254
FXUS63 KDMX 131147
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
647 AM CDT Thu Jun 13 2024

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

.KEY MESSAGES...

- Additional thunderstorms expected today with severe weather
  possible. Enhanced Risk (3/5) south central & southeast.

- Dry Friday, but intermittent chances for showers & storm
  return Friday night and persist into next week

- Elevated heat & humidity south this afternoon, and then a more
  prolonged period again next week

&&

.DISCUSSION...
Issued at 326 AM CDT Thu Jun 13 2024

Our weather will be driven by a fairly typical summer-like
pattern with the westerlies and jet just to our north in varied
placements, and the current Rockies upper ridge gradually
progressing eastward and taking hold across the eastern CONUS by
the middle of next week. This will keep intermittent chances
for showers and thunderstorms in the forecast, along with
seasonally elevated heat and humidity south today, and again for
at least several days next week.

Starting out early this morning, a weak short wave along the
MT/WY border noted in water vapor imagery through the subtle
stream of moisture topping the Rockies ridge has developed
elevated convection across SD early this morning, and is
expected to expand somewhat and track along the IA/MN border
later this morning per most 00z HREF members, generally staying
below severe limits as it encounters fairly low instability.
This won`t be our main show today however, as it is expected to
further enhance a frontal boundary now through the Siouxland
area as it drifts south through IA during the day. The airmass
ahead of the boundary is already fairly moist and unstable with
dewpoints into the 60s, and MLCAPEs 500-1500 J/kg. Insolation
later today is expected to increase that instability further
with weakly capped 3000-4000 J/kg MLCAPEs in place across
southern IA during peak heating. This coupled with 40+ kts of
deep shear should allow for robust and organized convection with
elevated damaging wind and large hail potential, and an
Enhanced Risk (3/5) of severe weather for those locations. While
weak low level flow will result in minimal 0-1km shear, SRH,
and streamwise vorticity ingest, the noted CAPE could still
result in an isolated tornado.

Details in this initiation location are not great, but many 00Z
HREF members blossom a noted MCS across either northern MO or
southern IA during the afternoon. Recent HRRR runs, even as far
back as this time yesterday, continue to depict small storm
clusters with significant wind gusts of 65+kts, which makes
sense considering fairly steep low level lapse rates and 1000+
DCAPEs. Soundings, both thermodynamically and kinematically, are
fairly similar to yesterday as well with higher based LFCs and
much of the CAPE above the freezing level suggesting supercells
in this environment could again lead to 2+" hail similar to
what occurred along the MO River yesterday. While locally heavy
rains are certainly possible in this airmass, 00Z HREF output
doesn`t reflect much in the way of that potential with little
if any probabilities of 2+" accums in 6 hours.

Elevated heat and humidity will be in place south preceding any
storm initiation as well with highs in at least the lower 90s,
and 65-70F dewpoints driving heat indices well into the 90s and
approaching 100F at times. Wet Bulb Globe Temperature (WBGT)
Risk is expected to reach the High category (3/4) and
approaching Extreme (4/4) values in a few spots.

The weather should be more tranquil by late evening however
with fair weather, more seasonal temperatures, and lower
humidities anticipated for Friday. The aforementioned Rockies
upper ridge will be on the move this weekend however, as will a
noted PV anomaly off the CA/Baja coast. This is expected to
reach the central Plains Saturday with its associated lift
beginning to affect Iowa as soon as Friday night central and
west. Limited instability should keep any severe weather
potential low, but slow movement at times, steady low level
moisture transport, elevated precipitable water values, and
3.5-4km warm cloud depths may all contribute to efficient rains
with locally heavy rainfall.

The short wave should be east of the area by Sunday however,
starting at least a few days of elevated heat and humidity.
Models are not in great agreement on the degree of amplification
with the western trough/eastern ridge keeping varied scenarios
in play from little precip and increased heat to more convection
but more tempered heat and humidity to start the next work week.
There is a better consensus of Iowa more fully in the warm
sector by the middle of next week however. Regardless, even the
blended NBM output suggests several days with highs and heat
indices in the 90s and low chances for showers and storms in the
forecast. WBGT Risks are often in the High (3/4) category over
the south half of the state, and the experimental NWS HeatRisk
index suggests Extreme values (4/4) by Monday driven by the
unseasonally elevated heat and humidity, but also influenced by its
longevity and little overnight respite as lows should be remain
in the lower to middle 70s. This will be the first prolonged
heat episode of the year, so those planning or participating in
outdoor activities for the early to middle portions of next week
should note heat safety rules and accommodations.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 647 AM CDT Thu Jun 13 2024

VFR conditions and mostly clear skies were in place across the
forecast area and this should generally be the case through the
morning. Scattered weak convection may skirt the IA/MN border
later this morning, but any confidence to include thunder or
non-VFR at any TAF site is insufficient at this time. More
robust convection is expected to develop across southern IA
and/or northern MO this afternoon and evening, with KOTM the
most likely affected TAF site. Anything that develops should
exit to the south later this evening however, leading to high
confidence in mostly clear/VFR conditions overnight.

&&

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

&&

$$

DISCUSSION...Small
AVIATION...Small