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