328
FXUS63 KDMX 110607
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
Issued by National Weather Service Duluth MN
106 AM CDT Fri Jul 11 2025

.KEY MESSAGES...

- Flood Watch for flash flooding in effect for most of 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.

&&

.UPDATE...
Issued at 727 PM CDT Thu Jul 10 2025

Expanded the Flood Watch east through the Waterloo, Oskaloosa,
Montezuma and Toledo areas tonight. Training convection will
move out of Nebraska with a strengthening low level jet in an
environment characterized by precipitable water values near 2.00
inches, and slow Corfidi vectors indicative of back-building
convection into the low level jet. Rainfall amounts of 2 to 3
inches, with locally higher amounts are expected by Friday
morning. Will monitor trends in the high resolution CAM guidance
and radar should an additional expansion to the Flood Watch
become necessary.


&&

.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 /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 106 AM CDT Fri Jul 11 2025

Thunderstorms will continue to move across the terminals for the
next few hours. DSM and OTM to be most strongly affected for the
next few hours, with strong wind gusts and IFR visibilities and MVFR
ceilings. The other terminals to get MVFR to potentially IFR
conditions in thunderstorms with heavy rain. A period of MVFR
ceilings are possible, a 50 to 70 percent chance, Friday morning,
mainly for KALO and KMCW. Rain shower and thunderstorm to
temporarily decrease Friday morning in the 12-17Z timeframe, before
increasing once again with daytime heating. Friday afternoon storms
to most likely affect KDSM, KOTM and KALO, with lower probabilities
for KFOD and KMCW.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Flood Watch until 7 AM CDT Friday for IAZ033>039-044>050-
057>062-070>075.

&&

$$

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