Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 090450

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1050 PM CST Fri Dec 8 2023


- Showers, a few thunderstorms are expected along/ahead of a cold
  front passing through the region this evening, continuing
  eastward overnight into early Saturday morning while decreasing
  in coverage. Most locations will receive less than 0.1" of
  rainfall but there is a 60 to 90 chance of over 0.25 in
  northeastern MO and west-central IL.

- A strong thunderstorm or two is possible in central and eastern
  MO this evening with gusty winds and small hail, but the
  probability of any severe thunderstorms is less than 20 percent.

- Other than a day of slightly below average temperatures on
  Sunday, near to above average temperatures will prevail through
  next week.


.SHORT TERM...  (Through Late Saturday Night)
Issued at 226 PM CST Fri Dec 8 2023

Deep southwesterly flow continues to transport moisture into the
region ahead of a cold front, stretching from southeastern KS
through west-central MO into southeastern IA as of 20z. So far, this
evolution has only resulted in an increase in clouds. But, as large-
scale ascent increases early this evening ahead of an upper-level
shortwave trough rounding a longer wavelength trough over the Great
Plains, showers will develop in the next several hours and increase
in coverage, especially along the cold front.

Greatest coverage of showers will be between 02 and 06z across
central, northeastern MO and west-central IL, coincident with
enhanced frontal convergence in proximity to an open wave surface
cyclone tracking northeastward along the front. There is a 15 to 20
percent chance of embedded thunderstorms as well, supported by
anywhere from 150 to 750 J/kg of MUCAPE advertised by model
guidance. There is still a signal for a narrow wedge of marginal
surface-based instability to exist along and immediately ahead of
the front across central into eastern MO, but there is some
uncertainty in the magnitude of this instability. CAM and short-term
model guidance depict a range of SBCAPE from below 100 to locally
approaching 500 J/kg. Given that 50 to 70 kt of deep-layer wind
shear and 30 to 40 kt of 0 to 3 kt wind shear will be present, at
least organization of showers and thunderstorms into a broken line
is expected but the amount of surface-based instability will
determine whether thunderstorms are capable of any damaging wind
gusts or not. But the current, most likely scenario is that a
thunderstorm or two will be strong with gusty winds and small hail
in those areas.

Additional scattered showers are anticipated ahead of the front
within broad low to mid-level WAA, but as the front continues
eastward through eastern and southeastern MO into southwestern IL
overnight, coverage of showers and any thunderstorms will decrease
as the shortwave trough departs and large-scale ascent decreases.
Therefore, most areas will receive less than 0.1" of rainfall but
there is a 60 to 90 percent chance of over 0.25" in northeastern MO
into west-central IL, per the HREF.

By 14z Saturday, the cold front will exit the CWA to the east along
with nearly all remaining showers, with the expectation for there to
be no post-frontal rain through that time. Breaking of clouds should
also occur through the morning; however, clouds may increase during
the afternoon and evening as the axis of the main upper-level trough
and perhaps a subtle shortwave trough pass. The HREF and global
ensemble model guidance also support a 20 to 30 percent chance of
light rain in southeastern MO and southwestern IL during that time
as wave of post-frontal rain attempts to extend northwestward, but
far greater probabilities exist well southeast of the CWA.
Otherwise, high temps on Saturday will be cooler than today, albeit
still above average as the core of the cooler post-frontal airmass
does not arrive until the evening.


.LONG TERM...  (Sunday through Next Friday)
Issued at 226 PM CST Fri Dec 8 2023

As the upper-level trough will be exiting the Mid-Mississippi River
Valley height rises aloft will favor a surface ridge to move in
Sunday afternoon. Deep northwesterly flow with the exiting upper-
level trough and approaching surface ridge will favor cool, slightly
below normal temperatures and dry conditions on Sunday. As this
surface ridge moves across the area by Monday, low-level southerly
flow will promote near to slightly above normal temperatures to
start the work week. The upper-level pattern will become more zonal
through the first half of the week.

Ensemble guidance members are in agreement that a surface low
pressure system near the Hudson Bay in Canada will extend an
associated weak cold front through the Mid-Mississippi River
Valley on Tuesday, pushing the surface ridge east. Given the lack
of low- level moisture, rain is not forecast. Confidence in this
dry forecast is further increased given that 90% of ensemble
members maintain no precipitation. Temperatures on Tuesday are
expected to remain near normal across the northern portions of the
CWA and above normal temperatures across the southern portions of
the CWA. These temperature differences will be largely due to the
timing of the passing cold front. Through mid-week, there is
medium to moderate confidence (50-70%) in these slightly above
normal temperatures given that the lower quartile of the NBM
members are at or a degree below climatological normal.

There is consensus among ensemble members that slight upper-level
ridging will begin to build back in by mid-week. At the surface,
high pressure will move into the region and continue to promote near
to above normal temperatures for the rest of the forecast period.
Despite deterministic guidances signaling vorticity maximums moving
through the CWA during this timeframe, the surface high pressure
will help limit low level moisture moving into region and keep
precipitation chances very low.


.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1049 PM CST Fri Dec 8 2023

Cold front is beginning to move through the forecast area. It
should move through KCOU, KJEF, and KUIN by 06z Saturday and then
through St. Louis metro area by 10z Saturday with winds veering to
the northwest. Otherwise, showers and a few storms should exit
KCOU and KJEF with just vicinity showers, but kept predominate
rain at KUIN through 08z Saturday. As for KSUS, KSTL, and KCPS
rain will be between 06z and 10z Saturday.

As for ceilings, MVFR cigs will lift and scatter out from west to
east between 10z and 12z Saturday. Gusty northwest winds will
diminish by daybreak and remain that way through the rest of the
forecast period, except at KUIN, KCOU and KJEF where winds will
become gusty once again by Saturday afternoon.




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