767
FXUS63 KLSX 080332
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1032 PM CDT Tue May 7 2024

.KEY MESSAGES...

- Isolated to widely scattered thunderstorms will dot parts of
  east-central and southeast Missouri and southwest Illinois into
  the early evening. A few of these storms could be strong to
  severe with large hail the dominant threat.

- Scattered to numerous severe thunderstorms are expected on
  Wednesday, primarily from late morning through the afternoon
  into early evening. The overall set-up is quite volatile
  supportive of large hail, damaging winds, and a few strong
  tornadoes.


&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 236 PM CDT Tue May 7 2024

Surface features are rather nebulous this afternoon with largely
veered southwesterly surface winds across the area and very weak
convergence within the flow across southwest IL into southeast MO.
The air mass in this area is currently quite unstable with SBCAPE
of 3000+ J/KG and deep layer shear on the order of 70 kts. If we
can get some sustained deep storms, which is more challenging
with the weak convergence/forcing, then supercells would be the
favored mode. Given the veered low-level flow, the hodographs are
rather straight favoring large hail as the main threat. The
convergence zone is expected to shift east of the area by early
evening with an ill-defined cold front settling southeastward
across the area as well and the cumulative boundary becoming
stationary across southern MO and southern IL later tonight. After
the current storms, things look rather tranquil tonight with
potential for some patchy fog.

Wednesday continues to be a day of big severe weather potential but
there is uncertainty on how it will all play out. The overall set-up
hasn`t changed much with a surface low moving through west-central
MO in the morning, and lifting through northeast MO and into west-
central IL by evening. Accompanying the movement of the low, the
aforementioned boundary to the south will move northward as a warm
front with a trailing cold front accompanying the surface low.

Undiluted, the warm sector will become rather expansive and very
unstable by afternoon with potential for MLCAPE of 2500-3000+ J/KG.
There is also very little CIN forecast within the warm sector due to
heating, thus convection could evolve not only along the boundaries
but seemingly throughout the open warm sector. The possibility of
morning storms however leads to uncertainty in how convective
evolution will occur. At least a few of the CAMS including the 12Z
HRRR develop robust convection across western MO rather early in the
day, growing upscale into a severe QLCS and racing this eastward
across the southern half of MO and southern IL during the morning
and early afternoon. The best I can tell this early development
seems to be in response to a weak vort max in the southwesterly
flow and convergence well ahead of the cold front/surface low.
While I certainly can discount this scenario, it seems a more
plausible scenario is that convection will develop from late
morning into the early afternoon throughout central/southwest MO
and points east. Given the lack of cap, this might result in waves
of storms. The aforementioned strong instability and deep layer
shear of 60+ knots would favor supercells, however as mentioned in
the SPC SWODY2 outlook, the convective modes could be messy with
a mix of supercells and organized bowing segments. The
environmental set-up is quite volatile with the aforementioned
CAPE/SHEAR space, low LCL heights, steep mid-level lapse rates and
anticipated convective modes favoring all severe weather hzards.
Forecast hodographs across eastern MO and southwest IL in the
afternoon depict impressive clockwise curvature in the lowest few
kilometers resulting in large low- level SRH, also supportive of
strong tornadoes. By early evening, all the activity should have
moved east of the CWA and into the Ohio Valley region.

Glass

&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 236 PM CDT Tue May 7 2024

An elongated positively-tilted longwave trof evolves from SE Canada
through the SW CONUS by early Thursday, anchored by upper lows in
the Great Basin and lower Great Lakes. The Great Lakes upper
low/trof continue to deepen across the eastern U.S. through Friday,
largely in response to upper ridging building from the Pacific into
the Pacific NW, while leaving a cut-off low in the vicinity of the
far southwestern U.S. While there are differences in details with
the eastern upper trof structure and depth amongst the deterministic
models/ensembles and LREF, there is a dominant signal for the
presence of a trof and persistence of NW flow aloft Friday
through the weekend.

This pattern supports near to below normal temperatures beginning
immediately post-frontal on Thursday and continuing through
Saturday as a series of shortwaves in the NW flow bring weak
reinforcing cold fronts through the area. Accompanying the
shortwaves could be periodic precipitation chances, especially on
Thursday, however there is no strong signal for high pops at any
point in time.

Heights aloft are on the rise by late Sunday as the eastern trof
progresses into the western Atlantic and by early next week the flow
aloft across the region is relatively weak by mid-May standards. In
response we should see temperatures moderate and return to above
normal. The aforementioned temperature trends are fairly well
depicted in the NBM temperature guidance and the IQR generally
ranges from 5-8 degrees in the Thursday-Tuesday time frame.

Glass

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1027 PM CDT Tue May 7 2024

Mainly dry and VFR conditions are expected through mid morning on
Wednesday with the exception of some patchy fog in low lying areas
including river valleys. Included a tempo group for fog at SUS and
CPS between 08-13Z. Then there will be an increasing chance for
showers and thunderstorms over Missouri and Illinois during the
late morning and afternoon hours. Have made minor adjustments to
the timing of showers and thunderstorms with 16-20Z at JEF/COU,
19-23Z at UIN, and 19-24Z at the St. Louis area terminals. Any of
the stronger storms tomorrow will be capable of producing MVFR,
possible IFR ceilings and visibilities, hail, and strong wind
gusts over 35 knots.

Britt

&&

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

$$

WFO LSX