351
FXUS63 KDMX 271738
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1238 PM CDT Sat Apr 27 2024

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

.KEY MESSAGES...

- Another round of severe weather is possible over southern
  into central Iowa later this afternoon and evening. All severe
  weather hazards are possible, but large hail and damaging
  winds are the primary threats.

- Showers and storms are likely tonight into Sunday. Additional
  severe weather is possible Sunday afternoon and evening, but
  the extent of the threat is uncertain at this time.

- Weather pattern remains active into next week with more shower
  and storm chances Tuesday/Tuesday night and again late in the
  week.

&&

.DISCUSSION...
Issued at 300 AM CDT Sat Apr 27 2024

We have transitioned into a quieter period overnight as the upper
low responsible for yesterday`s severe weather outbreak has
pushed north and east of the area. Subsidence on the backside of
the departing low has cleared out most cloud cover and
precipitation, although the lull will be short lived as another
potent upper wave is already churning through the four corners
region and ready to take aim on the Plains states later today.
A warm frontal boundary draped over southwestern and through
central Iowa serves as the focus for additional convective
development. Hires models percolate isolated showers and storms
early in the day as instability builds along and south of the
front, but this will likely have a tough time overcoming mean
deep layer subsidence. Much more robust convective initiation
holds off until 20z-22z as vigorous deep layer ascent
overspreads the increasingly unstable warm sector. Most of the
00z model suite has honed in on nw MO into sw IA for initial
development. These storms may quickly become severe as they work
within a favorable environment for large hail and damaging wind
gusts. Sounding analysis suggests a few tornadoes cannot be
ruled out with the initial development into southern Iowa given
0-1km SRH ~100-200 m2/s2, looping hodographs, plentiful CAPE
within the lowest 3km.

Convective evolution becomes increasingly uncertain into the evening
hours. A few hires members (HRRR/ARW) quickly grow upscale and race
the storms east, while others (NAM/FV3) take a more northerly
trajectory and eventually push the storms north of the warm frontal
boundary and into a less favorable environment for severe weather
over northern Iowa. Redevelopment through the night is also poorly
handled as some models favor the mesoscale storm outflow over MO
into SE Iowa, while others favor the synoptic warm front bisecting
the CWA. In either case, strong background upper level divergence
should support at least scattered shower and storm activity over the
forecast area tonight into Sunday as the upper low pushes
toward the Dakotas. Another window for a severe weather risk
including hail and a few tornadoes may materialize Sunday
afternoon and evening as the surface low moves and attendant
dryline move into western Iowa. The extent of the threat will
hinge on precip/clouds clearing out and allowing the boundary
layer to destabilize.

Pattern remains active into next week as models show multiple waves
traversing through the region. After another break in storm chances
on Monday, the atmosphere quickly reloads with return moisture
ahead of an approaching upper wave and surface frontal boundary.
Showers and storms are looking increasingly likely as the front
passes through Tuesday evening and night. Latest NBM PoPs are
likely too aggressive into Wednesday and may need to be trimmed
back if current trends continue. The next system is quick on
its heels as another shortwave digs into the central Rockies,
leading to additional shower/storm chances later in the week.
Models begin to diverge with pattern evolution by this point,
and ensemble data is noisy and does not provide many clues for
honing in on a particular time period for higher PoPs.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1235 PM CDT Sat Apr 27 2024

MVFR to VFR conditions will initially prevail across all sites
this afternoon before anticipated convection and approach of
next upper wave and surface low are expected to degrade
conditions back towards IFR. Best opportunity for TSRA/SHRA will
be in/around KDSM/KOTM around/after 21z, then expanding
northward to at least encroach all TAF sites with SHRA/VCSH
mentions. Ceilings should fall towards 1kft, but have held off
on the most pessimistic of guidance for this issuance.

&&

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

&&

$$

DISCUSSION...Martin
AVIATION...Curtis