503
FXUS63 KDMX 041734
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1234 PM CDT Fri Jul 4 2025

 ...Updated for the 18z Aviation Discussion...

.KEY MESSAGES...

- Very warm and humid again today. Heat index values may
  approach 100 degrees at times.

- Thunderstorm chances increase late tonight northwest and for
  much of the area on Saturday. A few strong storms may occur
  with damaging winds the primary threat.

&&

.UPDATE...
Issued at 343 AM CDT Fri Jul 4 2025

Upper ridging is in place across Iowa early this morning and
that ridging is outlined well with several areas of
thunderstorms/MCSs this morning on the outer periphery of the
ridge from Wisconsin to northwest Minnesota/ North Dakota and
back south into Wyoming and Colorado. Much of the Ridging is
capped by the old school 700 mb 12c or greater temperatures
though the northeast Iowa capping is situated closer to 800 mb.
The upper ridging will remain over central Iowa much of the day
which should prevent any storms today and will also lead to
another very warm and humid day with highs in the the upper 80s
to mid 90s. Heat index values will approach 100 at times with
dew points in the upper 60s to mid 70s.

The upper ridge will begin to move east tonight as an short wave
moves into the Dakotas and this will also drag a surface
boundary towards the state. Initially though this will result in
southwesterly flow into Iowa as the ridge departs and the low
level jet leaning into northwest Iowa along with saturating
profiles and convective instability. This will lead to
increasing thunderstorm chances over the northwest late tonight.
The surface boundary will move slowly across the region Saturday
with MLCAPE in vicinity of the boundary of 2000+ J/kg. Some
speed shear of 20-30kts will exist in the effective layer but
the directional shear isn`t great. A few strong storms will be
possible Saturday with the primary threat being damaging winds
while small hail could occur as well. The tornado threat is low.
Sunday will be cooler with highs in the 80s. A few storms may
linger over the far southeast.

&&

.DISCUSSION...
Issued at 243 PM CDT Thu Jul 3 2025

It`s a hot and humid afternoon over Iowa with blue skies and light
to breezy winds. Dewpoints have skyrocketed into the mid 70s this
afternoon with ambient temperatures already in the upper 80s to low
90s early this afternoon. This has bumped heat indices up to the
upper 90s to even over 100 in some locations, further emphasizing
the hot conditions. While there is likely some minor advective
component to these moist conditions, the main driver of these higher
dewpoints today is likely evapotranspiration from the corn. As
the boundary layer continues to mix this afternoon, drier air
aloft should mix down and slow or reverse the increasing
dewpoints. That said, temperatures will continue to increase,
which could bring a few hours of heat indices between 100 to 105
still this afternoon. Along with these warm, moist temperatures
this afternoon is a very unstable airmass overhead.
Fortunately, a small amount of capping and the lack of any
forcing/lift to displace parcels will help to negate widespread
thunderstorm development this afternoon and evening. A pop up
shower/storm or two could still crop up in the east as winds
back slightly from southwestern into northeastern Iowa this
afternoon, although convergence would be very weak if any occurs
at all. If a storm does develop, there is a marginal risk for
severe weather (level 1 of 5) across the northeast, with the
main concerns being wind and hail, although the lack of shear
means pulsey, isolated storms. Moderate rainfall would also be
produced by any stronger updrafts that develop, but again, the
risk of anything developing in our forecast area is low (15% or
less).

Temperatures remain warm into Friday, as the approaching upper level
wave (and coincident clouds/precipitation) has slowed down a bit now
arriving a bit later in the day on Friday. This should keep skies
clear through at least mid-day for much of the forecast area,
allowing temperatures to again climb into the upper 80s to near 90.
Dewpoints will be similar to slightly lower, putting heat indices in
the mid 90s to near 100 again. By the afternoon and into the
evening, cloud cover will move in ahead of the approaching wave. The
slower and drier arrival of the wave should keep conditions dry
through much of the Fourth of July/Friday.

Shower and thunderstorm chances then return late Friday night to
early Saturday morning as theta-e advection increases and a low
level jet noses into the state. The profile will have plenty of
moisture present as this occurs, with 2+" PWATs and strong moisture
transport. That said, models continue to trend downward with
precipitation coverage and amounts as the trailing front moves into
the state, likely due to the better forcing departing north and east
with the main wave. This should help mitigate prolonged heavy
rainfall from this system, as the environment will be favorable for
heavy rainfall but likely won`t have the lift to produce consistent,
widespread storms. However, will want to keep an eye on high
resolution guidance as it comes in for Saturday afternoon and
evening, as the atmosphere will be quite unstable and any lift along
the cold front will be enough to produce storms over central Iowa.
The wind profiles are relatively benign, which should limit
organized convection, but could still see some small hail and gusty
winds with any more efficient updrafts. Likewise, locally heavy
rainfall will be likely under any stronger storms. Of course, severe
or not, thunderstorms always have lightning, so it`s important to
stay weather aware this weekend if you have any outdoor holiday
plans. Intermittent precipitation chances continue Saturday night
and mostly depart by Sunday, barring a few lingering showers/storms
in southeast Iowa. With the cloud cover and scattered precipitation
on Saturday and northerly flow behind the front on Sunday,
temperatures will be slightly cooler through the weekend with highs
reaching the mid to upper 80s. More precipitation chances return
then toward the start of next week as weak shortwaves pass through
what will be a zonal flow pattern. However, the spread between
models in regards to timing and location is quite large, so will
discuss that more as we go through the weekend.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1231 PM CDT Fri Jul 4 2025

VFR conditions prevail with breezy south southwesterly winds.
Scattered cumulus at around 5000 to 6000 ft has begun to
develop early this afternoon, and will likely persist through
sunset. Winds die down overnight, but cloud cover and
precipitation will increase from west to eat into the morning
hours. Showers and thunderstorms are possible at all sites
tomorrow morning and into the afternoon, but confidence is
highest in sites over northern Iowa. Storms will also be
accompanied by MVFR to potentially IFR ceilings, again most
likely over northern Iowa.

&&

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

&&

$$

UPDATE...Donavon
DISCUSSION...Dodson
AVIATION...Dodson