317
FXUS63 KDMX 030809
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
309 AM CDT Thu Jul 3 2025

.KEY MESSAGES...

- Hot and humid today with isolated afternoon or early evening showers
  or thunderstorms possible in north central into eastern Iowa.
  Gusty winds or small hail possible.

- Much of Independence Day will be hot and humid. Thunderstorm chances
  will move in from west to east late in the evening if not
  after midnight for most central Iowa towns.

- Higher chances at 40 to 65% for thunderstorms Saturday into Saturday
  night. Severe weather and heavy rainfall threat continues to
  look low.

&&

.UPDATE...
Issued at 309 AM CDT Thu Jul 3 2025

GOES-East water vapor imagery shows some progression in the upper
level pattern from 24 hours ago with the ridge axis moving over the
High Plains while the trough is now over southern California.
Convection tonight has been rather limited upstream of Iowa with
just a very weak wave passing over the eastern Dakotas. Regional
radar shows a few storms festering between Sioux City and Sioux
Falls with this activity more forced by the 850mb warm air
advection, which can be seen as the brighter/lighter blue line in
the Nighttime Microphysics RGB moving northward over northern Iowa
and then the darker blue area moving northward over South Dakota.
Confidence in how these storms may fester into Iowa is low with any
convective allowing model (CAM) being farther north and/or delayed.
For now, have brought low PoPs into northern Iowa that then drop
into east central Iowa later this morning. These may need to be
adjusted southward based on current trends.

Whatever is left of this activity should diminish toward mid to late
morning. There will likely be some degree of residual cloud cover
over northern into eastern Iowa, but as that erodes through the day
surface instability grows to over 3000 J/kg with steep low and mid-
level lapse rates developing. Unless there is some residual boundary
to aid in convergence, the overall weak forcing from a subtly
passing shortwave trough will likely prove insufficient for much
more than isolated showers or storms over north central into the
eastern third or so of Iowa. Even then, deep layer shear is 25 to 30
knots with effective shear about half of that. So, storms may be
able to organize into multicells or may end up just being more
disorganized. Either way, forecast soundings show plenty of dry air
in the mid-levels and sub-cloud layer with downdraft CAPE values
over 1000 J/kg. Thus, there is potential of gusty downdraft winds,
which are currently being advertised as sub-severe per CAMs. The low
level instability upwards of 150 J/kg and the steep lapse rates
could result in small hail to perhaps 1" severe criteria hailstones.
Any storms will push off to the east or diminish by early this
evening.

As the shortwave trough moves into the Great Lakes region, the upper
level ridge will push over the state Friday and off to the east
Friday night. This will allow the flow to become from the southwest
or south in the low levels with the trough over the West Coast
lifting into the region Friday night through Saturday night. With
broad, low level theta-e advection along the 30 knot low level jet
axis, storms will develop late on the 4th of July. Compared to this
time yesterday and inherited forecast from the dayshift, timing has
slowed a bit and current expectation is that much of Independence
Day and the evening will be dry over many central Iowa locations
within our service area with storms move focused near and west of
the Missouri River. However, as phased low level thermodynamics and
kinematics push eastward later Friday night into Saturday, so will
the storm chances. While not a washout on Saturday into Saturday
night, chances of storms peak in the 40 to 65% range. Instability
somewhere between 1000 and 2000 J/kg will be sufficient for storms,
but storms may not be able to organize in the weak deep and
effective shear regime. The environment also looks to be one
supportive of efficient rainfall, though current QPF forecast from
global models point to 1 to 2 inches at most, though the NAM and
regional Canadian do have their higher pinpoint values.

&&

.DISCUSSION...
Issued at 250 PM CDT Wed Jul 2 2025

A remnant MCV over southern/southwestern Iowa continues to produce
isolated thunder and showers early this afternoon. While this
complex is moving into a less favorable environment in central
Iowa, boundary layer mixing will lead to destabilization through
the afternoon which may lead to additional showers/storms in
proximity to the MCV. Of course, short range models provide
little confidence to this afternoons forecast, as they`ve been
trying to dissipate this precipitation for the last couple of
hours. Therefore, will be keeping a close eye on radar trends as
this MCV continues to churn this afternoon. With more mixing in
the boundary layer, storms that develop in proximity to the MCV
could become surface based, which would provide a slightly
better shear profile thanks to the southwesterly low level
winds. SPC mesoanalysis data reflects this, with effective shear
values ahead of the MCV around 20 to 30 kts and modest but
increasing SBCAPE values of 500 to 1000 J/kg. This would be
conducive for a few stronger storms, especially further west
toward the higher instability. Fortunately, the MCV should
continue eastward away from the more favorable severe weather
environment to the north and west. This area will have
instability values in excess of 2000 J/kg and a much better
shear environment boasting 35 to 40 kts of effective shear, as
well as long hodographs aloft. This makes this region more
favorable for severe thunderstorms capable of producing large
hail. Despite this favorable environment, this area also looks
capped off with -50 to - 100 J/kg of mixed layer convective
inhibition (MLCIN) thanks to the warm layer aloft. With no
boundaries/convergence nearby, parcels will struggle to find any
source of lift to displace parcels and bypass this convective
inhibition. Therefore, there is a conditional threat for an
isolated severe storm or two in the north and west, but the
likelihood of producing one in this environment is low.

Models continue to struggle with resolving how convection will play
out in the overnight hours, as solutions vary on the strength of the
low level jet (LLJ). The HRRR and RAP have been in lockstep today,
struggling to generate any precipitation overnight tonight.
Conversely, more rambunctious models like the NAM Nest and FV3 show
storms developing along the nose of the low level jet through much
of the overnight hours. The GFS also indicates a slightly stronger
LLJ and convection overnight, in contrast to it`s higher resolution
siblings (HRRR/RAP), although it has backed off on the amount of
precipitation with it`s 18z run. This all to say, tonights
shower and thunderstorm chances are not a high confidence
scenario. Most unstable CAPE will remain in the 1000 to 2000+
J/kg range overnight, but the deep layer shear environment will
be waning as the surface decouples. This will help negate the
threat for organized storms/severe weather, but the higher
instability does still leave a concern for at least pulsey
convection and hail overnight. Moderate to heavy rainfall would
also be a concern if precipitation matches what is shown in the
NAM Nest/FV3, but this looks to be on the higher end of our
expectations at this time.

The low end chances for scattered showers and storms continues into
Thursday, but do drift farther northeast as the upper level
northwest flow regime continues further east and upper ridging
builds from the west. With a very similar environment to today,
and the same lack of real great forcing, expecting a similar
low end thunderstorm threat in the far northwest portions of the
forecast area on Thursday afternoon and evening. Shear will be
slightly weaker on Thursday, but there will also be less of a
warm layer to inhibit storms, producing less of a severe weather
threat but more opportunity for storms to develop. That said,
have kept precipitation chances lower given the potential for
storms to be displaced from our area entirely, depending on the
model you choose.

In addition to the scattered storm chances in the afternoon,
temperatures will be much warmer tomorrow with highs in the upper
80s to low 90s. With dewpoints in the low 70s, heat indices will
push into the upper 90s to near 100 in some locations, making for a
muggy day in Iowa. Temperatures then stay warm into the Fourth
of July holiday, but increasing cloud cover ahead of the
approaching upper level trough will drop temperatures down
slightly in the west. This cloud cover will precede the return
of shower and thunderstorm chances through the holiday weekend,
beginning in the west on Friday and persisting area-wide through
Friday night into Saturday. The severe weather threat will
continue to be assessed through the coming days, but the general
lack of shear being depicted in longer range models will make
it difficult for strong, organized storms to develop. Forecasted
PWATs over 2" and strong low- to mid- level moisture transport
suggests storms could be efficient rainers, which could lead to
aggravation on area rivers. However, forecast QPF isn`t overly
high at this range, so will continue to monitor this threat as
we draw closer and models begin to resolve things better.
Regardless, the most important takeaway is the potential for
thunderstorms/lightning during a busy holiday weekend.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1045 PM CDT Wed Jul 2 2025

VFR conditions are expected across the terminals through the TAF
period. Have removed shower chances into Thursday morning
across the northeastern sites as chances have further
diminished. Areas of high clouds are otherwise expected at
times, with light south/southeasterly winds turning more breezy
into late morning.

&&

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

&&

$$

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