730
FXUS63 KLSX 161955
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
255 PM CDT Thu Oct 16 2025

.KEY MESSAGES...

- Temperatures will remain above normal in the 80s on Friday.

- Showers and thunderstorms are expected ahead of a cold front on
  Saturday. Confidence in severe storms on Saturday afternoon and
  evening is increasing.

&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 253 PM CDT Thu Oct 16 2025

The upper level ridge which has been controlling our weather will
shift east and attenuate somewhat over the next 24-36 hours.  This
will cause the surface high which is currently over the Great Lakes
Region to drift over the Eastern Seaboard.  As this happens, the
closed upper level low over Wyoming and Montana will open up into a
progressive wave which lifts northeast into Canada by early Friday
morning. The cold front attached to the surface low will move slowly
east through the central Plains tonight and Friday, and stall in
central or eastern Kansas Friday night.  Our area will be sandwiched
between the retreating high and the approaching cold front through
Friday.  The changes to sensible weather here will be limited to
increasing southerly flow along with a slow increase in dew point
temperatures.  Highs on Friday will remain well above normal in the
low to mid 80s, with lows Friday and Saturday morning mainly in the
upper 50s to mid 60s.  Latest guidance also shows the low level jet
increasing to 30-35kts late Friday night ahead of the next upstream
short wave.  This produces weak to moderate moisture convergence
across parts of central and northeast Missouri into west central
Illinois. Most short range guidance spits out light QPF in response
to the moisture convergence so chance PoPs in those areas is
reasonable.


Carney

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 253 PM CDT Thu Oct 16 2025

Attention turns to Saturday, and the first good chance at a
significant rainfall we`ve had in a long while.  The first short
wave that moves into Canada on Friday leaves behind a long wave
trough stretching from the Dakotas all the way through southern
California and Arizona.  As this moves relatively slowly eastward,
another short wave dives through the Plains and phases with the long
wave trough Saturday afternoon into Saturday night.  The resulting
combined long wave forces cyclogenesis along the front over the
Midwest which pushes the cold front through the Mid Mississippi
Valley.  The deterministic GFS has consistently been building 1000+
J/Kg MLCAPE over Missouri by 18Z for the last few runs.  The 10/16
12Z run is the strongest I`ve seen yet, with MLCAPE of 1500 to
nearly 2000 J/Kg stretching from just west of St. Louis, across
central Missouri into eastern Kansas at 18Z Saturday in the pre-
storm environment.  I think this is on the higher end of what`s
likely though as the LREF 75th and even 90th percentile SBCAPE
generally tops out around 1500 J/Kg. With that said, 1000-1500 J/Kg
is very respectable when coupled with 40-50kts of deep layer shear.
Hodographs are pretty linear though, so any discrete convection
should grow upscale into an MCS.  However, the initial discrete
cells will have enough environmental shear to rotate and any
supercells will be capable of producing severe winds and large hail.
Low level conditions don`t appear particularly conducive to
tornadogenesis with 10-15kts of 0-1km shear and less than 100 m2/s2
helicity, as well as LCL heights AOA 3500ft.

All of this severe weather potential is predicated on the idea that
morning clouds and showers will clear out early and daytime heating
can produce the afore mentioned levels of instability.  As opposed
to the GFS, the deterministic ECMWF barely cracks 500 J/Kg of MUCAPE
Saturday afternoon with showers continuing all morning, which .
limits insolation.  This would obviously be the more benign solution
with beneficial rainfall and little if any chance for severe
weather.  LREF mean SBCAPE tops out just over 1000 J/Kg at 18Z
Saturday as does the 50th percentile.  This doesn`t help lean toward
either solution at this point, but the severe solution feels more
likely than it did yesterday due to the run-to-run consistency of
the GFS.  Any severe threat should end during the evening as storms
burn through available instability.

One final note about Saturday, precipitable water values will be
relatively high...in excess of 1.5 inches.  While the MCS and
individual storms should be progressive, there is a chance that some
areas could receive multiple rounds of rain.  Some locations could
receive 2+ inches of rain in a short period, and perhaps 4+ overall.
There could be some localized nuisance flooding of poor-drainage and
low-lying areas. However the soils are very dry, so widespread
flooding/flash flooding is not expected at this time.

The remainder of the forecast looks relatively uneventful.  Cool
high pressure builds across the Mississippi Valley behind the front
on Sunday, and another cold front moves through late Monday or
Monday night.  The second front looks moisture-starved with the cool
high pressure system settled along the Southeast U.S. and Gulf
Coast, so little if any precip is expected.  Temperatures drop to
near or just be low normal in the 60s for Sunday, then warm up a bit
into the 70s  Monday ahead of the second cold front.  Highs return
to the 60s for the remainder of the week behind the second front.


Carney

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1230 PM CDT Thu Oct 16 2025

VFR flight conditions are expected to prevail through the period.
Winds will gradually veer to the southeast and then to the south
by by Friday morning.

Carney

&&

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

$$

WFO LSX