021
FXUS63 KDMX 021121
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
621 AM CDT Wed Jul 2 2025

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

.KEY MESSAGES...

- A few weakening showers or storms arriving from the northwest
  may survive into north central Iowa this morning. A higher
  chance (30-50%) for scattered storms is forecast over north
  central, northeast, and east central Iowa later this afternoon
  into tonight. There is a marginal risk of severe weather with
  hail and gusty winds the main threats if storms do form.

- Warm through the rest of the week, shower and storm chances
  return on Friday into the weekend.

&&

.UPDATE...
Issued at 249 AM CDT Wed Jul 2 2025

Upper level pattern per GOES-East water vapor imagery shows a trough
over the West Coast States with an upper low over central California
with a ridge downstream over the Rockies. A mesoscale convective
system (MCS) is over Nebraska with another cluster of storms over
northeastern South Dakota both being forced by subtle shortwave
troughs riding down the crest of the ridge with a broad trough over
the Eastern US. The MCS over Nebraska is spreading clouds into
western and now central Iowa, most of which are high level. RAP
analysis shows this shortwave induced MCS is also being fueled by a
40 knot low level jet (LLJ) at 850mb. Most convective allowing
models (CAMs) are struggling to resolve both features, but the RAP
and HRRR are handling the MCS well given current placement of
lightning.

As both areas of convective activity move in a southeastwardly
direction as the LLJ veers through the night, they will be heading
into an environment less conducive for their maintenance and a
downward trend is expected. Previous discussion below mentioned
the potential of sub-severe gusty winds, but more recent runs
have this removed. A few showers or weak storms may reach our
service area and linger with those chances highest over
northern Iowa at 20%. More so, expecting to have leftover cloud
cover from both areas of weakened storms that persists into at
least the early afternoon. This could set up a differential
heating boundary that could be the focus for any possible
development in the afternoon hours. Forecast soundings from the
GFS and RAP show by mid to late afternoon that mixed layer
instability is somewhere between 1000 and 2500 J/kg with very
steep low-level and steep mid-level lapse rates. Deep layer
shear is around 35 knots and thus supportive of storm
organization, but effective shear is less than 20 knots and does
raise question about how organized convection could become.
Mid-level dry air contributes to large downdraft CAPE values
over 1000 J/kg. While this environment would suggest strong to
severe storms with hail and gusty wind potential, the forcing is
weak provided by a glancing shortwave trough. A majority of the
guidance is muted on storm chances this afternoon. Into
tonight, a weak LLJ develops and points into Iowa. This weak
forcing contributes to a variety of CAM output from nearly
nothing to varying placement of storms over Iowa. For example,
the 0z HRRR really mixes out the boundary layer in the afternoon
and in the night the elevated warm layer proves sufficient
against the forcing and thus is unable to convect. The 0z
NAMNest and FV3 are more aggressive, which is not uncommon, with
a small cluster of storms dropping out of north central into
eastern Iowa with the FV3 training these storms and producing a
copious, and likely an unrealistic amounts, of QPF. This all to
say that PoPs were trended slightly lower given the variety of
solutions, but for now have focused the highest PoPs up to near
50% over north central, northeast, and east central Iowa from
late afternoon and moreso the overnight hours. Whatever
activity is left Thursday morning should diminish through the
day as the ridge axis from the Rockies pushes over the state.
Highs on Thursday should be well into the 80s to around 90
degrees.

&&

.DISCUSSION...
Issued at 250 PM CDT Tue Jul 1 2025

A look out the window in central Iowa shows a quiet and pleasant day
with mostly clear skies overhead. Cumulus clouds have begun to pop
up early this afternoon, thanks to a thin layer of instability and
saturation around 5000 to 6000 ft. Temperatures are warm but
seasonal in the upper 70s to low 80s with light winds out of the
west to northwest. This same picture can be found throughout the
state today as surface high pressure sits overhead; a much needed
breather to what`s been an active couple of weeks in Iowa.
Unfortunately, this moment of serenity is short-lived as shower and
thunderstorm chances return to the forecast tonight into tomorrow,
then yet again through the holiday weekend.

The 500 mb flow pattern shows generally northwest flow overhead,
which will see a weak shortwave traverse through it tonight into
tomorrow. This wave will pull warm, moist air to our west and
southwest up towards the state, as well as provide weak forcing for
shower and storm chances over the Dakotas tonight. Convection
tonight over the Dakotas will first develop along a 35 to 40 kt LLJ
oscillating southeastward with the shortwave. Short-range models
show this activity being mostly multi-cellular, but some do suggest
upscale growth into at least a weak MCS. This MCS then rides
southeastward along the instability gradient, fueled by the LLJ. The
CAPE/shear environment is meager once this complex arrives in Iowa,
which will help to mitigate severe weather concerns by the time it
arrives around daybreak. HRRR deterministic output does imply
at least some sub-severe wind gusts over northwest Iowa with
storms tomorrow morning, but these generally diminish before
reaching the northwest portions of our forecast area. This is
echoed by HREF max wind gust output which also peters out as
storms arrive in the less favorable severe environment in
central Iowa tomorrow morning. Given the less favorable
environment in central Iowa and support from both ensemble and
deterministic output, severe weather appears unlikely at this
time.

As is typical for these weakly forced systems, these morning storms
will dictate how convection the rest of the day plays out. In the
event that a cold pool/MCS progresses southeast into the state, a
residual boundary could set up and provide lift for parcels in an
increasingly unstable environment tomorrow. In this scenario, storms
would fester along the boundary as the warm, moist west
southwesterly flow collides with it throughout the day. This becomes
especially true during peak heating in the late afternoon and as a
relatively weak LLJ ramps up in the evening. With 2000-3000 J/kg of
instability in northwest Iowa tomorrow afternoon, roughly 30 to 40
kts of deep layer shear overhead, and a well mixed boundary layer
(DCAPE of 500-1000 J/kg), any surface based storms could become
organized and produce strong winds and hail. That said, model
soundings show an elevated warm layer working to negate storm
development within this better afternoon environment. Will want to
watch how mixing impacts this tomorrow, as surface based convection
would lead to our greatest chances for severe weather, especially in
northwest Iowa. Into the evening, instability begins to diminish but
a 20 to 30 kt LLJ will help provide lift for sub-severe storms into
the evening and overnight. Moderate to heavy rainfall will be
possible with any stronger storms as well, and training convection
along a boundary could lead to isolated pockets of heavier rainfall
amounts. Of course, this is all contingent on a boundary, or other
lifting mechanism, being present in the otherwise neutral
environment. While the HRRR/RAP are generally reliable, other
convection allowing models aren`t nearly as aggressive with storms
tomorrow, some of which remain almost entirely dry through Wednesday
night. Therefore, while there is a potential for isolated
strong to severe storms, the threat is certainly conditional and
dependent on how mesoscale features evolve.

Our northwest flow regime generally persists into Thursday, but will
steadily drift eastward as troughing builds over the western CONUS
As this occurs, the thermal ridge will build into the state,
bringing warmer temperatures in the upper 80s and 90s through the
second half of the week and into the first part of the holiday
weekend. A brief period of upper ridging will be in place overhead
Thursday night into Friday morning, providing at least a short
period of dry weather on Fourth of July morning. Unfortunately, as
we go later into the day Friday, the mid-level trough will begin to
knock on the doorstep, which could bring shower and thunderstorm
chances to the western half of the state as soon as Friday
afternoon, with chances persisting along a trailing cold front
Friday evening and through the day Saturday. While it`s a bit far
out to be ironing out any specific details regarding severe weather,
there will be plenty of instability present to produce storms Friday
through Saturday, although initial indications show shear may be
lacking for widespread severe weather. Regardless of severity,
lightning poses a risk to outdoor plans on the busiest summer
holiday of the year, so will be watching convective trends this
weekend closely.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 621 AM CDT Wed Jul 2 2025

Light and variable winds early this morning will become from the
southwest peaking at 6 to near 12 knots this afternoon before
becoming from the south and a bit lighter tonight. While prevailing
VFR conditions are forecast, scattered storms may develop late
this afternoon into tonight. Have continued PROB30 groups at
FOD, MCW, and ALO tonight adjusting the timing and will
continue to monitor forecast trend for future adjustments.

&&

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

&&

$$

UPDATE...Ansorge
DISCUSSION...Dodson
AVIATION...Ansorge