377
FXUS63 KLSX 050957
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
457 AM CDT Sun May 5 2024

.KEY MESSAGES...

- Scattered strong to severe thunderstorms are expected late
  Monday night into early Tuesday morning mainly along and west of
  the Mississippi River.

- Additional rounds of severe thunderstorms are possible on
  Tuesday and Wednesday across portions of the area. However,
  severe weather on each day will depend on exactly what happens
  the day prior, which is uncertain at this time.


&&

.SHORT TERM... (Through Monday Night)
Issued at 319 AM CDT Sun May 5 2024

A shortwave can be seen in water vapor imagery drifting slowly
northeastward across the Southern Plains within southwesterly flow
that extends across the Midwest. At the surface, high pressure
extends southward out of the Northern Plains and Upper-Midwest into
the Middle Mississippi Valley. Through the day today, this high will
gradually shift eastward as the shortwave and its surface reflection
move toward the area. Guidance has been trending this system slower
and further south in recent initializations, leading to most of the
day today being dry for much of the area. Rain chances will increase
this evening mainly over southeastern Missouri and southwestern
Illinois and continue through the overnight hours as the shortwave
slowly pivots across the region.

Tomorrow, the shortwave will continue east-northeastward into the
Ohio Valley as southwesterly flow aloft resumes over the CWA. A warm
front will lift northward through the area as southerly low-level
flow ushers in warm, moist air. As a result, afternoon temperatures
across the CWA will see 5-10 degree bump over what is expected
today. The increase in moisture will also increase instability over
the CWA during the day into the overnight hours. Subtle ridging in
the wake of the shortwave is expected to keep convection mostly at
bay across much of the area. However, I can`t rule out some isolated
afternoon showers and weak thunderstorms mainly over portions of
southeastern Missouri and southwestern Illinois given the closer
proximity to the slowly departing shortwave.

Lapse rates become increasingly conducive for convection Monday
night, and MUCAPE will hover around 1,000 J/kg across the area.
Afternoon convection from the Plains is expected to congeal into
multiple line segments as it moves eastward toward our CWA during
the overnight hours. Given aforementioned MUCAPE and 0-6 km of
around 30 kts, this convection will be able to sustain itself at
least through central Missouri and toward the Mississippi River
during the overnight hours. Damaging wind gusts and a tornado or two
are possible before convection weakens.

Elmore

&&

.LONG TERM... (Tuesday through Saturday)
Issued at 319 AM CDT Sun May 5 2024

At the start of the period on Tuesday, southwesterly flow will
reside over the Middle Mississippi Valley as a shortwave pivots over
the region. Guidance has been trending more aggressive with the
Monday night/Tuesday morning convection rolling into the area from
the Plains, which raises some questions as to how much we will be
able to destabilize Tuesday afternoon. Where the early morning
convection doesn`t stabilize the atmosphere, 50+ kts of 0-6 km shear
and around 2,000 J/kg of SBCAPE will create an environment conducive
for severe thunderstorms. However, the early morning convection may
cause this environment to be located east and/or south of the CWA.

On Wednesday, the trough will reload as another shortwave pivots
through the Central Plains and into the Midwest. In turn, a warm
front will lift through the Middle Mississippi Valley as a surface
low moves toward the region. Within the warm sector ahead of the
low, SBCAPE of at least 1,000-1,500 J/kg per deterministic guidance
and ensemble means will reside among about 50 kts of 0-6 km bulk
shear. Assuming this environment is realized, severe thunderstorms
capable of all hazards will be possible. However, where exactly this
threat occurs depends on how far north the warm front reaches and
the timing of the cold front.

In the wake of Wednesday`s cold front on Thursday, ensemble clusters
show flow aloft over the Middle Mississippi Valley will become
increasingly northwesterly as the upper-level trough expands
eastward and shears apart, forming a cutoff over the western CONUS.
Beneath the western periphery of the main trough now over the
eastern CONUS, deep northwesterly flow will advect cooler air into
the region, with confidence increasing that temperatures will run
below normal to end the week into next weekend, as ensemble means
have been trending cooler.

Elmore

&&

.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 453 AM CDT Sun May 5 2024

VFR flight conditions are expected through a majority of the day
today. This evening, a weather system will approach the terminals
from the south, bringing scattered showers and thunderstorms to
the region. Given the expected coverage of this convection,
confidence is low in direct impacts to local terminals. However,
if terminals are directly impacted, brief drops to MVFR or IFR
visibilities are possible. This convection is expected to remain
south of KUIN. Lower ceilings will also accompany the rain
chances, with IFR ceilings possible at the very end of the TAF
period. However, given uncertainty in the timing at the end of the
period, these conditions have been left out for now.

Elmore

&&

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

$$

WFO LSX