903
FXUS63 KDMX 201153
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
653 AM CDT Sun Apr 20 2025

 ...Updated for the 12z Aviation Discussion...

.KEY MESSAGES...

- Widespread showers and some thunderstorms starting this
  morning and lasting into tonight north. Moderate to locally
  heavy rains may occur, but river and flash flooding unlikely.

- Small area of severe weather potential south, mainly
  southeast, including tornadoes

- Cool and dreary today, then normal to above normal temps Tue
  into the weekend

- Intermittent chances for showers and some storms return Monday
  night lasting through at least the work week

&&

.DISCUSSION...
Issued at 351 AM CDT Sun Apr 20 2025

April will end with an active pattern starting tonight, and
lasting off and on through the week as we transition from our
current higher amplitude flow to a more progressive minor SW
flow regime until upper level ridging builds next weekend. Our
attention this morning turns to a noted NM/TX PV anomaly as it
lifts into the MO Valley by 00z. The deep, phased kinematic and
thermodynamic forcing, including theta-e advection and
frontogenesis contributions, will reach southern IA this
morning and persist through the day while the deformation zone
and trowal matures during the afternoon. This will lead to
prolonged rainfall in many locations before exiting to the north
overnight. The primary 1-2km moisture transport surge will pass
just to our east into the immediate MS Valley, and warm cloud
depths this far west aren`t notable, but the prolonged light to
moderate rainfall will accumulate. 00z EPS and GEFS peak
ensemble median 24hr QPF depicts widespread 1.0-1.5" amounts,
likely an indicator of the potential for general amounts,
especially in the deformation zone. The higher resolution 00z
HREF peak 24hr local probability matched mean QPF suggests
isolated locations could see 2-3" amounts after multiple 3hr
0.50-0.75 amounts begin adding up.

Across southern IA, severe weather remains some concern. The
00Z HREF is suggesting low top supercells only one or two
counties removed from the forecast area to the southeast with
several member updraft helicity tracks in mean MUCAPEs only a
few hundred J/kg. The tornado parameter space jumps dramatically
just several counties removed however per the 03z RAP sounding
at Burlington. Thermodynamically: 700 J/kg MLCAPE, 181 J/kg
0-3km CAPE, MLLCLs 285m. Kinematics: 500m SRH 302 m2/s2, 32 kts
of bulk shear, 0.034 streamwise vorticity/93% of total vorticity
(7% crosswise). These values are a fairly textbook low
cape/high shear tornado environment. Some of these models have a
known bias of inflated values, but recent RRFS, HRRR, HRW ARW,
HRW FV3, MPAS-HN, MPAS-HT, MPAS-RT runs all have 0-3km updraft
helicity streaks one to three counties removed from Davis County
in our far southeast forecast area.

Farther west closer to the surface low and parent upper low the
effective shear and low topped supercell potential will be
less, but the potential for funnels/landspouts increase some
closely attendant to the triple point and even farther east
along the occluded front approaching the IA border. Toward 00z
elevated 0-3km CAPE, Enhanced Stretching Potential, and Non-
supercell tornado parameters increase, suggest a minimal but
non-zero tor potential east of the current SPC outlook. These
values appear to diminish with the loss of heating post 00z,
thus the most likely scenario across southern Iowa, in our
forecast area, remains a lower tornado potential, but a more
favorable, northward thermodynamic enhancement near the triple
point or a very minor NW shift of the MS Valley instability axis
could increase that potential. To use a Field of Dreams
analogy, it will likely be low and away, but watch out for in
your ear.

The departed baroclinic zone from today`s system will quickly
move northward again ahead of a Dakotas short wave with H85/H7
theta-e advection returning Monday night. Generally the
potential for strong to severe storms appears low. Examination
of 00z GFS soundings suggests weak convection rooted about 1km
Monday night with MCAPEs +/- 500 J/kg and effective shear 30-40
kts. With LFCs above the freezing level, hail generation may be
enhanced, at least sub-severe. Additional surface based
convection may occur into peak heating with the passage of the
weak boundary. ~1000 J/kg MLCAPEs with minimal CINH may occur
late in the peak heating with similar effective shear 30-40 kts.
More favorable low level lapse rates and DCAPEs approaching
four figures will introduce some potential for gusty winds, and
mainly sub-severe hail may occur again as well with LFCs again
near the freezing level. Chances for showers and some storms
persist later in the work week as well, but forcing mechanisms
are more nebulous and ill-defined so the extended period of
chances is more due to uncertainty than a prolonged period of
notable rains. This pattern will also lead to temps at or above
normal for several days with highs Tue-Thu 70F+ in many
locations.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 653 AM CDT Sun Apr 20 2025

Overcast VFR conditions were in place across central IA at 12z
but they will not last long as showers and lower clouds will continue
advancing south to north across the state this morning. A
prolonged period of at least showers is anticipated through the
day and evening with widespread IFR conditions due to both
visibilities and ceilings. Some periods of thunder are possible
as well, but only mentioned at KOTM later today for now until
confidence at other locations increases.

&&

.HYDROLOGY...
Issued at 351 AM CDT Sun Apr 20 2025

Mainly within back rises are anticipated with this
precipitation based on RFC contingency forecasts whose median
and expected QPF values are Action Stage or below. HEFS output
is similar with median/50% exceedance probabilities noting all
responses below Action Stage, and 30% exceedance showing Action
Stage or less responses. The flood potential is non-zero however
if the high end of ensemble QPF is realized with max
contingency QPF and HEFS 10% exceedance noting a few spots just
reaching Flood Stage in the Shell Rock and Cedar Basins later
this week. The most expected impacts will be reduced
visibilities in rain, standing water on roads, fields, and
ditches.

&&

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

&&

$$

DISCUSSION...Small
AVIATION...Small
HYDROLOGY...Small