076
FXUS63 KLSX 151000
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
400 AM CST Fri Nov 15 2024

.KEY MESSAGES...

- A Dense Fog Advisory remains in effect for central and eastern
  Missouri as well as west central and southwest Illinois until
  900 AM.

- Periods of rain are expected across the area late Sunday night
  through Monday evening as a storm system moves through the
  Great Plains and Midwest.

- The weather will turn cooler for the latter half of the week
  with a continuing chance for rain (30-50%) Wednesday and
  Thursday.

&&

.SHORT TERM... (Through Sunday)
Issued at 348 AM CST Fri Nov 15 2024

A sprawling ridge of high pressure is centered over eastern Missouri
this morning.  Areas of dense fog have developed in the light and
variable wind under the influence of the ridge, and the clearing
sky across Missouri and western Illinois. The fog has spread east
of the Mississippi River into parts of west central and southwest
Illinois, so we`ve expanded the Dense Fog Advisory accordingly.
The relatively low sun angle of mid November will allow the fog to
stick around for a few hours after sunrise, but it should
dissipate by mid-morning. Temperatures today should be warmer than
Thursday`s, particularly across Missouri and western Illinois
where the sky has cleared out and the only thing hampering a full
diurnal warm up will be morning fog. Latest guidance shows
temperatures rising into the upper 50s to low 60s in these areas
which looks reasonable given yesterday`s highs across western
Missouri and eastern Kansas. Some low clouds will likely linger
over south central Illinois at least through the morning and
potentially part of the afternoon as well. This will suppress the
diurnal warm up and should keep highs in the low to mid 50s.

The surface ridge drifts east tonight which will cause the wind to
swing around to the southeast.  The increasing southeast flow should
be enough to preclude widespread fog development across most of the
area tonight.  With that said, I wouldn`t be surprised to see fog in
river valleys and other sheltered spots, and possibly across parts
of south central Illinois where the pressure gradient and winds will
be weakest through 12Z Saturday.  Southerly flow continues to
increase on Saturday ahead of a wave moving through the northern
Plains.  There`s some disagreement in temperature guidance with
highs on Saturday.  This looks like it`s due to excessive cloud
cover in some of the models, most notably the NAM among others. This
appears to be suppressing NBM forecast highs for Saturday.  Given
the synoptic pattern with southerly flow at the surface veering to
southwest aloft, I`ve leaned heavily on the 75h percentile of NBM
temperatures which pushes highs up from the mid 50s and low 60s to
the low to mid 60s across most of the area.

The low level reflection of the northern Plains short wave lifts
northeast from eastern North Dakota into Canada Saturday night into
Sunday morning.  The cold front trailing the low pushes into
northwest Missouri and Iowa before stalling.  Deterministic guidance
shows low level moisture convergence along and ahead of the front on
Saturday night, and spits out some very low QPF across parts of
northern Missouri.  However the moist layer is pretty shallow
according to forecast soundings, the Gulf will remain closed due to
the blocking ridge to our south and east, and persistent southerly
flow in the boundary layer will continue to bring in relatively dry
air from the lower Mississippi Valley.  Therefore I think the chance
for any measurable precipitation will be very low Saturday night
into Sunday.  The aforementioned persistent southerly flow will also
help to warm temperatures on Sunday over Saturday`s highs.  Latest
NBM is giving us highs in the 65 to 70 range, which looks reasonable
given the pattern.

Carney

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 348 AM CST Fri Nov 15 2024

Precipitation chances increase early next week as the upper level
pattern becomes more active.  The first in a series of short waves
ejects from the Southwest U.S., across the Plains and into the
Midwest Monday into Monday night, bringing us our first shot at
precipitation for the week.  Guidance is in relatively good agreement
with respect to timing and strength of this first wave.  Although
the LREF is showing 4 clusters, the position of the trough at 00Z
Monday across the Rockies and western Plains is very similar in all
of the clusters, as is the down stream ridge over the Mississippi
Valley.  Additionally, EOF patterns show only minor variance in the
position/amplitude of the wave as it ejects northeast through the
Plains and Midwest Monday into Monday night.  As opposed to the wave
over the weekend, the Gulf of Mexico will be open so there will be
plenty of moisture to produce rain in the warm sector of the system
so the likely/categorical PoPs in the forecast for Monday and Monday
night continue to look reasonable.  The latest NBM bumped up thunder
probabilities for Monday afternoon and evening, however the GFS is
only showing a very small area of 100-200 J/Kg MUCAPE during this
period.  Given we`re now well into November, I prefer to err on the
side of lower thunder chances so will continue thunder-free at this
time.

After a quiet Tuesday, attention turns to the next wave which looks
like it will cool us down significantly for the latter half of the
week.  Medium range guidance shows there will be a high amplitude
pattern over the CONUS, though there is little agreement on the
position of the trough/ridge pattern.  The LREF is showing 4
clusters, two of which show the trough east of the Mississippi at
00Z Thursday, and two show it west of the Mississippi at the same
time.  Three of the clusters show a very high amplitude trough/ridge
pattern while one is much less so.  This translates to lots of
uncertainty with respect to the sensible weather at the surface. For
instance, high temperatures on Wednesday show a spread of as much as
15 degrees between the 25th and 75th percentiles.  Precipitation
amounts in the LREF members for Wednesday and Thursday range from as
little as 0 to over an inch.  With that said, the details may be
very uncertain, but the over all trend should be cooler,
particularly for Thursday with temperatures falling well below
normal.


Carney

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1027 PM CST Thu Nov 14 2024

MVFR clouds have cleared from west to east more quickly that
previously expected. While MVFR ceilings are now east of all
terminals with VFR conditions for the time being, it may not last
long. Radiational cooling and calm to light and variable surface
winds are expected to support fog development farther east than in
the last update. While all sites have the potential to have some
extent of fog late tonight into early Friday morning, KCOU, KJEF
and KSUS stand the best chance to see dense fog (1/4SM) with IFR
conditions for a few hours.

There is some question with the westward development near
KCOU/KJEF with the noted eastward shift. For now, prevailing
groups were continued until there is enough confidence to remove
the conditions. The remainder of the site may see MVFR fog
development given the later clearing and slightly milder
temperatures. VFR conditions are forecast to return by mid-morning
Friday.


Maples

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Dense Fog Advisory until 9 AM CST this morning for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
     Gasconade MO-Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln
     MO-Madison MO-Marion MO-Moniteau MO-Monroe MO-Montgomery MO-
     Osage MO-Pike MO-Ralls MO-Reynolds MO-Saint Charles MO-
     Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte
     Genevieve MO-Shelby MO-Warren MO-Washington MO.

IL...Dense Fog Advisory until 9 AM CST this morning for Adams IL-
     Brown IL-Calhoun IL-Greene IL-Jersey IL-Madison IL-Monroe
     IL-Pike IL-Randolph IL-Saint Clair IL.

&&

$$

WFO LSX