479
FXUS63 KDMX 010857
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
357 AM CDT Mon Sep 1 2025

.KEY MESSAGES...

- Shower and weak storms possible again today, with the highest
  chances(50-70%) this morning southwest before gradually
  drying

- Low potential for showers and weak storms north Tuesday
  afternoon to evening (20-30%), with a strong cold front
  bringing better chances (50-70%) late Tuesday through
  Wednesday. No severe storms are expected at this time

- Cooler and generally drier conditions to follow with high
  temperatures expected to only reach the 60s, with overnight
  lows in the 40s (maybe even some upper 30s in areas across the
  far north!).

&&

.UPDATE...
Issued at 350 AM CDT Mon Sep 1 2025

Observations this morning indicate that fog is quite isolated over
parts of northern Iowa, particularly at Estherville where
visibilities of a mile are being reported. With skies expected to
remain clear over the area with light winds and residual moisture
northwest in particular, the expectation still remains that some
additional locations may see fog development, as guidance continues
to indicate. Any additional fog should remain more patchy in nature,
so not anticipating much impact but will continue to monitor through
the next few hours through sunrise.

Outside of the fog, the overall pattern hasn`t changed very much
over the past 24 hours, with the shortwave responsible for much of
the rain showers in Iowa shifted a bit further south and east into
southeast Nebraska and western Missouri today. This will generally
keep the main stratiform rain shower activity over portions of
southern and western Iowa for the rest of the morning, before the
moisture axis slowly descends further south of the state and rain
chances gradually diminish this afternoon and especially into the
evening. Otherwise, the weak boundary that has been over northwest
to north-central Iowa over the past few days in relation to the
shortwave is expected to be displaced a bit further west today. Weak
convergence mainly over north/northwest Iowa into the afternoon to
evening and peak heating in the same area may bring at least one
more chance for weak storms and even a few funnel clouds over that
area, as weak instability builds and shear remains fairly weak.
Additional rainfall totals by the end of today are expected to
generally be less than an inch, though again cannot rule out locally
higher amounts in locations where slow moving storms may occur.
Highs are expected to warm into the mid to upper 70s for most,
though slightly cooler southwest where thick clouds will limit the
heating, with values expected in the upper 60s.

With the shortwave largely south of the state by Tuesday, dry
conditions are generally expected into the morning as low level flow
shifts westerly. However, midlevel moisture from the upper midwest
is signaled to descend into Iowa, with a weak boundaries moving
across southern Minnesota and another extending into Iowa by
late/morning to early afternoon as CAMS suggest, which looks to
bring a potential for some isolated showers and storms mainly near
and east of I-35 into the afternoon and evening. Looking further
into the environment shows again fairly weaker shear and
instability, so would expect a very low potential for any storms to
hold together for very long. Highs Tuesday are expected to reach the
mid to upper 70s.

By Tuesday evening, a deepening Canadian low pressure system is
expected to drop south into the Upper Midwest, with a more defined
cold front sinking into Iowa by late Tuesday into Wednesday morning.
Showers are generally expected with this frontal passage throughout
the day, ending from northwest to southeast into Wednesday evening.
Considering storm potential, the ideal parameter space will be into
Minnesota and the Dakotas given the location of the front during
peak heating, where instability values over 1000+ J/kg and better
shear are looking to be co-located. The severe potential diminishes
into the evening, when the front would be further south getting into
northern Iowa, though with at least some lingering unstable
conditions early on cannot rule out some storms initially, though
not looking severe at this time. Will have to monitor trends as
timing and location could shift slightly, which may result in a
shift in the better storm potential and so on. Following the frontal
passage, a cold advection regime then takes over that is
characterized by increasing northwesterly flow. High pressure will
descend into the Western Plains following this frontal passage, with
cooler temperatures expected.

&&

.DISCUSSION...
Issued at 251 PM CDT Sun Aug 31 2025

The blocking pattern and resultant stagnant progression remains
locked in across the CONUS as a broad ribbon of nearly stationary
shortwave energy extends NW-SE from the Dakotas thru the mid-
Mississippi River Valley. It`s here that a broad area of moist
ascent is contributing to intermittent rain showers and embedded
thunderstorm chances that will persist thru midday Monday mainly
central and west. Similar to yesterday, short-range CAM guidance
points to a NW-SE oriented convective band forming north and east of
DSM this afternoon along the differential heating boundary. Rainfall
has been highly localized as slow motions and training
showers/storms resulted in large gradients over short distances,
which will likely persist with tonight and early Monday`s activity.
Like last night, some areas may pick a quick several inches of rain.
However, do not anticipate any widespread flooding concerns given
recent run of dry weather.

The shortwave finally begins to depart on Monday as the downstream
Rex Block finally breaks, allowing the pattern to become progressive
once again. Rainfall chances will remain during the morning hours,
however will see a general decrease by Monday afternoon as the
system departs. High temperatures Monday look very similar to today,
reaching into the 70s for most locales.

Tuesday will be a transitional day as we await the arrival of the mid-
week cold front. Well upstream, the stationary low over the eastern
Pacific retrogrades, amplifying the ridging across western NOAM and
steepening the meridional flow into the center part of the country.
By Wednesday morning, this will bring a strong cold front
strengthening winds, turning them northwesterly, and advecting a
cooler air mass into the state. There will be no shortage of forcing
along the front, so rain chances will increase in proximity of the
cold front moves through. Behind the front, 850mb temperatures are
progd to fall to 0 to 2C across northern sections of the state. This
means high temperatures will generally only warm into the 60s during
the afternoon; with overnight lows falling into the 40s. There are
some indications that upper 30s will be a possibilitiy across the far
north as well.

A second cold front appears to pass through Thursday night
reinforcing the cool air mass while bringing the possibility of
another quick shot of rain. Otherwise, expect dry conditions through
the remainder of the period.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1052 PM CDT Sun Aug 31 2025

Scattered showers and isolated thunderstorms will linger through
Monday, but mainly across western and southwestern Iowa with
probability significantly lower (20% or less) at our TAF sites.
Therefore, there is no mention in the outgoing 06Z TAFs. Areas
of fog will likely develop late tonight/early Monday morning as
well, but again will primarily be west of the terminals with
some impact possible at MCW/FOD but more unlikely at
ALO/DSM/OTM. Will advertise MVFR at the former two sites, but
amendments may be needed overnight based on short-
term/observational trends.

&&

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

&&

$$

UPDATE...Bury
DISCUSSION...Hahn
AVIATION...Lee