793
FXUS63 KDMX 291739
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1239 PM CDT Mon Apr 29 2024

 ...Updated for the 18z Aviation Discussion...

.KEY MESSAGES...

- Thunderstorm chances return Tuesday afternoon into Tuesday
  night. Strong to severe storms possible with large hail and
  damaging winds the primary threats.

- Additional showers and storms likely Wednesday evening through
  Thursday. Severe threat uncertain. Much of area may see 1-2"
  of rain.

&&

.DISCUSSION...
Issued at 228 AM CDT Mon Apr 29 2024

Precip is all but over for now as an upper low is pushing away
toward the Canadian border. Deep moisture transport has been
deflected well east and synoptic subsidence will further take
hold over the state into the today, leading to a lull after an
active stretch of days. Moisture wrapping around the backside of
the upper low will keep a low stratus deck in place to start
the day before starting a northward retreat this afternoon.
Clearing skies and light winds as a surface ridge axis moves
through tonight will lead to ideal radiational cooling
conditions. Portions of northern Iowa and other favorable river
valleys and low lying areas will see lows dip into the 30s along
with the potential for some areas of fog.

Pattern activity picks right back up Tuesday as a potent
shortwave and attendant jet streak emerge over the central
Plains. Southerly winds usher moisture back northward ahead of
an approaching cold front. An increasingly unstable boundary
layer ahead of the frontal boundary pushes MLCAPE to near 1500
J/kg by mid afternoon when storms fire over eastern Nebraska and
move into western Iowa by late afternoon or evening. Effective
shear of 40-50 kts oriented near normal to the frontal axis
will support discrete storm modes initially, potentially
supercellular and capable of producing large hail and damaging
winds. The tornado threat is more questionable due to moisture
return uncertainties as some models only increase dewpoints into
the mid 50s before storms roll through. Model soundings show an
inverted-V attribute below 800mb, which may support a localized
threat for significant wind gusts especially as cold pools
begin to merge with upscale growth into the evening. Eventually
the severe threat wanes as instability diminishes by late
evening and early overnight.

Cold front stalls out near the MO border on Wednesday, then
slowly begins to retreat back northward as a warm front late in
the day as another upper low approaches from the west. Models
initiate convection along the frontal boundary by mid to late afternoon,
potentially strong to severe given a favorable shear and
instability parameter space along and south of the front. A
further south frontal placement would limit the severe threat
into southern Iowa. Shower and storm coverage expands northward
Wed evening and night as the LLJ/northward moisture transport
intensifies and phases with background kinematic forcing
downstream of the approaching upper low. High precip chances
continue into Thursday. Any severe threat appears low at this
time unless there is a break in showers/cloud cover and more
instability is realized. Main upper jet stream is also lagging
west and displaces the most favorable shear environment. 50th
percentile QPF from both the GEFS/ECENS has 1-2" of rain for
much of the forecast area through the middle of week. Higher
amounts are possible in areas that see repetitive rounds of
deeper convection.

Models are in much better agreement late week into weekend with a
more progressive pattern evolution. Upper low sweeps north and east
by Friday and drier air pushes into the state. NBM is latching on to
this trend with lowered PoPs in the latest run. Scattered precip
chances return for the weekend as a weak shortwaves interact with a
surface boundary waffling over the region. Predictability of
timing and spatial coverage of potential precip is low at this
time.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1238 PM CDT Mon Apr 29 2024

Low clouds continue to circulate across the majority of the
state into the afternoon, leading to remaining MVFR conditions.
Through the rest of the afternoon, clouds are expected to
gradually lift north/northeast with clearing expected generally
after 00z-03z across the terminals. Breezy W/SW winds will
subside into the evening, though will increase towards the end
of the period out of the south. A low signal is in place for
some isolated fog over KFOD Tuesday morning, though confidence
is not very high on occurrence, so decided to leave out at this
time.

&&

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

&&

$$

DISCUSSION...Martin
AVIATION...Bury