188
FXUS63 KLSX 021941
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
241 PM CDT Thu May 2 2024

.KEY MESSAGES...

- Showers and thunderstorms will continue through the afternoon
  and evening. There is potential for an isolated severe
  thunderstorm across central through northeast Missouri and west-
  central Illinois between 2-8pm. Should a thunderstorm become
  severe, the hazards will be damaging winds up to 60 mph and
  quarter sized hail.

- An active weather pattern will continue through at least mid-
  week, with strong to severe thunderstorms possible at points
  between Saturday and next Wednesday.

&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 239 PM CDT Thu May 2 2024

Radar imagery is showing two areas of shower and thunderstorm
activity, one across southwest Missouri and another ahead of a cold
front extending from west-central Missouri through northeast
Missouri. Current water vapor imagery and surface analysis shows the
Mid-Mississippi River Valley in deep southwesterly flow.

Amidst southwesterly flow is a shortwave trough that is pushing
its way into southwest Missouri and is the current focus of the
convection in that area. Ahead of this shortwave, from central
Missouri through northeast Missouri, is a region of vorticity
advection. With increased deep moisture convergence and ample mid-
day heating due to a decrease in cloud cover this morning, these
areas are the prime location for shower and thunderstorm
development and intensification over the next few hours.

It is possible that a few of these thunderstorms could become strong
to severe with the primary threat being damaging winds however
quarter sized hail cannot be ruled out. 18Z environmental analysis
indicates SBCAPE values between 1500-2000 J/kg and deep-layer shear
values maximized between 20-25 kts across the aforementioned areas.
This low shear environment will help keep thunderstorms
disorganized and further limit the severe chances. The most likely
time for isolated severe thunderstorms will be between 2 - 8 pm.
The strong to severe thunderstorm potential will diminish quickly
after sunset with the loss of instability and weaker shear further
to the east.

These showers and thunderstorms will continue to move east-northeast
through the afternoon and evening hours as the shortwave treks to
the northeast and the cold front shifts east. Showers with a few
rumbles of thunder are expected overnight and will diminish on
Friday. Despite the passage of the cold front and northeasterly
flow, high temperatures on Friday will remain near to slightly above
normal for early May.

MMG/Jaja

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 239 PM CDT Thu May 2 2024

Our ongoing active pattern will continue this weekend and into the
middle of next week as multiple shortwaves pass through the Mid-
Mississippi Valley. The first of these shortwaves will arrive
Saturday afternoon into evening, bringing with it the potential for
strong to severe thunderstorms. At the surface, a cold front will be
approaching the region from the northwest with abundant moisture
return ahead of it. Deterministic guidance is quite uniform in
depicting mid-60s dewpoints across most of the CWA on Saturday, with
70-90% of ensembles corroborating this. If this comes to fruition,
dewpoints on Saturday will be above the 90th percentile of
climatology and jeopardizing records at KSTL/KCOU/KUIN. Despite
anomalous moisture, there is uncertainty regarding the amount of
instability we`ll have on Saturday. Ensembles range from 500-1000
J/kg, but incoming cloud cover, warmer or cooler temperatures, and
timing of the frontal passage could all sway these values higher or
lower. With upper level ascent, abundant moisture and instability,
and a surface trigger all present, confidence is high that we will
see convection on Saturday. What`s uncertain is the strength of
thunderstorms that do develop.

Another shortwave is forecast to pass over the region sometime
Sunday into Monday as a surface low and warm front push north
through the CWA. This will bring more precipitation to the area, but
the potential for severe will be much lower than on Saturday.
Anomalous moisture and heat will be wiped clean from the area by
Saturday`s cold front and deep layer shear will be unimpressive.
Anything that initiates along the warm front would be elevated in
nature and quite shallow according to model soundings. With guidance
trending later into the evening on the arrival of the shortwave,
this only dampens the potential for severe.

A deep mid-level trough will swing through the Intermountain West
and into the Plains early next week. This feature will be the point
of focus for severe weather next week as it enhances upper level
ascent and deep layer shear, and at the surface advects warmer air
and more abundant moisture back into the Plains and Midwest,
increasing instability. There are still too many differences among
guidance (as highlighted in very different WPC clusters for next
week) to discuss specifics, but this is a set-up conducive for
severe weather. Just when and where that happens is unclear as of
yet.

Jaja

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1254 PM CDT Thu May 2 2024

Current radar imagery shows two areas of showers and thunderstorms,
one extending from west central MO through north central MO and
another across far southwest MO. These showers and thunderstorms
will continue trekking to the north and east through the afternoon
and evening. Development and intensification is expected in central
and northeast Missouri over the next few hours. With the shower and
thunderstorm passage, MVFR to IFR flight conditions are likely along
with gusty winds upwards to 30 knots. If a thunderstorm does become
strong to severe, locally higher gusts are possible. As the showers
and thunderstorms continue to the east, it is expected that
thunderstorm intensity will decrease around sundown. With the
passage of a cold front, showers are expected linger with IFR to
low MVFR stratus through the overnight and Friday morning hours,
especially at the St. Louis Metropolitan terminals. Although
guidance is trending with less IFR ceilings, there is confidence
in IFR ceiling occurrence due to increased low level moisture.
Flight conditions will gradually improve on Friday as drier air
moves in.

Winds will become variable with the passage of the cold front and
favor northeasterly winds on Friday.

MMG/Jaja

&&

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

$$

WFO LSX