300
FXUS63 KLSX 220344
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1044 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 06z TAFs through 06z Sunday Night)
Issued at 1044 PM CDT Sat Sep 21 2024

VFR conditions are expected for a few more hours with isolated
showers moving through eastern sections of Missouri prior to 06z.
Conditions deteriorate around 06z with showers and thunderstorms
moving in from west to east. Just prior to 04z this evening,
showers and thunderstorms spread from near the MO/IA border
through west-central MO and into the southern Plains. This broad
mass of activity will translate eastward overnight through into
Sunday morning. MVFR ceiling/visibilities will be the primary
impact with localize pockets of IFR possible at any terminals
directly impacted by thunderstorms. This potential is greatest at
KCOU/KJEF. While these conditions cannot be ruled out at KUIN and
metro terminals, thunderstorms will likely weaken somewhat as they
track east early Sunday morning.

Though a bulk of the precipitation was originally expected to
shift east through mid-morning Sunday, some of the latest trends
in data show a slightly southward shift in the axis of heavier
rainfall. If this trend continues, impact may extend slightly
later into the morning. There is a brief lull behind this initial
wave and the arrival of the cold front Sunday evening, when
activity flares up again along and ahead of the front. The
position of the front will determine exact placement of higher
potential Sunday evening, but the general consensus if for
rainfall to shift southeast of KUIN/KCOU/KJEF around 00z and metro
terminal after 02z-03z. A few isolated showers may linger with
MVFR ceilings along and behind the front. VFR could make a return
at KUIN before the end of the TAF period.

Maples

&&

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

$$

WFO LSX