075
FXUS63 KDMX 182254
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
554 PM CDT Fri Jul 18 2025

 ...Updated for the 00z Aviation Discussion...

.KEY MESSAGES...

- Slight Risk (level 2 of 5) for severe weather tonight north
  half and again tomorrow in the south and east. Damaging winds
  are the main threat.

- Slight Risk (level 2 of 4) for excessive rainfall into Sunday.
  Hydro threat low tonight, but may increase with subsequent
  rounds through the weekend.

- Warming temperatures into mid next week with return of 100+
  degree heat indices, especially south.

&&

.DISCUSSION...
Issued at 249 PM CDT Fri Jul 18 2025

Active weekend of weather ahead but subsequent rounds of storms are
all conditional and dependent on how the round ahead plays out
making the forecast for the weekend complex and challenging to say
the least.

As expected, surface high pushed off to the east early this morning
with increasing mid-level flow out of the southwest bringing the
return of moisture as dew points this afternoon have surged into the
60s to 70s. Although temperatures are only in the 70s to low 80s,
conditions certainly feel much more humid out there compared to our
pleasant day yesterday with heat indices well into the 80s,
especially south.

Our first shortwave in a train of shortwaves beneath the upper west
to northwesterly flow into next week is a mid-level low tracking
through the Dakotas/MN into tonight with a trailing boundary that
will move through the area overnight. Additionally, the boundary to
the south is also lifting back north this afternoon, expected to
make it through much of the area into tonight as well. The
environment in the warm sector ahead of the boundary diving
southeast late will be characterized by capping early, as seen in
SPC Mesoanalysis data early this afternoon, but also with lingering
lower clouds from the overnight limiting the storm threat this
afternoon into evening though some showers are trying to kick off in
parts of northwest Iowa this afternoon. Forcing overall as we head
into tonight is weaker, but theta-e advection continues to increase
with the ample moisture translating to 1000-2000+ J/kg of MUCAPE
(higher to the north and west) with bulk shear of 30-40 knots, with
higher values to the north of the IA/MN border at 3Z. Although some
timing discrepancies remain from run to run and model to model,
general thoughts are still on track with storms developing in SD/MN
and tracking southeast with time late tonight into the overnight.
The LLJ prior to today wasn`t looking particularly robust, but
latest data has increased speeds through NE and SD pointing into
where convection is supposed to develop helping to initiate
convection before it tracks further south and east. Initial
storms may have a large hail and tornado threat, but this is
more likely outside of our area with storms expected to grow
upscale into a line largely by the time it gets into our CWA.
This will make the primary threat overnight strong to damaging
wind gusts as storms congeal and track through the area.
However, will need to watch conditions closely, especially far
north to northwest where the tornado threat may still maintain
into our area pending exactly where and when storms develop.
CAMs largely have come into agreement that this line of storms
will be maintained through much of the area (though as noted at
different times) the strength of which should wane with time as
it will become cold pool dominant the further south/east with
the better shear to the north, but exactly where storms start to
weaken remains uncertain and the SPC Day 1 Slight (Level 2 of
5) generally covers the better threat area well, with the
Marginal (Level 1 of 5) through the rest of the area. Although
locally heavy rain is also possible with storms tonight given
favorable warm cloud depths and pwats nearing 1.5-2", the
overall hydro threat through the night remains lower with
expected progressive storms, several dry days now, continuing
maturing crops, and QPF values largely below 1-2", though
locally higher amounts of 2-3"+ are possible as seen in the 12Z
HREF localized probability matched mean data into 12Z Saturday.
3-Hr flash flood guidance is around 3" in southern Iowa and 2-3"
in other areas but given our mature crops, this is likely
underdone, so overall thinking we have the capacity for the
expected QPF tonight and will evaluate further periods with
time.

Beyond overnight, additional storms are forecast at times through
the rest of the weekend but timing is highly dependent on where the
boundary is located and how the atmosphere recovers. The 12Z HRRR is
certainly concerning given the environment that could be in place
later Saturday, but this is also conditional on our potential
instability being realized. All this to say, there are certainly
additional opportunities for locally heavy rain and some strong to
severe storms, but will need to dive into the mesoscale details
following this initial round overnight to really have the full idea
of what and when the next potential threat will be. With each
subsequent round over the same area, the hydro threat does increase,
but this is not a guarantee that areas will see every round of
storms that may be possible through the weekend. Southern Iowa is
the most likely location for additional storms later Saturday, and
then another round could be fueled overnight into Sunday morning.
NBM PoPs remain overdone through the weekend accounting for the
uncertainty in timing of storm development but it won`t be
raining/storming at every moment of the weekend. See the WPC and SPC
Day 1-3 risks for further details, but any with outdoor plans should
keep a close eye on the weather this weekend and with overnight
storms as well, folks should have multiple ways to receive warnings.

Temperatures will generally be on an increase into next week but the
weekend remains more uncertain with how storm chances play out.
However, the heat looks to build into mid next week where 100+
degree heat indices return to the forecast in especially southern
Iowa, pending additional storm threats at times with a
continued active pattern.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 554 PM CDT Fri Jul 18 2025

MVFR to VFR ceilings persist across the area this afternoon and
will last through much of the TAF period. Thunderstorms across
northwest Iowa late tonight will move across the area through
the overnight and Saturday morning. Strong storms will bring a
damaging wind threat overnight which will be addressed in more
near term amendments later in the period. Uncertainty remains in
the exact timing and placement of storms overnight, so have
continued with a prob30 mention within TAFs. Visibility and
ceiling reductions are also likely with storms. Additional
storms possible later Saturday but at the very end of this TAF
period and dependent on how first round plays out overnight
into early Saturday.

&&

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

&&

$$

DISCUSSION...05
AVIATION...Hagenhoff