821
FXUS63 KLSX 031711
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1211 PM CDT Fri May 3 2024

.KEY MESSAGES...

- An active weather pattern will continue this weekend and through
  much of next week, bringing multiple chances of showers and
  thunderstorms to portions of the area.

- This pattern will lead to a chance of isolated strong to severe
  thunderstorms on Saturday, with additional strong to severe
  thunderstorm chances next week. However, details pertaining to
  any severe thunderstorm threat next week are unclear at this
  time.


&&

.SHORT TERM... (Through Saturday Night)
Issued at 313 AM CDT Fri May 3 2024

Current water vapor imagery shows a trough over the western CONUS
with a shortwave moving through the Upper-Midwest and Northern
Plains. At the surface, a low is in a similar location, with its
cold front seen in surface observations moving into northeastern and
central Missouri. Forcing from the front and a weak shortwave aloft
continue to drive scattered showers across the area. As the shortwave
progresses eastward and the front exits the area later this morning,
rain chances will wind down and end west to east.

Despite easterly-northeasterly winds behind the front today as high
pressure settles into the Midwest, mid- to upper-level flow from the
south-southwest will reduce the potency of this post-frontal airmass
and keep temperatures at to just above climatological normals.
Despite the front clearing the CWA, it will not have scrubbed
southern portions the area of boundary layer moisture completely. As
a weak shortwave moves through the Mid-South during peak heating
this afternoon, this low-level moisture will yield upwards of around
1,000 J/kg of SBCAPE that will lead to isolated thunderstorm
development mainly over far southeastern Missouri. Thunderstorms are
not expected to be strong, as weak 0-6km bulk shear of roughly 15-20
kts will produce updrafts that are weaker and short-lived.
Convection will dissipate during the early evening as the sun begins
to set and instability wanes.

Tonight, the aforementioned surface high will move eastward and
return southerly low-level flow to the area. In turn, low-level
moisture will begin advecting into the CWA ahead of a cold front
moving across the Great Plains toward the Middle Mississippi Valley.
Convection associated with the front will approach the CWA early
tomorrow morning, but will be in the process of decaying. So, while
I can`t rule out very isolated damaging wind gusts across
northeastern Missouri, strong to severe thunderstorms Saturday
morning are unlikely.

The better chance for severe weather comes Saturday afternoon as the
cold front moves into the area and a subtle shortwave travels
overhead. Then, guidance consensus is that roughly 1,500 J/kg of
SBCAPE will be present among 25-30 kts of 0-6 km bulk shear. Another
round of showers and thunderstorms is expected along and ahead of
the front in this environment, leading to an chance for isolated
strong to severe thunderstorms capable of large hail and damaging
wind gusts during the afternoon and evening. The amount of
instability is conditional on how much clearing of convective debris
from the early morning storms can occur prior to peak heating. If
clouds stay thicker and more widespread, we could see less
instability and weaker storms.

Elmore

&&

.LONG TERM... (Sunday through Thursday)
Issued at 313 AM CDT Fri May 3 2024

At the start of the extended period, guidance consensus is that
southwesterly flow aloft will still be in place over the Middle
Mississippi Valley. Both deterministic guidance and ensemble
clusters show a shortwave within this flow moving out of the
Southern Plains and into the Middle Mississippi Valley through the
day on Sunday and into Monday, though, there remain differences in
the phasing of this feature. A quicker solution would lead to a
faster surface low and rain chances during Sunday afternoon mainly
along and south of I-70, while a slower solution would lead to much
of Sunday being dry across the area with rain moving in during the
overnight hours. Regardless, guidance consensus is that the surface
low will weaken as a warm front stalls somewhere across the Midwest.

As we get into early next week, confidence in the evolution of the
upper-level pattern and thusly local surface conditions decreases.
In general, a trough will deepen over the western CONUS as a
shortwave swings through the Great Plains and into the Upper
Midwest. In turn, a surface low will follow a similar trajectory and
occlude as it swings a cold front through the Great Plains and
toward the Middle Mississippi Valley Monday into Tuesday. Guidance
differs on the progression of this front through the region, with
some stalling it west of the CWA while others have it clearing the
CWA. However, if the latter solution materializes, the front will
quickly lift northward as a warm front for Wednesday. Narrow
temperature spread among ensemble guidance for Wednesday showing
temperatures rising compared to Tuesday provides high confidence
that in some way, the CWA will be in the warm sector on Wednesday as
the western trough reloads. Additional shortwaves will round the
trough as it begins to edge eastward, sending another surface low
through the Midwest Wednesday into Thursday. This low will swing a
cold front through the CWA, with a majority of guidance having it
clear the CWA. Through this stretch (Monday-Wednesday), the CWA at
times will be in the warm sector multiple times beneath
southwesterly flow and in proximity to a warm and/or cold front.
This does set the stage for the potential for strong to severe
thunderstorms, and severe probability guidance from Colorado State
University shows portions of the area with heightened probability
through these days. However, severe thunderstorm potential will
hinge on the location and timing of the fronts - something that is
uncertain at this lead time.

As a result of the cold front passing through the area sometime
Wednesday into Thursday, ensemble consensus is that temperatures
will cool by the end of the week into next weekend as the trough
continues eastward and northwesterly flow aloft sets up over the
Midwest. By how much temperatures will cool is uncertain, however,
as temperatures range by about 10 degrees between the 25th and 75th
percentile. Confidence is high in temperatures being at or below
climatology given the 75th percentile hovering around seasonal
normals.

Elmore

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1204 PM CDT Fri May 3 2024

IFR/MVFR ceilings have been slow to scatter this morning.
Observations have shown very gradual improvement, but restricted
flight conditions will likely last into early afternoon. Otherwise,
winds will veer to southeasterly overnight and to southwesterly by
18z Saturday. By this time, a cold front will be approaching the
forecast area, introducing the chance of showers and thunderstorms
and a wind shift to northwesterly to all terminals Saturday
afternoon into evening.

Jaja

&&

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

$$

WFO LSX