836
FXUS63 KDMX 102321
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
Issued by National Weather Service Duluth MN
621 PM CDT Thu Jul 10 2025

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

.KEY MESSAGES...

- Flood Watch for flash flooding in effect for portions of west-
  central Iowa this evening through tomorrow morning. Very heavy
  rainfall amounts in excess of 5 inches possible in localized
  areas within the watch. Rainfall rates of 2 inches per hour
  possible.

- Enhanced Risk for severe weather in portions of western and
  northwest Iowa. Damaging winds in excess of 70mph possible
  with storms (10% chance locally) along with a tornado or two
  possible. The severe threat extends past sundown.

- Additional severe weather, primarily winds and a tornado or
  two, possible in the southeast Friday. Scattered storms could
  persist much of Friday, becoming more widespread in the
  afternoon through Saturday morning southeast.

&&

.DISCUSSION...
Issued at 128 PM CDT Thu Jul 10 2025

A complex setup featuring both synoptic and mesoscale features will
drive the short term forecast today. At the synoptic scale, there
are two noticeable shortwaves visible in satellite imagery: one
associated with the cloud swirls across southern MN and the other
collocated with the EML and surface low in Nebraska. The MN
shortwave will be the culprit for a cluster of thunderstorms across
northern and northeast portions of the state this afternoon with
wind being the main severe threat with these. Localized enhancements
from a nearby MCV increase confidence in storm coverage. Further
west, as expected, storms have sustained themselves along the
gradient of the EML, even mixing some of it down in the form of heat
bursts in places like Lincoln, NE. The parent shortwave will eject w
est-to-east through the day and pull the instability axis with it,
bringing this thunderstorm activity into western and central Iowa
late this afternoon. Enough midlevel saturation exists above the EML
that there could be thunderstorm development behind the gradient.
The confidence in surface-based severe threats has increased given
the clearing over western Iowa this morning with a damaging wind
threat existing with storms on the gradient, plus with the narrow
enhancement of deep-layer shear over 40kts thanks to the influence
of both shortwaves nearby. Theta-e differences of -40K are
forecasted for this same area of concern due to the dry air
pocket from the EML. Can certainly foresee 70mph+ winds in parts
of the line that forms. The Enhanced Risk upgrade is warranted
for the wind risk. Another big concern, however, is the extra
round of rainfall that only exacerbates soil moisture conditions
for the heavy rainfall setup tonight, increasing the amount of
rain that will become runoff.

To that point, more moisture is on the way for tonight, seeing the
+14C dew point at H850 at Dodge City. The MCS with wind in the
evening will get caught along the warm front of the main cyclone.
This boundary will land somewhere across central Iowa draped west-to-
east. The MCS will then follow the forward propagation vector
and likely move along the front. The boundary ingestion as well
as the very moist profile will sustain at least a marginal wind
threat through the night. With dew points sitting in the 70s,
the surface- based potential will be extended past sunset due to
the PBL taking longer to cool. Paired with the 0-3km shear
reorienting to westerly around 30kts, can`t completely rule out
a tornado threat near where the warm front intersects or where
the line becomes favorable aligned. Boosting confidence in this
is the elongation of low-level hodographs from the LLJ. Emphasis
again, however, that heavy rainfall will be the main concern at
this point. The overnight updates have often compared the setup
to the Maddox Heavy Precipitation setup which rings true. The
mean flow will also be oriented from W-to-E, the same as the
front. Flow will also be light, making storm motions slower. The
LLJ will angle right into the warm front, increasing moisture
pooling and convergence along it. There we will find PWATs
exceeding 2.25" and warm cloud depths around 4000m. Corfidi
vectors will point to the ESE, meaning there will be some
training of storms along the boundary, adding more to the heavy
rain threat. How much rain? The overnight update captured the
upper echelon well with a push towards 10 inches in localized
areas. As far as spatial coverage of rain goes along the line,
would side with the more broad coverage of global guidance like
the GFS and NAM which have 2"+ all along the front in central
Iowa. The HREF members` QPF has a more accurate representation
of magnitude from mesoscale processes, but is not widespread
enough along the line due to simulated cold pool processes
artificially extinguishing CI. The most notable HREF feature is
the HREF 6 hour QPF NP>3" lighting up over west-central Iowa.
All-in-all, all collocated with the front, a widespread amounts
over 2 inches are expected with localized amounts residing in
the 5-10" range. We saw rainfall rates pushing 2 inches per hour
last night so a repeat of these high rates is certain tonight.
Rates like this over an urban area would lead to flash flooding
issues and river rises are possible over basins that receive the
highest amounts. A Flood Watch for flash flooding has been
issued for portions of west-central Iowa.

The evolution into Friday is less certain as mesoscale features will
end up shoving the front south. The triple point will move into Iowa
during the day Friday with storms reigniting on the surface warm
front with peak heating. Current thinking is that southern Iowa has
the highest chance of seeing storms as the persistent LLJ will
counteract cold pool momentum, decreasing confidence in CAM
solutions that shove the front into MO. The Slight Risk from the SPC
is well-placed as well as the continued Slight Risk ERO for
continued heavy rain along the warm front. Barring cloud debris,
MLCAPE values along the warm front push 2,000 J/kg. Deep-layer shear
will be at least 30kts nearest the parent low, but the higher
magnitude found in models is questionable due to potential
convective contamination. Shear vectors point parallel to the front,
meaning that cold pools will quickly become unbalanced. Still, there
will be a tornado threat early in storms` lifecycles, especially
those that can form rotating updrafts. This is due to surface
vorticity along the front paired with enhanced stretching potential
from >8 C/km low-level lapse rates. Then the wind threat takes over
as strong cold pools undercut updrafts. The cold front then replaces
the warm front overnight with some training storm potential across
mainly the south. Another inch or two could fall in the southeast,
this thankfully being a region that would be able to take more
rainfall. There has been an overall slowing trend with the exit of
this last system, extending precipitation potential more into
Saturday morning as well, mainly across the east. The cold front
will then stall in northern MO through at least Sunday, then our
next best shot at low-level moisture returns around midweek.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 616 PM CDT Thu Jul 10 2025

Thunderstorm coverage will increase through the late evening
and into the overnight. MVFR to IFR conditions in thunderstorms
with heavy rain are expected. Wind gusts over 30 kts are likely
with the strongest storms. MVFR ceilings are possible, a 50 to
70 percent chance, Friday morning. Rain shower and thunderstorm
coverage may temporarily decreasing Friday morning in the
12-15Z timeframe, before increasing once again with daytime
heating.

&&

.HYDROLOGY...
Issued at 334 AM CDT Thu Jul 10 2025

Soil moisture has recovered to a bit over northern Iowa, but there
are still areas in the Cedar, upper Iowa, upper Des Moines, and
Raccoon basins where the 0-10cm and 10-40cm soils are above the 80th
percentile. The 1, 3, and 6 hour flash flood guidance (FFG) over
central Iowa ranges from 1.5 to 2 inches, 2 to 2.5 inches, 2.5 and
to 3 inches, respectively. Given the favorable meteorological
environment of efficient, high rain rates along with potential
for back-building storms that repeat rounds of rainfall over the
same areas, low lying and poor drainage urban areas are likely
to see ponding. Further, these FFG values may be exceeded and
result in flash flooding with this risk perhaps highest over
northern Iowa given the more saturated soils. While we take into
account mature agricultural crops, which does lessen, but does
not fully remove, the flash flooding potential tonight/early
this morning. This will likely not be true later Thursday into
Friday with the flash flooding chance increasing. Thus, WPC has
slight risks of excessive rainfall (level 2 out of a max of 4)
for both Thursday and Friday.

Looking at rivers, many basins across the state have reduced channel
capacity with USGS streamflows at any of the 7, 14, or 28 day
averaging period showing at least normal with northeastern and parts
of north central Iowa above normal to much above normal. With rivers
not at baseflows and a few still in or near action stage, this
heightens the potential for river flooding. As with any river
flooding, what basins will be impacted the most will depend on the
location of the highest rainfall totals. With the official river
forecasts only taking into account 24 hours of rainfall, this
evening`s forecasts did not take into account much of the rainfall.
This will change with this (Thursday) morning`s river forecast cycle
as rainfall through early Friday makes it into the forecast. Now, we
typically look at the experimental, five-day Hydrologic Ensemble
Forecasting Service (HEFS) and the ensemble rainfall hydrographs to
provide some idea of expectations. While these often perform well
and may in parts of our service area, they could be underdone due to
model averaged rainfall. For example, the Wednesday 12z
deterministic GFS had a swath of 2 to 4 inches between Highway 3 and
I-80. Meanwhile, the 12z GEFS had no more than 2 inches through
early Monday. Thus, the 12z run of the HEFS, which is based off of
the GEFS rainfall, is likely not showing a realistic look at the
potential for river flooding. If we look at the 5 day HEFS 10%
chance of exceedance, numerous river forecast points reach action
stage with a handful into minor flooding. For the ensemble QPF
hydrograph, it does a bit better with rainfall when looking at the
95th percentile, 72 hour QPF that shows widespread rainfall of 3 to
4 inches. While this probably overplays the spatial extent and
underdoes the max rainfall, it shows that most river forecast points
do not reach flood stage/minor flooding unless the high end rainfall
occurs. The National Water Model (NWM) forced by the GFS, which has
the higher QPF mentioned above, does show low annual exceedance
probabilities (AEP)/higher streamflows on a few areas in the western
half of the state. So, the expectation is a minimum of stream rises
on some of our basins with minor flooding possible in a handful of
locations with these all dependent on where the higher rain
falls.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Friday morning for IAZ033>036-044>048-
057>060-070>073.

&&

$$

DISCUSSION...Jimenez
AVIATION...DLH
HYDROLOGY...Ansorge