Area Forecast Discussion

000
FXUS63 KLSX 270947
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
447 AM CDT Sat Apr 27 2024

.KEY MESSAGES...

-Widely scattered showers and sub-severe thunderstorms are
 possible during the day today. There will be ample dry time in
 between, but any given location impacted by thunderstorm could
 receive a brief downpour.

-Two more distinct rounds of showers and thunderstorm arrive late
 tonight into early Sunday and late Sunday morning into Sunday
 night. The strongest storms could be severe with large hail,
 damaging winds, and isolated tornadoes. Heavy rainfall could
 produce localized flash flooding. .


&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 446 AM CDT Sat Apr 27 2024

The CONUS view on water vapor shows yesterday`s upper level low
continuing to depart to northeast through the Upper Midwest with the
next upper level low rotating east-northeast into the Four Corners
Region. The approaching upper level low will bring additional
potential for more significant shower and thunderstorm activity to
the central U.S. Until then, the local area gets a brief reprieve
from widespread rainfall.

The last bit of shower and thunderstorm activity weakened and moved
out of the forecast area just prior to midnight. All has been quiet
since then. However, another weak mid-level impulse is expected to
pass north-northeast over an area of weak surface convergence that
resides at the northwest edge of the southeastern ridge. MUCAPE will
gradually build overhead, eventually shrouding much of the CWA with
500-800 J/kg of instability through peak heating this afternoon.
While this isn`t much, it`ll be enough for widely scattered,
disorganized showers and thunderstorms to develop in a region of
relatively weak 0-6km shear (30-35 kts). There will be ample dry
time between showers and thunderstorms, but any given area that gets
under one a thunderstorm could see a brief, localize downpour. It
seems there is enough of a northward shift in the warm front near
the MO/IA border that better potential over northeast Missouri and
west-central will shift into Iowa this evening. This should be
watched, however, as some CAMs have shown the potential for
intensifying thunderstorms in an area of relatively higher MUCAPE
(1000-1200 J/kg) and 0-6km shear (around 40 kts).

Widespread showers and thunderstorms are expected develop again over
the central Plains this evening as a N/S oriented LLJ jet
strengthens to 50-60 kts this evening into tonight. CAMs are in
fairly decent agreement showing showers and thunderstorms growing
upscale and organizing into a linear structure as they track
eastward into western Missouri around and after 00z this evening.
While 2000 to as much as 3500 J/kg of MUCAPE extends into northeast
Missouri and west-central Illinois late this afternoon, it begins to
wane through the evening into late tonight. This, along with mid-
level lapse rates (5.5-6C) along and east of the Mississippi River,
will likely lead to weakening in the line as it tracks into east
central Missouri late this evening. How long it survives may depend
on development of a cold pool as some CAMs show the line beginning
to bow out from west to central Missouri. The hail threat, while not
zero, is likely to be lower as it transitions to a wind and tornado
threat as soundings showing surface winds being backed ahead of the
line over central Missouri. The evolution of the line will need to
be watched later today into this evening to more accurately assess
the overall threat over western sections of the CWA. It is expected
that the line will continue to weaken as it approaches the
Mississippi River with some indications that it may even collapse
entirely before reaching the St. Louis Metro Area.

Ongoing strong to severe thunderstorms eject northeastward out of
Oklahoma early Sunday morning, bringing the potential for widespread
activity to the local area late Sunday morning into Sunday
afternoon. How this evolves will have a domino effect on severe
potential heading into Sunday afternoon and Sunday night. The upper
level low tracks northeast through the central U.S. in a similar
manner as the prior upper low. This motivates some eastward movement
in the cold front, but begins to slow and eventually become more
east-west oriented in time. Prior waves of showers and
thunderstorms, along with their coverage, could result in a
relatively more stable environment by the time any surface
convergence reaches central Missouri. Given the latest trends,
confidence is not high in severe potential Sunday afternoon into
Sunday night.


Maples

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 446 AM CDT Sat Apr 27 2024

Sunday`s upper low will continue to exit to the northeast Monday as
a surface cold front becomes more diffuse and stalls over southern
sections of the forecast area. This suggests additional chances for
showers and thunderstorms mainly over southeast Missouri and
southwest Illinois, especially as better upper ascent remains well
south in relation to an upper shortwave that traverses Arkansas.
This coincides with better instability, where MUCAPES might climb
above 1000 J/kg. Further north and west, surface high pressure
attempts to build into the area with modest mid-level height rises
behind the front. Instability never fully recovers, limiting the
northward extent of rainfall potential as surface flow briefly turns
westerly.

The best potential for dry time is Tuesday as we get between the
departing system and any additional systems that track across the
central U.S. The quasi-zonal flow remains active with multiple
system tracking through the central Plains into the Midwest. Among
the main topics could very well be the well above normal
temperatures. There isn`t a whole lot of spread (5-8 degrees) in NBM
temperature data through midweek with the 75th percentile hinting at
the potential for low to mid-90s by Tuesday and Wednesday. The
southwest flow component may be in play, which is climatologically
supportive for above normal warmth. Active weather will be the key
with any rainfall or resulting cloud cover inhibiting diurnal
heating in this period. Nonetheless, even the 25th percentile is
suggesting mid to upper 80s, which gets within reach of record
highs. Cooler looks to hold off until late week, as a cold front
approaches the region from the northwest.


Maples

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1119 PM CDT Fri Apr 26 2024

Isolated showers and thunderstorms will continue to be possible
across much of the area through the period. The most likely times
will be right at the beginning of the period from 06Z to around
10Z, then again on Saturday afternoon/early evening. Did not
mention thunder in this TAF cycle due to the timing uncertainty
and likely low coverage of storms. Otherwise, VFR ceilings which
will occasionally scattered out at times and southerly flow
gusting up to 25-30kts is expected to prevail. There will also be
a few patches of MVFR ceilings early in the period, mainly across
central and northeast Missouri.

Carney

&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion