046
FXUS63 KLSX 012300
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
600 PM CDT Mon Jul 1 2024

.KEY MESSAGES...

- An active weather pattern will begin late Tuesday with
  opportunities for showers and thunderstorms including the threat
  of strong to severe thunderstorms and locally heavy rainfall
  with isolated flash flooding.

- The initial threat of strong to severe thunderstorms Tuesday
  evening is greatest across northeastern, central MO and west-
  central IL.

- Hot and humid conditions are expected across portions of the
  area Tuesday through Thursday. Heat index values could exceed
  100 F in central MO Tuesday afternoon.


&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 339 PM CDT Mon Jul 1 2024

A very slowly decaying MCS has persisted across western MO and is
now tracking more progressively eastward toward central MO this
afternoon; however, the MCS should continue decaying and eventually
dissipate by the time it reaches the CWA due to the combination of
decreasing instability and increasingly pronounced dry low-level
easterly flow with eastward extent. Therefore, only sprinkles and
light rain are anticipated to reach central MO. Otherwise, clouds
associated with the MCS and weak CAA has kept temperatures 5 to 15 F
below average.

Aside from MCS remnants this afternoon, dry conditions will prevail
through most of Tuesday across the region under the crest of an
upper-level ridge shifting eastward tonight into Tuesday. With much
greater insolation and onset of low-level WAA by Tuesday,
significantly warmer temperatures are anticipated with high
temperatures returning to above average, in the upper 80s to mid-90s
F. A warm front will also be lifting northeastward into central and
northeastern MO on Tuesday but should act more as a dewpoint
discontinuity than temperature difference. Most model guidance
indicate that dewpoints on the warm (west) side of the front will be
around 70 to the mid-70s F, with afternoon heat index values rising
above 100 F. However, confidence is low that values will rise to
Heat Advisory criteria since there is still some variability on how
high dewpoints and heat index values will get with slight
differences in front position.

Thunderstorms are forecast to develop in the vicinity of a cold
front Tuesday afternoon across northwestern MO and IA ahead of an
upper-level trough, growing upscale into an MCS reaching
northeastern MO/west-central IL. Model guidance generally indicate
2500 to 3500 J/kg of MLCAPE and 30 to 40 of deep-layer wind shear
available as thunderstorms initially arrive, sufficient for at least
a few strong to severe thunderstorms with a primary hazard of
damaging winds, but marginally severe hail and an isolated tornado
are possible. Although the threat of severe thunderstorms will
gradually decrease through the evening, the strongest thunderstorms
may tend to become focused toward the southwest with time since
instability will decrease with eastward extent and low-level
convergence will be maximized where the LLJ interacts with the
southwest side of any surging outflow. If this evolution can take
place and encourage periods of training thunderstorms, locally heavy
rainfall and isolated flash flooding will also become potential
hazards across northeastern MO into central MO with PWATS above 2"
and deep warm cloud depths. Antecedent soil conditions are dry but
the latest HREF LPMM of 24-hour QPF is 3 to 7" in those areas, with
the disclaimer that it is heavily skewed toward a small membership
with high rainfall amounts. These factors preclude higher confidence
in flash flooding.

Pfahler

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 339 PM CDT Mon Jul 1 2024

An active weather pattern will persist through Wednesday and
Thursday as the cold front moves into the CWA and wavers beneath a
series of shortwave troughs/perturbations within upper-level west-
southwesterly flow. Thunderstorms are not anticipated continuously
through this entire period, but there are several timeframes that
are more favorable for showers and thunderstorms. The first is
Wednesday afternoon and evening along and ahead of the cold front
that will be near the I-44 (MO) and I-70 (IL) corridors during that
time. Some of these thunderstorms could become severe since MLCAPE
of 2000 to 3500 J/kg will coincide with deep-layer shear of 20 to 30
kt. The primary severe thunderstorm hazards will be damaging winds
and large hail. It is unclear exactly how widespread thunderstorms
will become with weak upper-level forcing but upscale growth of
thunderstorms into an MCS is possible. The next timeframe is
Wednesday night as the front retreats back north and is overrun by a
LLJ. The expectation is that these thunderstorms will be largely
evaluated, but high, anomalous PW and potential for some training of
thunderstorms in the event an MCS forms, leads to at least some
threat of locally heavy rainfall and isolated flash flooding. The
location is not yet clear with differences in front position,
although the threat of flash flood would increase if any heavy
rainfall falls in areas that receive heavy rainfall Tuesday night.

On Thursday (Fourth of July), model guidance is in agreement that a
more potent upper-level shortwave trough will traverse the Central
Plains into Midwest, forcing the front back through the CWA late in
the day as a cold front after it lifts north of the CWA, providing
another opportunity for showers and thunderstorms. The exact timing
of showers and thunderstorms on Thursday is uncertain with the warm
front possibly remaining in or near the CWA, but the chance of
showers and thunderstorms increases during the evening as large-
scale ascent ahead of the trough increases and the cold front
arrives.

Temperatures Wednesday through Thursday will be largely dictated by
which side of the front a given location is:  to the north of the
front temperatures are forecast to be near average and along and
south of the front temperatures are anticipated to be above average.
With humid conditions also present, afternoon heat index values
could reach above 100 F in some locations south of I-70. However,
exact values and whether or not they reach dangerous thresholds will
be influenced by thunderstorms.

After Thursday, global model guidance remain in relative agreement
that upper-level flow over the Mid-Mississippi River Valley will
become broadly cyclone as longwave troughing develops over the north-
central CONUS. Embedded shortwave troughs within this flow will
provide additional opportunities of showers and thunderstorms at
times with a series of frontal passages. However, there is spread in
ensemble model guidance on the when showers and thunderstorms will
occur as well as coverage and rainfall amounts, due to differences
in moisture return quality. Temperatures are also favored to be
generally cooler and near average in this pattern.

Pfahler

&&

.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 555 PM CDT Mon Jul 1 2024

Dry VFR conditions will persist through this TAF period, with
winds veering out of the south overnight and strengthening
Tuesday. Showers and thunderstorms will approach KUIN and the
central Missouri terminals after 00z on Tuesday evening, outside
of this current forecast window. There is some question as to how
widespread they will be once they reach those terminals, if they
manage to reach them at all.

MRB

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX