882
FXUS63 KDMX 301137
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
637 AM CDT Sat Aug 30 2025

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

.KEY MESSAGES...

- Patchy fog may continue to develop over parts of northeastern
  into north central Iowa toward sunrise this morning.

- Isolated showers or storms possible (<20%) this afternoon from
  northwest to southeast Iowa, with slightly better chances
  (<30%) west. Lightning is the main hazard with a funnel cloud
  or wind gust to 30 mph from a dying shower or storm less
  likely.

- 40-60% chance of showers and storms Sunday with these highest
  chances over western Iowa fading with eastward extent into
  central and eastern Iowa. Clouds, showers, or weak storms
  linger into Labor Day.

- Much cooler and drier air Wednesday through Friday.

&&

.UPDATE...
Issued at 358 AM CDT Sat Aug 30 2025

Much of Iowa early this morning remains dry, though a few showers
with some lightning as seen from radar returns is continuing to
drift southeastward over far eastern Iowa, which is expected to
gradually fade through the morning. Otherwise, fog has begun to form
over portions of northern Iowa, with Hampton and Estherville
reporting visibilities around to less than a 1/2 mile over the past
hour. Given the light winds and low-level moisture present, the
potential of fog will remain but will still largely depend on how
thicker clouds observed over northeastern Iowa into
Minnesota/Wisconsin track, as this will limit the extent of fog
formation this morning. Model guidance still covers much of
northeastern Iowa with patchy to dense fog, but may be overdone if
clouds remain over portions of the state. Cannot rule out remaining
isolated areas of more dense fog at times through the morning, but
will continue to monitor closely. Any fog that does remain by
sunrise will fade, clearing into mid-morning.

Following the morning fog, the focus then shifts back to the
shower/storm potential later today. Deterministic model guidance
depicts the shortwave over the western Central Plains approaching
western Iowa today and Sunday, gradually dropping into Missouri by
Monday. A weak boundary now roughly over northwest to southeast Iowa
will be the main focus again for shower and storm development.
Models seem to generally agree that development of any activity
would occur by early afternoon through the evening in
correlation to the diurnal pattern given warming temperature,
along with presence of some mid-level moisture, but more limited
given the dry surface high pressure that still remains mainly
over the Upper Midwest. However, convergence along the boundary
should develop some shower activity though more isolated in
coverage given fairly weak forcing northwest to southeast across
Iowa. Further parameter space analysis shows weak shear and
lapse rates, though some notable instability(around 500+ J/kg),
which could result in some lightning production from any showers
that end up developing. The potential does remain for non-
severe gusty winds and a few weak funnel clouds into the
afternoon and evening as well. At the same time, the main
moisture and instability axis associated with lift from the
shortwave itself may result in some shower/storm activity to
reach west/southwest Iowa as CAM guidance generally shows,
though the severe weather potential is quite low given weak
shear and meager instability over western/southwestern Iowa. A
general thunder risk is outlined per SPC to account for storm
potential near the boundary which makes sense.

By Sunday, with the shortwave grazing west/southwest Iowa as it
moves into Missouri, additional showers and non-severe storms
are looking more likely into western Iowa through much of the
day. Models are not in agreement on the eastward extent of this
activity, with the NAM covering western Iowa, while the GFS/Euro
suggest more widespread development across the CWA given that
the moisture axis makes it further east. The forecast reflects
the more widespread solutions, with highest chances (50-60%)
over western Iowa. The parameter space looks to remain roughly
similar to how conditions look for today, which will lead to
little concern for severe weather. Through early next week, the
shortwave looks to stall out as the mid-level trough over the
eastern CONUS deepens while also slowing down, keeping rain
chances in the forecast through the holiday and Tuesday as well.
Will continue to refine these details over the weekend forecast
packages, though concerns are overall low.

&&

.DISCUSSION...
Issued at 234 PM CDT Fri Aug 29 2025

Upper air charts show that a longwave trough is over
southeastern Canada with a flattening ridge over the southern
Plains and central Rockies. In between these features, there is
fragmented, elongated shortwave energy from the northern Rockies
to just west of Iowa. The downstream trough has been and will
continue to slow down the eastward progression of this
fragmented energy. In addition, there is a residual surface
front from southeast Iowa/Burlington area up to east of Mason
City. Day Cloud Phase Distinction RGB shows this area as having
more vertically developed cumulus clouds. While most locations
will stay dry, this boundary has been the focus for widely
isolated showers and a few storms so far this afternoon over our
far northeastern and eastern service area into northeast and
eastern Iowa. While won`t rule out a funnel cloud in this weak
flow regime, the more favorable parameters look to set up over
northeastern Iowa. What showers or storms do form will likely be
of an airmass variety and may produce a wind gust up to around
30 mph. This is the result of dry air in the mid-levels and to a
lesser degree in the subcloud layer per forecast soundings.
Much of this activity will diminish after sunset.

Fog may once again form after midnight tonight over northeast into
north central Iowa, though mid and high level cloud cover may limit
the extent of this fog coverage. Any fog that does form will
dissipate shortly after sunrise. The stalled north to south boundary
once again will provide a focus for isolated showers or storms in
the afternoon and early evening hours with the focus this time over
northern Iowa with some into western Iowa as the fragmented energy
somewhat congeals into a closed mid-level low. No severe weather is
forecast , but lightning, the potential for a few funnel clouds, and
the potential of a few wind gusts from shower/storm collapse up to
around 30 mph would be possible.

The closed mid-level low will shift southeastward over Nebraska
Sunday into early next week. With the more focused energy and closer
proximity of this low, more coverage of showers and storms is
forecast on Sunday into Sunday night over western Iowa with chances
lessening into central and eastern Iowa. Again, not expecting severe
weather with the main hazard being just lightning on this day with
the additional cloud cover and moisture in the soundings mitigating
the funnel cloud and gusty wind potential that is found on Friday
and Saturday.

As this closed mid-level low begins to move away early next week,
cloud cover will linger along with showers or weak storms on Labor
Day. Confidence in timing and location of these on Labor Day is low
at this time. An upstream trough over central Canada will dive
southward sending a surface cold front through the state late
Tuesday into early Wednesday. Moisture along this front along with
synoptic lift should be sufficient for some degree of shower and
storm coverage as the front moves through. Behind the front, surface
high pressure will fill in over the Plains directing much cooler and
drier air on breezy winds into the region late in the week. Extreme
forecast index of high temperatures shows high ECMWF ensemble
support for unusually cool temperatures late next week. This is also
reflected in the NAEFS 500 mb height that show heights at the -0.5
percentile or lower. Dry conditions and high temperatures in the 60s
to low 70s will be common by late week with lows early Thursday
morning around 40 degrees over northern Iowa.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 636 AM CDT Sat Aug 30 2025

Areas of dense fog and low stratus continue to result in low
visibilities and ceilings leading to IFR/LIFR conditions at the
northern terminals, which will remain over the next few hours
before conditions gradually improve. Otherwise, mid to upper
level clouds will pass through, with low chances (<20%) for
isolated to scattered showers and storms this afternoon to
evening, though given the lower confidence have kept direct
mentions out at this time and will continue to monitor trends.
Winds will be light out of the east/southeast through the
period.

&&

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

&&

$$

UPDATE...Bury
DISCUSSION...Ansorge
AVIATION...Bury