Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 060950

Area Forecast Discussion
National Weather Service Saint Louis MO
350 AM CST Wed Dec 6 2023


- Early morning clouds will give way to sunshine across most of
  the area today. Wind will turn to the south-southwest which will
  help temperatures warm up to near or slightly above normal.

- Persistent southwest flow will produce temperatures 15 to 20
  degrees above normal on Thursday and Friday.

- A strong storm system will bring rain to the area Friday night
  and Saturday. There is a 20-30 percent chance for thunderstorms
  during this timeframe as well. Additionally, there is still
  around a 20-30 percent chance for the rain to mix with or change
  to snow late Saturday night into Sunday morning.


.SHORT TERM...  (Through Late Tonight)
Issued at 343 AM CST Wed Dec 6 2023

A ridge of high pressure will move southeast from western Missouri
into eastern Arkansas today.  The The wind will turn back to the
south-southwest as the ridge axis traverses the forecast area
through the morning.  The southerly flow on the back side of the
ridge will bring warmer temperatures to central and northeast
Missouri as well as west central Illinois today.  The warmer air
will also help break up the low cloud deck from west to east this
morning.  East central Missouri and southwest/south-central Illinois
will see ceilings persist the longest this morning, and miss out on
a few hours of insolation.  Therefore temperatures won`t rebound
quite as much.  This will result in about a 10 degree temperature
gradient from central Missouri east to south central Illinois with
highs in the low 50s in central Missouri to low to mid 40s in south
central Illinois.  Southwest flow will continue tonight which will
result in warmer low temperatures in the mid to upper 30s on
Thursday morning.


.LONG TERM...  (Thursday through Tuesday)
Issued at 343 AM CST Wed Dec 6 2023

Little has changed in the forecast for Thursday through Sunday.  The
strong upper ridge currently over the Western US will move east
ahead of a deep trough.  The combination of persistent southwest low
level flow and the upper ridge will produce temperatures 15 to 20
degrees above normal in the upper 50s to mid 60s on Thursday and
Friday.  Meanwhile the aforementioned trough will be digging into
the western Plains.  The resulting surface low is forecast to move
northeast from Oklahoma/Kansas into the Midwest Friday night/early
Saturday morning.  The positively tilted upper trough will continue
to dig and drive the cold front southeast through Missouri and
Illinois Saturday.  While the operational 06/00Z GFS and ECMWF
appear to be in very good agreement on the position of the trough as
well as the timing of the cold front Saturday, the EOF patterns in
the ensemble cluster analysis indicate uncertainty in both timing
and amplitude of the trough.  Additionally, there are 3 clusters
centered around Saturday and 4 clusters centered around Sunday
indicating increasing uncertainty with the trough as it moves east.
This translates into uncertainty with respect to the timing of the
cold front, and indeed the 25th percentile high at STL is 48 on
Saturday, while the 75th percentile is 63.  Also of note, the LREF
mean is showing 100-200 J/Kg of CAPE ahead of the front, and the
operational GFS and ECMWF are showing 400+ J/Kg. If the front ends up
being slower those CAPE values could be woefully underdone.  The
high shear environment will be supportive of severe storms if
there`s enough instability, so we will have to keep a close eye on
this as the forecast progresses.  One other thing to watch is the
possibility of snow behind the front Saturday night into Sunday.
Currently the GFS and ECMWF dry out before it gets cold enough, and
the LREF shows a maximum of only 25-30% chance for accumulating
snow.  However there are a few members of the ensembles that produce
quite a bit of accumulating snow in the cold sector.  Again, this
looks like a very low probability at this time.  The remainder of
the forecast through Tuesday looks relatively quiet behind the
weekend system with near normal temperatures.



.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1026 PM CST Tue Dec 5 2023

Low clouds with ceilings between 2500-3500 feet AGL will persist
at all of the terminals overnight before clearing out on Wednesday
morning between 14-17Z. Dry weather is expected at the terminals
through the period. Otherwise, northwest winds will back out of
the south to southwest by Wednesday afternoon.





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