929
FXUS63 KDMX 181150
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
650 AM CDT Sat May 18 2024

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

.KEY MESSAGES...

- Although there will be a few breaks, the pattern will
  generally be active through at least Tuesday with several
  periods of storms possible

- Strong to severe storms and locally heavy rains possible at
  times, especially Tuesday

- Outside of warm temps today, a fairly seasonal temperature
  regime in place through next week

&&

.DISCUSSION...
Issued at 436 AM CDT Sat May 18 2024

Our weather pattern isn`t expected to change a whole lot
through the weekend and through the next week with progressive
mainly lower amplitude flow resulting in systems of varied
strengths every few days. The first of which will affect mainly
eastern sections later today. At onset this morning short waves
exiting the Dakotas into MN, and the parent wave moving from MT
into ND have already spread decent QG forcing into the Siouxland
area this morning. Moisture return is limited however with
little sensible reflection here and the better warm/theta-e
advection farther north resulting in only scattered convection
near the Twin Cities. This forcing, and a noted cold front
aloft, will push into IA later today along with the upper
wave`s attendant cold front during peak heating and highs well
into the 80s, likely the warmest day of the forecast period.
Moisture pooling along and ahead of the front is expected to
result in 1500+ J/kg uncapped MLCAPEs by late afternoon. Some
models such as the RAP are more aggressive, with surface
dewpoints to near 70F, but that seems overdone considering
upstream conditions per the 08z surface analysis and the current
state of evapotranspiration. Guidance varies with respect to
coverage, but at least a few storms are expected to develop
along the front east during the late afternoon and evening
hours. Deep shear is weak keeping any severe potential on the
low side, but the degree of instability and especially higher
cloud bases and four figure DCAPEs could result in some strong,
gusty winds until the front exits by late evening leading to
dry conditions and surface high pressure overnight.

Chances for showers and storms will increase once again west to
east Sunday however with weak short waves crossing the Rockies
into the Plains and moisture return behind the departing 1-3km
ridge. While the best instability and severe potential will
remain to our west tomorrow and tomorrow night, sufficient
instability and just enough deep shear may be present for a few
stronger storms. Any surface based storms to our south and west
may transition to more elevated and continue to be fueled by a
40kt low level jet and moisture transport into the night. While
the environment doesn`t appear too extreme, seasonally high
precipitable water and specific humidity values, and 3.5km warm
cloud depths, may result in some locally heavy rainfall with
isolated 1-2" amounts.

By Monday what looks to be the strongest system of the period
will begin with enhanced southwest flow aloft ahead of the
maturing western trough. Instability will again build into the
MO Valley with the western fringe of a warm front into IA.
Plains convection may again develop during peak heating and
sustain itself into IA through the night with another cycle of
deep moisture convergence on the nose of a 35-40kt low level jet
focused into the south. Deep shear may just be sufficient at
>30kts for organized storms and some wind and hail potential.
The heavy rain parameter space pushes well through elevated
climatological percentiles resulting more potential for isolated
heavy rains.

The best potential for severe weather will be Tuesday when
synoptic forcing, instability, and deep shear all increase
during peak heating as the surface wave deepens and moves into
IA, coincident with a strong mid level speed max. 0-6km shear
increases to near 50kts with 2000-3000 J/kg MLCAPEs and reduced
CINH. Discrete supercells are possible at onset of development
with deep shear vectors normal to the advancing boundary.
However a more linear structure may evolve with time as the
0-6km shear backs as the storms move eastward. With this in
mind, the new SPC Day 4 outlook has increased severe
probabilities to 30 percent central and southeast, with a
notable signal in the latest CSU machine learning guidance. As
mentioned in the previous discussion, previous MCSs have the
potential to contaminate the environment at times early next
week, so repetitive rounds of convection and questionable thermodynamic
recovery will need to be monitored late Sun into Tue before
details of any particular event become more clear in the coming
days.

Although the signal is much weaker for the rest of the forecast,
active progressive flow will result in at least some chances for
additional precipitation off and on through the end of next
week. The 00z GFS has a fairly strong, faster short wave and
resultant surface system through the Siouxland Friday, but as if
often the case this may be a bit fast with much of the other
deterministic, ensemble, and AI guidance suggesting a slower
progression.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 649 AM CDT Sat May 18 2024

VFR conditions are anticipated through early afternoon with
areas of mid clouds transitioning to afternoon cumulus
development along and ahead of an approaching cold front. There
is some potential for a few storms along the front during the
late afternoon and evening hours east near KALO or KOTM, but
confidence is too low for any mention beyond VCSH for the time
being. Any showers or storms that develop would likely be higher
based and VFR with clearing skies overnight.

&&

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

&&

$$

DISCUSSION...Small
AVIATION...Small