036
FXUS63 KDMX 090923
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
423 AM CDT Wed Jul 9 2025

.KEY MESSAGES...

- Several rounds of thunderstorms possible Tonight through
  Friday night.

- Locally heavy rainfall and a few strong storms may occur
  tonight into Thursday.

- More widespread storms with the potential for greater areal
  coverage of heavy rain along with a few strong storms Thursday
  night through Friday night.

&&

.UPDATE...
Issued at 421 AM CDT Wed Jul 9 2025

Mainly focusing on convective trends and associated potential
hazards though Friday night with this update. Quick touching on
current weather as some fog has developed in the river valleys again
this morning and is currently most prevalent in part of the
Nishnabotna river valley though the typical Fort Dodge drop has
occurred with the sensor on the low end of the site close to the Des
Moines river. A band of ACCAS has developed over north central Iowa
this morning and there was even a few showers earlier near Fort
Dodge, that have since dissipated. This has developed in a region
where a ribbon of theta-e advection aloft is moving through and has
resulted in an area of Showalter values near 0 and some very modest
MUCAPE values and instability. Note the NAM is way over saturated
near 800-850 mb resulting in much higher MUCAPEs. The main axis of
the low level jet(LLJ) remains to the west and extending through
western Kansas,central Nebraska and north into the Dakotas. There
are a few clusters of storms scattered about within this LLJ axis.
This region is lacking any short wave support and it has limited the
areal coverage of storms in addition to profiles still lacking full
moisture return from the south.

Much like Tuesday, it remains possible today that a very isolated
shower or storm develops but at this point the chance is low enough
at any one site to exclude mention. The focus is on tonight through
Friday night with numerous chances for thunderstorms. While the 500
mb flow currently has upper ridging to the US Rocky Mountain region,
there is a strong upper level short wave moving over northern
California. That short wave will flattened the ridging by tonight and
will allow for several energy fragments to move into Iowa over the
next several days. One of these fragments of energy is ahead of this
system and currently over Idaho. Analysis of the Potential Vorticity
1.5 pressure surface allows these features to stand out. The Idaho
PV anomaly will arrive into western Iowa tonight and move across the
area on Thursday. The LLJ and instability will tilt into Iowa as
this occurs. It will also allow more moisture to lift back into Iowa
with pwat values approaching 2 inches once again. Should an MCS
develop, the overall propagation of the system will be south into
the instability axis and this could lead to a band of heavy rainfall
somewhere west of Interstate 35. The short wave energy will move
across central and eastern Iowa on Thursday with additional
thunderstorms possible upper level system. By Thursday afternoon,
there is an 850 mb dew point minimum over central Iowa which is part
of an Elevated Mixed Layer (EML) settling in. Despite the presence
of the EML, there will be instability present above this layer which
could lead to weak convective development in the afternoon and
evening. Note, heavy rainfall rates will occur at times with
individual cells.

Thursday night and Friday timeframe is still the period to watch as
that primary stronger upper level short wave arrives. As of current,
still expected a warm front to lift into Iowa and there are still
signals that a Maddox Frontal heavy rain event could occur given
overrunning moisture over that boundary and 850 mb dew points
approaching 20C in addition to ample warm cloud depths. Much of the
guidance is pushing pwats above 2" at times in some areas. Therefore
still monitoring the potential for a band of very heavy rainfall.
Storm motion speeds are less certain at this point. Also monitoring
the severe weather potential, especially with the warm front in the
region. Surface winds are relatively light but there is enough
curvature and speed shear in the 0-3 km layer to it potentially
interesting with rotating storms. Additional storms will continue
into Friday night before the system passes east of the state leading
to a mostly dry weekend.

Another system is on the horizon around next Tuesday/Wednesday which
could bring another round of strong storms to Iowa.

&&

.DISCUSSION...
Issued at 247 PM CDT Tue Jul 8 2025

Clouds have been clearing out of the state after storms rolled
through much of western and southern Iowa early this morning. This
afternoon, with the clearing sky, temperatures have once again
warmed well into the 70s to low 80s. Have seen cumulus development
over southern Minnesota ahead of a weak front and this has dropped
into northern Iowa. The temperatures are pushing towards the
convective temperature, but there is an elevated warm layer. A
glancing shortwave trough to the northeast may provide enough to
overcome this along with the surface boundary, but not expecting
more than an isolated storm or two over northern Iowa where those
cumulus clouds are dropping down this afternoon. High pressure will
pass over the state tonight with very light winds as the surface
front settles just south of the state. With an inversion setting up
and the recent rainfall, this may foster an environment conducive
for fog formation. However, forecast soundings show that full
saturation is not achieved, but surface and low level dewpoint
depressions are just a few degrees. For now, have added some patchy
fog mentions in eastern portions of the forecast area.

As the high moves off to the east Wednesday, will be looking at a
more active period based on pattern recognition along with AI/ML
guidance from late Wednesday night/early Thursday morning through
late Friday. However, there is lack of strong synoptic forcing and
thus the details on hazard type and timing will rely on mesoscale
features like leftover boundaries or depend on the degree of
convective cloud debris that are largely unknown at this time. For
Wednesday night/early Thursday, low level warm air advection may
bring some sort of convective cluster towards the state. While
severity likely will be waning into Iowa, efficient, heavy rainfall
parameters will be favorable with a 30 knot low level jet (LLJ)
funneling moisture into the state. There is the potential for a
corridor of higher rainfall totals, which are reflected in the 12z
GFS, FV3, and CMC along with the latest HREF max field over a small
portion of western into central Iowa. This may not be the most
probable outcome given the rest of the guidance, which shows amounts
around or less than an inch. However, WPC will have a marginal over
portions of western into a bit of central Iowa for this potential.

As we move into the day Thursday, depending on the amount of
clearing and any outflow boundaries lingering, could have several
thousand J/kg of instability by late afternoon with the front south
of the state now lifting back into Iowa. A lead shortwave trough
topping the ridge will drop into the state and provide for some
higher deep layer shear values in the 30 to 40 knot range. This
parameter space would suggest strong to severe storms by later in
the afternoon into the evening hours being possible. The LLJ will be
more pointed into Iowa Thursday night so expecting the severe storm
potential will lessen, but the heavy rainfall and flash flooding
concerns will be increasing into the nighttime hours. The
aforementioned front will be perpendicular to the LLJ axis and thus
provide for repeated rounds, or training, of storms over the same
area with deep, favorable warm cloud depths and precipitable water
values of 1.5 to around 2 inches. This will add additional rainfall,
perhaps several more inches, and lead to flash flooding, especially
if the rain falls over more vulnerable basins from  the previous
night`s rainfall.

The Thursday night/early Friday storms should diminish to some
degree in the morning hours of Friday, but a shortwave trough
translating eastward from the West Coast will bring some additional
forcing into the region. While cannot rule out stronger storms in
the vicinity of the stalled boundary, deep layer shear will not be
as high at just around 30 knots on Friday afternoon/evening with
plenty of instability. The bigger concern will be flash flooding as
the rainfall adds up with little change in the overall favorable
setup for heavy rainfall on Friday into Friday evening. While there
are timing differences in how quickly the shortwave trough exits,
conditions will trend drier on Saturday.

While details such as the location of the heavy rainfall cannot be
certain this far out, the soil capacity is the least in northern
Iowa, but has also now been reduced in southern Iowa with last
night`s rainfall. River channels across the state also have reduced
capacity with USGS streamflows at any of the 7, 14, or 28 day
averaging period showing at least normal if not much above normal,
especially over northeastern into north central Iowa. While the
ensemble QPF hydrographs are likely suffering from not have the full
window of rainfall as it only goes through early Friday, the
experimental five-day Hydrologic Ensemble Forecasting Service (HEFS)
does show more response at the 10% chance exceedance level. Five day
forecast of the high magnitude flow from the National Water Model
(NWM) forced by the GFS, which has the higher QPF, shows low annual
exceedance probabilities (AEP)/higher streamflows on some streams in
central Iowa. This is all pointing toward rises or renewed rises on
some streams in central Iowa late this week if not this weekend.
Whether it is river or flash flooding or severe weather later this
week, Iowans should continue to monitor the forecast and have a
means to receive weather alerts whether NOAA Weather Radio and/or a
notification service like Alert Iowa.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1035 PM CDT Tue Jul 8 2025

VFR conditions are likely for much of the forecast period.
Patchy light fog chances remain for Wednesday morning, but with
low confidence in occurrence and placement have kept mentions
from TAFs. Thunderstorms chances return beyond the current TAF
period overnight Wednesday to Thursday and will addressed in
later updates.

&&

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

&&

$$

UPDATE...Donavon
DISCUSSION...Ansorge
AVIATION...Hagenhoff