902
FXUS63 KLSX 202308
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
608 PM CDT Mon May 20 2024

.KEY MESSAGES...

- Isolated severe thunderstorms are possible through early this
  evening across parts of southeast and east-central Missouri and
  southwest Illinois. Damaging winds to 60 mph and quarter size
  hail is the primary threat.

- Severe thunderstorms appear likely on Tuesday night, especially
  across central and northeast Missouri and west-central Illinois
  during the evening with all severe weather hazards possible.

- Another round of severe thunderstorms is possible Wednesday
  afternoon and evening across primarily southeast Missouri and
  southwest Illinois.

- Well-above normal temperatures are forecast on Tuesday with a
  record high possible at St. Louis (the daily record high in St.
  Louis is 93F in 1941.


&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 301 PM CDT Mon May 20 2024

The remnant MCV from last nights storms was drifting across
eastern MO at mid-afternoon. Thus far we haven`t seen any storms
attendant with the MCV, but the potential for isolated storms will
continue across eastern MO and southwest Illinois into the early
evening as the MCV moves east within an environment of 1000-1500
MLCAPE. There is an area of slightly better shear associated with
the MCV that could promote some storm organization including
multicells or even a short-lived supercell if we get storms to
develop. Damaging winds to 60 mph and quarter hail would be the
main threat.

Most of the remainder of tonight looks dry however there is some
potential for a few elevated showers and thunderstorms from
central into northeast MO and west-central IL later this evening
into overnight associated with an increasing and veering
southwesterly LLJ.

Tuesday looks largely like a dry and very warm day with gusty
southwest winds, a good warm advection regime, and lots of sunshine
through mid-afternoon. Highs will be 10-15+ degrees above normal and
the record high may be in jeopardy at St. Louis.

It still appears we will be in for an active Tuesday night,
especially during the evening with scattered to numerous severe
storms. There is some question on storm organization and mode with
only glancing large scale forcing as the main lifting short wave
largely moves from the central Plains into the Upper MS Valley
during the evening. The current expectation is the passing wave
should reduce CIN enough that a broken line of thunderstorms will
develop along the advancing cold front during the late afternoon
into the early evening. This line is expected to have the greatest
impacts during the 23-05z time frame, predominately across central
and northeast MO and west-central IL. Convective mode looks like it
could be mixed with supercells and small-bowing segments within the
broken line. While all severe weather hazards are possible, the low-
level flow becomes veered and more unidirectional during the evening
and especially through the night ahead of the advancing cold front
lowering the overall tornado threat. The lack of forcing moving into
the overnight hours also suggests the coverage and intensity of
storms will diminish as activity pushes towards the southeast and
eventually impacts southeast MO and southwest IL, with a more
isolated severe threat consisting of discrete, scattered storms.

Glass

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 301 PM CDT Mon May 20 2024

All indications are the cold front will move slowly across
southeast MO and southern on Wednesday, owing to continued
southwest upper level flow and the absence of a formidable short
wave to drive it much further south. The front should then ooze
southeastward a bit by Wednesday evening due to convective outflow
and stall. Showers and thunderstorms are expected to become more
numerous by Wednesday afternoon, spanning both sides of the front
in association with weak frontal forcing and a migratory
disturbance. Across southeast MO and southern IL the afternoon
airmass is forecast to become very unstable and deep layer shear
of 40 kts would support an organized severe weather threat with
multiple convective modes and all hazards possible. There is also
an increasing overrunning component of the low-level flow forecast
to occur from the afternoon into Wednesday night supporting
potential for showers and storms on Wednesday night well north of
the boundary including central and east-central MO. If we see
repeat thunderstorms in either pre or post-frontal regime, then
locally heavy rain will be possible.

The period from Thursday through the Holiday Weekend is best
described as warm and unsettled. There are differences in the
guidance suite in details of the synoptic regime but the general
flavor advertised in the ensembles is that west-southwest flow aloft
will persist through this time period with shortwaves traversing the
region that originate from a slowly progressive broad western trof.
The timing and amplitude of these waves is varied but again the
overall flavor is these progressive short waves will result in a
wavering frontal boundary for this stretch and continued chances
of showers and thunderstorms. Ultimately there could be some
periods of stronger storms and heavy rain as well.

Glass

&&

.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 600 PM CDT Mon May 20 2024

VFR categories are expected to prevail at local terminals for most
of the 06Z TAF period, with only a few possible exceptions. At
Quincy (UIN), there is a low probability of a few showers and
thunderstorms developing overnight tonight, but confidence is low
that this will directly impact the terminal. Overnight storms are
not expected to impact any other terminals through 00Z. Gusty
southerly winds can also be expected over the latter 12 hours of
the TAF cycle (00-12Z TUE).

Near the end of the St. Louis TAF cycle (STL, 04-06Z TUE),
thunderstorms are likely to move into the area from the northwest.
The timing and strength of these storms may need to be adjusted
in future TAF updates, but confidence is high that this activity
will eventually reach the terminal. In addition to lowering
ceilings and reduced visibility, other thunderstorm hazards like
gusty winds and hail will be possible if a storm directly impacts
the terminal.

BRC

&&

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

$$

WFO LSX