewpoints
continue to cool as the lower levels dry out each day. Likewise,
sunny skies and warm temperatures during the day have allowed soils
to continue to dry out, further limiting the moisture at the
surface. Low stratus isn`t helping the case this morning either, as
radiational cooling is less effective under areas with cloud cover.
The net result is another morning of shallow fog potential, which
could briefly reduce visibility to under a mile, but will be limited
in spatial coverage and unimpactful to most areas. Fog will then
burn off after sunrise.

Conditions remain dry today as we turn our attention to our next
chance for precipitation beginning Friday morning and tracking
southeastward across the area into early Saturday morning. A
deepening 500 mb Canadian upper low will bring the potential for
thunderstorms along an elongated surface boundary into the
Upper Plains region tonight, with storms eventually tracking
east southeastward toward the area. Fortunately, upper ridging
overhead and surface high pressure over the Great Lakes should
keep any precipitation chances in Iowa at bay overnight, with
the better deep layer moisture and mid-level lapse rates not
arriving until the boundary and upper trough does on Friday
morning.

Upon it`s arrival in northwest Iowa on Friday morning, the initial
surface boundary looks fairly diffuse, with weak low level flow and
limited convergence. Given the weak convergence, storms will likely
struggle to form initially, but if a storm can take off, instability
values will be sufficient for stronger updrafts, especially over
northwestern Iowa where MUCAPE values will be around 1000 to 2000
J/kg. The deep layer shear looks modest, with roughly 30 to 40 kts
of bulk shear in northwest Iowa. Organized storms are certainly
possible with this environment, with the main concerns being strong
winds and small hail. As we get later into the afternoon and early
evening, better forcing will arrive in the form of forced ascent
along the trailing synoptic cold front. Depending on the timing of
the trailing cold front, storms may still be able to take advantage
of the better instability before the atmosphere stabilizes along the
boundary overnight. This gives us a rough window in the late
afternoon and early evening where the better environment could phase
with the stronger forcing, making this the most likely timeframe to
see strong to severe storms. The storm prediction center has a
marginal risk for severe weather in place over northwestern into
central Iowa today, with wind and hail as the main threats. Showers
and storms may fester along the cold front as it progresses
southeastward overnight, but the risk for severe weather will be
quite a bit lower. By Saturday, cooler, drier air fills in behind
the boundary.

&&

.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 /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 611 AM CDT Thu Aug 21 2025

Fog has generally avoided TAF sites so far this morning, but may
see some brief MVFR impacts as the sun rises this morning,
mainly over northern Iowa, so have kept brief visibility
reductions in for a few more hours. In addition to the fog this
morning, stratus around 3k to 4k ft has developed north and
northeast, which may bring brief MVFR conditions at KMCW and
KALO. Both fog and stratus will diminish after sunrise, giving
way to VFR conditions through the remainder of the day and this
evening.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 8 AM CDT this morning for IAZ058-059-
071>073-081>084-092>095.

&&

$$

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