131
FXUS63 KLSX 131142
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
642 AM CDT Mon May 13 2024

.KEY MESSAGES...

- Waves of numerous showers and thunderstorms are expected from
  late this morning through Tuesday afternoon. An isolated strong
  thunderstorm with gusty winds, small hail, and locally heavy
  rainfall is possible this afternoon and evening.

- The next opportunity for showers and thunderstorms will be
  between Wednesday night and late Friday.


&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 344 AM CDT Mon May 13 2024

An upper-level closed low, apparent across the Central Plains in
early morning GOES-16 water vapor imagery, is progged to approach
the Mid-Mississippi River Valley today and slowly pass overhead late
tonight into Tuesday, leading to an extended period of showers and
thunderstorms across the region. Coverage will increase this morning
as large-scale ascent ahead of the closed low overspreads the CWA,
but one or more waves of numerous showers and a few thunderstorms
are expected between late morning and late this evening as subtle
shortwave troughs round the closed low in tandem with bands of
enhanced low-level WAA and moisture convergence. Showers and
thunderstorms may be relatively more persistent in northeastern MO
and west-central IL today, in proximity to a stationary front.
Widespread cloud cover and rounds of precip will limit high
temperatures to the low to mid-70s as well as overall instability.
However, some surface-based destabilization through weak insolation
in thinner/broken clouds is indicated to generate SBCAPE up to
locally 1000 J/kg, with highest probabilities of SBCAPE over 1000
J/kg in the latest HREF across central, east-central, and
southeastern MO late this afternoon. Deep-layer wind shear will
remain rather weak (20 to 30 kt) for this amount of instability, but
some loose organization/aggregating of thunderstorms into a few
multicell clusters could occur. Modest low and mid-level lapse rates
will limit thunderstorm intensity, but an isolated thunderstorm with
gusty winds or small hail is possible this afternoon and evening in
central, east-central, and southeastern MO.

A relative lull in showers and thunderstorms is anticipated late
this evening into Tuesday morning with a nocturnal decrease in
instability and a gap in deeper large-scale ascent between shortwave
troughs and the arrival of the closed low itself Tuesday morning.
However, abundant low-level moisture and low-level
convergence/ascent associated with a weak surface cyclone tracking
into the CWA will be favorable for areas of drizzle and fog to
develop overnight into Tuesday morning. Redevelopment of showers and
thunderstorms is expected late Tuesday morning through the afternoon
across much of the area behind the closed low and surface cyclone
within weak large-scale deformation and daytime destabilization.
Showers and thunderstorms will end rather quickly Tuesday evening as
the sun sets the closed low continues to pull away. Similar to
today, clouds and precip will keep temperatures cool and slightly
below average.

The HREF interquartile range of total rainfall from today through
Tuesday evening is between 0.25 and 1" across the CWA, with the
highest amounts being achieved where there are multiple rounds of
thunderstorms. Locally heavier rainfall cannot be ruled out where
thunderstorms become slow-moving or can backbuild, captured by the
HREF localized probability matched mean (LPMM) containing very
localized amounts of 2 to 4" of total rainfall. However, the threat
of any flash flooding appears low, since soils have recently had
some time to dry and any heavy rainfall should be very localized.

Pfahler

&&

.LONG TERM...  (Tuesday Night through Sunday)
Issued at 344 AM CDT Mon May 13 2024

Wednesday through the upcoming weekend, there are still differences
across global model guidance regarding the upper-level wave pattern
over the CONUS, but there is at least better agreement on the
longwave pattern. An upper-level ridge is anticipated to dominate
the Mid-Mississippi River Valley with dry conditions on Wednesday,
followed by passage of an upper-level trough around Thursday and
Friday. Uncertainties associated with this trough are how it
interacts with a southern stream trough/closed low ejecting out of
the Inter-Mountain West and the presence and timing of embedded
shortwave troughs. Despite a range in potential trough evolutions, a
vast majority of ensemble model membership contains showers and
thunderstorms across the CWA at some point between Wednesday night
and late Friday but coverage will be impacted by trough structure. A
small portion of ensemble model members continue showers and
thunderstorms through the upcoming weekend with slower ejection of
the southern stream trough, others are dry with upper-level ridging.
Temperatures through the rest of the weekend into the upcoming
weekend are more clear--a gradual warming trend after Tuesday with
temperatures returning to average at the end of the work week and
then above average over the weekend. This notion is well-supported
by warming of NBM interquartile temperature ranges through that
time.

Pfahler

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 642 AM CDT Mon May 13 2024

Isolated to scattered showers and a few thunderstorms will be common
throughout the TAF period along with one or more waves of more
numerous showers and thunderstorms tracking through the region. The
first wave is forecast to arrive midday into afternoon, accompanied
by flight conditions deteriorating to MVFR to possibly briefly IFR.
The timing and track of a potential additional wave of showers and
thunderstorms this evening is less confident and has not been
detailed.

MVFR stratus will likely linger this evening, especially at St.
Louis metro terminals before more widespread IFR stratus and patchy
fog develops overnight and impacts all terminals for a period
overnight into Tuesday morning. Drizzle is also possible but it is
uncertain how much visibilities will lower. Stratus will lift
Tuesday morning with some improvement in flight conditions; however,
another round of more numerous showers and thunderstorms is
anticipated by Tuesday afternoon.

Pfahler

&&

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

$$

WFO LSX