364
FXUS63 KDMX 221129
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
629 AM CDT Fri Aug 22 2025

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

.KEY MESSAGES...

- Showers and storms possible today and tonight. A marginal risk
  for severe weather exists over northwest Iowa with the overall
  threat low.

- Becoming much cooler this weekend and into next week.

&&

.UPDATE...
Issued at 420 AM CDT Fri Aug 22 2025

Surface ridge of high pressure is centered over northern Illinois
and into the southern half of Lake Michigan early this morning. The
high is ridged through the southern half of Iowa with light and
variable winds under the ridge. A few areas of river valley fog are
occurring under the ridge. Satellite imagery is showing fog in the
Des Moines river valley below Lake Red Rock and to the Mississippi
River. The Chariton River below Rathbun Lake is also showing fog.
The fog is more expansive into east central Iowa and southwest
Wisconsin and it may expand more into central Iowa over the next few
hours.

A frontal boundary is over north central Minnesota and into southeast
South Dakota and western Nebraska. This boundary has been the focus
for showers and thunderstorms over South Dakota overnight with that
convection weakening as it moves southeast and into a more stable
region. The convective debris from this storms has been moving into
northern Iowa and that may continue at least this morning as there
are more storms in central South Dakota. This may inhibit heating
today, especially over northern Iowa and may limit instability even
further than what will be discussed below. Forcing will be modest as
the boundary arrives and mid-level lapse rates will be in the 5.5 to
6 C/km range which will limit updraft strength potential. Despite
moisture pooling along the boundary and potential 70 degree dew
points, the elevated mixed layer will contain surface based
instability until at least mid-afternoon and it is possible the EML
holds through the day. The greatest chance for showers and storms
may be delayed until the main convergence and column cooling and
moistening occur with the main boundary passage this evening. At
this time, the threat for severe weather appears low. SPC does have
a marginal risk for severe weather but this is a courtesy stripe for
the stronger development in South Dakota that may not reach the more
stable west central to north central Iowa.

Much cooler temperatures will follow into the weekend and early next
week as the 850 mb temperatures drop into the single digits celsius
and that will be reinforced on Sunday with the passage of another
boundary. Could this be the end of the 70+ dew points for this year?
Perhaps as it appears dew points in the 50s and even 40s at times
will be arriving and at this time there is no significant push of
moisture back into Iowa through the remainder of the month. With the
cropland evapotranspiration coming to an end as well.

&&

.DISCUSSION...
Issued at 247 PM CDT Thu Aug 21 2025

Dry and quiet conditions have set up nicely across Iowa following
this morning`s fog, with surface analysis showing the area of high
pressure centered over the Great Lakes, with its western extent
leading to east/southeast surface flow over Iowa. Scattered cumulus
has developed given peak heating and destabilization, which should
gradually fade this evening as cooling gradually occurs. Otherwise,
highs are expected to top out in the upper 70s to low 80s. Given
light winds and mostly clear skies mainly east closer to the
departing high pressure tonight, a chance for patchy fog will be in
play again into Friday morning. However, given another day of dry
conditions, the soil moisture content should be less so the extent
of fog coverage should generally be less as well, though will be
monitoring trends over the next several hours. Low mentions were
added to the forecast to highlight the potential that is there.
Otherwise, overnight lows are expected to drop into the upper 50s to
low 60s.

The synoptic setup into Friday remains fairly similar from the past
few days, with a large area upper level high pressure circulating
over the southwestern CONUS, while further north into Canada is an
upper level low that will eventually drop its associated frontal
boundary further south into the Central Plains, with it effects
expected to impact at least northwestern Iowa by early Friday
morning. The influence of the slowly departing high pressure over
portions of central and eastern Iowa Friday morning should keep much
of the state dry, with the boundary expected to remain more
stationary into far northwestern Iowa during this time, which models
generally agree on in terms of area. That said, the expectation
remains that forcing with this boundary as a 20-25 knot jet develops
in the over western Nebraska into South Dakota is expected to kick
off showers and storms mainly over over that area into Minnesota,
even bubbling into far northwest Iowa into Friday morning. In terms
of the parameter space, the favorable ingredients are expected to be
generally north and west of Iowa where bulk shear around 30+ knots
and CAPE values over 1000+ J/kg reside, though values around 500
J/kg with shear closer to 20 knots over northwest Iowa may be enough
to a least develop some storms, though the severe potential is not
very likely into Iowa. Dry air in the lower levels into Iowa will be
another limiting factor early on to consider. Into Friday afternoon
and evening though, particularly around 3pm and later, instability
is expected to build with values up to 1000-1500 J/kg and bulk shear
values around 30-35 knots overhead, with highest values along the
western portion of the state that will allow for a higher potential
to see storms develop, even some strong to severe storms over
northwest into western/central Iowa as the frontal boundary
gradually tracks southeastward across the state. Given lapse rates
generally expected in the 6.5-7 C/km range and warm cloud depths,
the hail potential overall is not very great, but some potential
does remain early on with afternoon development over northwest Iowa
if any supercell development can become sufficient enough for hail
growth to occur. Otherwise, the wind threat remains primary and
covers areas further south and east later into the evening as the
storms look to gradually congeal more into a line. Lingering storms
look to remain generally across portions of south central to
southern Iowa overnight until around sunrise Saturday, though the
severe potential will be little if any given the loss of favorable
parameter space and the lack of an additional forcing mechanism as a
weaker low level jet remains west of the state.

A descending area of surface high pressure with widespread
northwesterly flow and dry air moving into the Central Plains will
lead to dry conditions through the remainder of Saturday. Highs will
be noticeably cooler and more comfortable with values in the 70s and
dewpoints in the 50s. The dry and cooler pattern will persist this
weekend, and even beyond as northwesterly flow remains with surface
high pressure overhead. Highs will generally be in the 70s, and lows
in the upper 40s to 50s, coolest north. Breezy winds at times this
weekend are also expected given the tightened pressure gradient over
the Upper Midwest as the deepening Canadian low pressure moves
eastward, with occasional gusts up to 20-25 mph, especially north. A
very low chance (<10%) for showers is possible into far southern
Iowa with a boundary nearby late Sunday into Monday, but for now the
forecast remains dry given the presence of dry air overhead. Dry
conditions generally remain at least through midweek before the next
chances for rain, generally less than <20% probability, may occur
south. More on these details in the coming days but concerns, if
any, are low at this time.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 626 AM CDT Fri Aug 22 2025

Localized areas of fog to begin the period with KOTM being
impacted the most though brief periods of fog may occur at KALO
also through 14z. Showers and storms currently over southeast
South Dakota may reach KFOD/KMCW yet this morning into early
afternoon. Confidence remains low enough at this time to not
place into the forecast. Additional showers and storms are
possible later this afternoon and into the evening ahead of a
boundary. The wind will be light much of the day and will
become northwesterly with fropa.

&&

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

&&

$$

UPDATE...Donavon
DISCUSSION...Bury
AVIATION...Donavon