285
FXUS63 KLSX 071948
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
248 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 certaintly 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 suports 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 18z TAFs through 18z Wednesday Afternoon)
Issued at 1224 PM CDT Tue May 7 2024

VFR flight conditions are expected to dominate most of the
forecast period. Diurnal instability driven cumulus clouds will
persist across east-central MO into southwest IL during the
afternoon impacting KSTL/KSUS/KCPS. There is also a threat of a
strong isolated thunderstorm in this region through 23-00z,
however the anticipated coverage/probability of a TAF impact is
low and thus it has not been mentioned. Overnight there could be
some MVFR vsbys from radiational fog at KSUS/KCPS from around
09-13z. The next significant round of showers and thunderstorms
will largely be centered on Wednesday afternoon, however I have a
mention as early as 16z at KCOU/KJEF but there is a good amount of
uncertainty in whether they will develop this early in the day.

Glass

&&

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

$$

WFO LSX