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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KLSX 231722
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1222 PM CDT Tue Apr 23 2024

.KEY MESSAGES...

- A weak cold front brings a round of showers across the area
  today. A few rumbles of thunder are also possible. Overall
  rainfall amounts will be light.

- A shift back toward warmer and more unstable weather is expected
  later this week. Multiple rounds of thunderstorms will be
  possible, some of which could be severe, although the timing and
  intensity is still uncertain. The most likely periods for
  thunderstorms will be Friday and Sunday.


&&

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 310 AM CDT Tue Apr 23 2024

A trough moving through the Great Lakes today will send a weak,
rather ill-defined cold front southward through our area today.
Ahead of this front, an area of moist advection at about 700MB will
lead to a round of showers developing and tracking through the
region. In the low levels we remain pretty moisture starved, but the
moist advection aloft will be enough to generate at least some light
rain. The first batch of moist advection moves east across northern
Missouri into Illinois later this morning. Some showers and
thunderstorms are already evident on regional radar across Nebraska
and northwest Missouri and this will translate eastward this
morning. Instability is pretty meager and entirely elevated, so
while we could see some rumbles of thunder we do not expect strong
storms today.

While the first batch of showers moves east this morning, another
round develops ahead of the front across central and southern
Missouri into southern Illinois this afternoon. Again we`re seeing
some meager instability aloft which could lead to some rumbles of
thunder especially in the initial activity, but we are not expecting
strong storms. Most areas will only see a few hundredths of rain
today, but areas that do get locally heavier downpours could see
more than 0.25 inch. Temperatures will be near seasonal normals, a
bit cooler to the north where clouds and showers are there to start
the day, while warmer to the south where at least some sun is
expected before the showers arrive.

Our brief shot of moisture ends quickly with the passage of the
front this evening. Winds turn to the north and a slightly cooler
and drier air mass moves in for Wednesday. We should see more
sunshine but highs likely top out a few degrees below normal, mostly
in the 60s.

Kimble

&&

.LONG TERM...  (Wednesday Night through Monday)
Issued at 310 AM CDT Tue Apr 23 2024

The main focus in the long term is on the return of moisture to the
region and the chances for severe thunderstorms which will accompany
it. Ridging moves out of the Rockies and through the Plains this
week with a trough moving onshore in southern California. As the
ridge moves east we`ll see our temperatures rise back above normal
through the weekend. With a persistent lee trough in the High Plains
this week we`ll see a continual resurgence of Gulf of Mexico
moisture northward through the Plains and eventually spilling into
our area as well. With this moisture will come greater atmospheric
instability as well, and with the passage of a few shortwave troughs
and related frontal features we`ll see chances for thunderstorms
increase late this week and this weekend. There will certainly be
periods in which instability and shear parameters line up for
organized thunderstorms and severe weather, but whether that threat
materializes in our forecast area is more in question. The greatest
threat time periods for our area look to be Friday and Sunday.

Moisture returns to the area in the form of a warm front lifting
northeast Thursday night into Friday morning. This will be our first
opportunity for showers and thunderstorms. With this being the
initial surge of moisture, the thunderstorm threats would likely be
centered around large hail depending on the magnitude of the
instability available (it could be elevated ahead of the front).
It`s a bit less clear at this point how much activity will redevelop
in the warm sector Friday afternoon and evening. A rather robust
shortwave trough will be tracking out of the Rockies and through the
Central to Northern Plains on Friday. Much of the synoptic scale
forcing with this passes by to our north, but there will likely
still be a belt of moderate to strong instability beneath a
weakening cap further south. Thunderstorm initiation could focus in
the vicinity of the dryline or trailing cold front mainly across
Oklahoma and Kansas, but potentially advecting eastward into our
area as well. There will be a greater all hazards severe weather
threat with this activity as instability is more likely to be
surface based in the presence of moderate deep layer speed and
directional shear. The greatest uncertainty is on forcing to focus
convective initiation and thence where it will move.

On Saturday in the wake of the departed shortwave trough we will see
at least brief shortwave ridging. The cold front in the wake of the
departing wave is unlikely to make it into our area, so we`ll remain
in the moist air mass. With more sun we`ll likely see our warmest
temperatures of this air mass on Saturday when much of the area
reaches the mid 80s. The combination of the moist air mass and
strong heating will lead to strong surface based instability
developing, but it will be occurring beneath a capping inversion
making it unlikely that we will be able to generate convection in
our area. NBM PoP is lowest on Saturday as a result, but may still
be too high owing to some timing differences in the models and the
relatively poor resolution of the longer range models. To our west,
though, another shortwave trough crosses the Rockies on Saturday.
The resulting surge of moisture northward will result in renewed
thunderstorm development along the remnant front to our northwest as
well as along the dryline in Oklahoma and Kansas. Some of the
convection in northwest Missouri could spread into parts of our area
as well Saturday evening, with the greatest potential across the
northwestern part of our forecast area. The severe weather threat
with this will likely be decreasing as it arrives into the overnight
hours.

As this trough moves northeast toward the Great Lakes on Sunday it
will send a cold front southeast behind it. Additional thunderstorm
development along this front is possible during the day on Sunday
and would present another all hazards severe weather threat
considering the unstable air mass ahead of it and the available
shear. This likely represents the period in which we could see the
greatest threat for severe weather in our area, but it`s still
unclear whether it will focus in our area or just outside. Models
are still trying to resolve the timing of this wave and the
associated frontal features, and as a result there is considerable
variability in where convective initiation occurs on Sunday. It`s
possible this could be focused more to our west or even to our
southeast depending on the speed of the trough and front.

Despite the passage of a cold front Sunday into Monday, we won`t see
temperatures drop all that much behind it. The main trough has its
origins in the Pacific so the air mass behind it is not particularly
cold and ridging builds back in quickly behind it. We should,
though, get at least a couple days break from the moisture and
threat for thunderstorms.

Kimble

&&

.AVIATION...  
-- Changed Discussion --
(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
-- End Changed Discussion --

&&

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

$$

WFO LSX