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000
FXUS63 KLSX 231936
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
236 PM CDT Tue Apr 23 2024

.KEY MESSAGES...

- Multiple rounds of thunderstorms, some of which could be strong
  to severe, remain in the forecast Friday-Sunday across the area.
  Details still need to be worked out but the best chance for
  severe weather will be Friday and Sunday, with conditions not as
  favorable for widespread thunderstorms Saturday.


&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 235 PM CDT Tue Apr 23 2024

Showers and scattered thunderstorms continue to push through the
forecast area this afternoon along an elevated cold front with
support from the axis of a mid-level trough swinging through the
area. Precipitation will continue to push through the area along the
front into the evening, with the strongest cells capable of pea-
sized hail. The front will continue to push south of the forecast
area overnight, with mid-level ridging and a surface high pushing
into the area in its wake. Widespread subsidence will result in dry
conditions and a mostly clear sky on Wednesday. Cold air advection
will increase along the northwesterly flow, but the clear sky and
bountiful solar insolation on Wednesday will provide a
counterbalance, resulting in temperatures similar to today with
highs near to just above normal.

Overnight into early Thursday morning the low-level high will shift
eastward, veering winds towards the south and increasing moisture
return. The previously mentioned front will return northward through
the lower Mississippi River Valley as a warm front overnight, and
warm air advection showers will start to fill in in the vicinity of
the front across southwestern Missouri overnight. These showers are
expected to stay largely southwest of the forecast area, but this
will depend on the location of the front.

MRM

&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 235 PM CDT Tue Apr 23 2024

Height rises associated with a mid-level trough will dominate the
central CONUS on Thursday, which will dampen at least somewhat the
chance for warm air advection showers and thunderstorm during the
day. Behind this mid-level ridge a trough will form over southern
California overnight Wednesday into Thursday, strengthening Thursday
as it shifts into the Four Corners region. By Friday morning the
trough will extend across the western half of the CONUS, with a
shortwave pushing northeast into the mid/upper Mississippi Valley.
At the same time the LLJ will shift east into the region, and
combined with WAA and an elevated warm front, will produce showers
and thunderstorms across the region Friday. Instability will
increase through the day, with the best instability (500-1000 J/kg
of MUCAPE) currently expected along and west of the Mississippi
River. Details surrounding instability remain uncertain at this
point. Deterministic model soundings show an elevated cap during the
day, which would inhibit thunderstorm development. Additionally,
morning rain chances could work over the environment, leaving little
ability for the instability to rebound. If enough instability is
available, strong to severe thunderstorms are possible Friday where
organization can occur in the 40-60 kts of 0-6km shear (mainly west
of the Mississippi River).

There will be a relative lull in thunderstorm chances Saturday as
mid-level height rises in the wake of Friday`s shortwave produce
widespread subsidence across the area. Warm air advection will
persist, so while widespread thunderstorms are not expected in the
absence of strong lift, I can`t rule out a stray shower or
thunderstorm during the day. In the meantime, another shortwave will
move through the broader mid-level trough from the Four Corners
region northeast into the mid-Mississippi River Valley by Saturday
night, bringing better chances for showers and thunderstorms to the
area.

This trough will slide northeast of the forecast area Sunday in
lockstep with a surface low that will form over the Southern Plains.
Thunderstorms will blossom along the warm front to the north of the
forecast area, and the cold front to the west overnight Saturday
into Sunday. This will leave the Missouri-Illinois region within the
warm sector, which will destabilize during the day. Deterministic
model guidance indicates at least 1000 J/kg of MUCAPE is possible
during this time. Uncertainty remains in how unstable we get due to
the location of morning convection and cloud cover, both of which
could limit destabilization. Uncertainty also remains in how much of
a cap will be present during the afternoon, which could inhibit
widespread thunderstorm development. The surface low will push into
the upper Mississippi Valley during the afternoon and evening,
dragging the cold front through the forecast area. If there is
enough instability and a small enough cap, showers and thunderstorms
will be possible within the open warm sector during the afternoon
and along the cold front as it moves through the region.
Thunderstorms that develop will be able to organize within the 40-60
kts of 0-6 km shear, and a few will become strong to severe.

The large mid-level trough will lift out of the area Monday, but
details on how quickly it does so remains uncertain. Ensemble
guidance splits the solutions roughly in half, with scenario 1
lifting sharply out with a negative tilt. This scenario would keep
chances for showers and thunderstorms lingering into Monday morning.
Scenario 2 lifts the trough out in a neutral orientation, which
would shift chances for showers and thunderstorms out of the
forecast area quicker. What is certain is that by mid-week we will
be largely precipitation free under an approaching mid-level right
and surface high.

MRM

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1218 PM CDT Tue Apr 23 2024

Showers and an isolated thunderstorm are moving through the region
and will impact the St. Louis metro terminals (KSTL, KSUS, KCPS)
this afternoon. Underneath any showers in the St. Louis metro,
MVFR visibility is expected briefly. Once showers exit the
terminal conditions will improve back to VFR conditions. The mid-
Missouri sites (KCOU, KJEF) may see a stray shower as well, but
confidence in MVFR conditions with these showers is very low.

Otherwise, VFR flight conditions will prevail through the
remainder of the period. A cold front will slide through the area
this evening, and winds will shift from southwesterly to
northerly and decrease in speed to near 5 kts.

MRM

&&

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

$$

WFO LSX