299
FXUS63 KLSX 212358
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
658 PM CDT Sat Sep 21 2024

.KEY MESSAGES...

- Our best opportunity to receive widespread beneficial rainfall will
  arrive tonight and last through Sunday morning. Additional
  chances for showers and thunderstorms exist until Tuesday.

- There is a chance of a few strong to severe thunderstorms on Sunday
  afternoon ahead of a cold front. The primary threat with the
  strongest cells will be damaging winds.

- Temperatures next week will start out well below average behind Sunday`s
  cold front and will moderate through the week.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 255 PM CDT Sat Sep 21 2024

Surface analysis shows that yesterday`s cold front has regressed
back to the north, clearing the area as a warm front. Most of
Missouri remains fairly cloudy from ongoing showers and
thunderstorms, limiting temperatures. Across the Mississippi River,
temperatures have had more freedom to climb with current
observations near 90 degrees in some locations. There is a 20
- 30% chance of showers and thunderstorms areawide this afternoon
and evening, but plenty of dry time is expected. The greatest
opportunity for stronger thunderstorms will be in southern Illinois
where sunshine has been more abundant today. 3000+ J/kg of SBCAPE is
present here, and bulk shear values are 20 - 25 kts. Ongoing
convection that has moved into this environment has strengthened,
becoming capable of small hail and gusty winds. Additional
development here will likely be capable of the same.

The better chance for showers and thunderstorms will arrive very
early Sunday morning. This will also serve as our best opportunity
to see widespread beneficial rainfall within the next week. In the
mid-levels, multiple series of vorticity maxima will eject into the
Mid-Mississippi Valley from a cutoff low entering the central
Plains. In the lower levels, a southwesterly 20 - 30 kt low-level
jet will develop overnight as a cold front pushes southeast into
northern Missouri. Both the LLJ and cold front will bring additional
moisture into the CWA, with PWAT values forecast to reach nearly 2".
This is corroborated by the HREF grand ensemble which shows a swath
of at least a 50 - 70% chance of this occurring Sunday morning. For
reference, the maximum PWAT value at SGF for this date is 1.79".

Although ample moisture and forcing will be in place tonight, the
concern for flooding is slim to none. Six hour flash flood guidance
values are nearly 5 inches since antecedent conditions are so dry.
To achieve these values, we will need very slow moving
thunderstorms, which deterministic guidance is quite confident won`t
happen. We`ll then need training thunderstorms, which is possible
given the orientation of the low-level jet. However, with the mix of
showers and thunderstorms expected and the progressive nature of the
system, it seems unlikely that any given location will see a series
of thunderstorms that would produce 2" - 3" of rain per hour or 5"
in 6 hours. Because of this, the largest impact of this system will
be a drought-improving rain.

Sunday morning`s showers and thunderstorms are forecast to exit the
forecast area late in the morning, with some clearing expected in
its wake. By this point the cold front will be well within the CWA,
and afternoon highs will reflect that. A near 20 degree temperature
range is forecast, with temperatures in northeast Missouri pushing
70 degrees and temperatures in southeast Missouri nearing 90
degrees. Southeast Missouri is also where the most instability will
reside. Mid-level forcing and low-level convergence will be much
more pronounced than they are today, promoting the potential for
thunderstorm development in the afternoon. Most guidance is in
consensus that this will happen, but confidence decreases concerning
the potential for severe thunderstorms. How much instability we get
will be tied to cloud cover, which most models have been bulking up
on in recent runs. Southwest Missouri has the greatest chance for
late morning clearing, so stronger thunderstorms may develop there
and shift east into our area. There is still the possibility that
convection that develops in our CWA becomes strong to severe given
the amount of forcing and bulk shear (30 - 40 kts) we have in store
already. We may not need an excess amount of CAPE when we have so
much else in place. If a thunderstorm does become severe, damaging
winds will be the primary threat.

Jaja

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 255 PM CDT Sat Sep 21 2024

The potential for showers and thunderstorms will continue Monday and
into Tuesday as vorticity maxima keep ejecting out of a mid-level
shortwave in the central Plains. Instability will be at a minimum,
however, as low-level cold air advection tanks our temperatures
behind the front. Highs on Monday are forecast to remain in the 60s
for nearly half of the CWA, with the warmest locations only reaching
the upper 70s. These values are below average for late September,
and do not promote the development of strong thunderstorms. The
shortwave axis is forecast to pass the region sometime on Tuesday,
ending our rain chances until at least late in the work week. Low-
level moisture will be scoured out, leaving dewpoints in the 50s and
low 60s. Those dewpoints alongside highs in the 70s will create a
seasonable feel to accompany the quiet weather. The slight chance
for showers and thunderstorms returns late in the week as a cutoff
low spins somewhere over the central or southern CONUS, but guidance
varies wildly concerning this feature so there`s not much to
discuss.

Jaja

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 648 PM CDT Sat Sep 21 2024

A small cluster of showers and isolated thunderstorms over
central and northeast Missouri could lead to brief periods of
MVFR visibility with occasional lightning this evening. Aside from
from this, VFR is generally favored through until later this
evening.

There is high confidence that widespread showers and thunderstorms
will encroach on central Missouri terminals around or just after
midnight, spreading eastward to KUIN and metro terminals only a
few hours afterward. Steady rainfall, locally heavy at times, will
result in MVFR visibilities through at least mid-morning. The bulk
of this activity will move eastward by early afternoon with
scattered showers and isolated thunderstorms linger through the
afternoon and evening, but overall, conditions briefly improve to
VFR until later in the afternoon and early evening when a cold
front drifts in from the northwest. Cigs drop to MVFR with another
round of showers and thunderstorm just beyond the 24 hour period.
While occasional lightning is possible at any time through the
period, VCTS was only carried through the early morning to better
handle potential in subsequent updates.

Maples

&&

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

$$

WFO LSX