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FXUS63 KLSX 212314
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
514 PM CST Thu Nov 21 2024

.KEY MESSAGES...

- Seasonably cold conditions will continue through Friday night
  before temperatures warm back up above normal by Sunday.

- A cold front moves through on Monday, cooling temperatures below normal.
  A storm system brings increasing chances of precipitation
  around the Thanksgiving holiday, with rain the most likely
  precipitation type.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 233 PM CST Thu Nov 21 2024

Earlier morning stratus has become more cumuliform, lifted, and
scattered out for the most part over the past few hours. A vigorous
vorticity maxima is currently evident in water vapor imagery over
southern Lake Michigan and is moving due south. Widespread
precipitation is associated with this feature in northeastern
Illinois. Some of this precipitation may sneak into parts of south-
central Illinois through early/mid evening. The precipitation should
fall as snow given very low wetbulb zero heights. However, any snow
that does fall looks light and will not accumulate given the
intensity (or lack thereof) and warm air/ground temperatures. Some
flurries are also possible further west, possibly as far west as the
Mississippi River this evening.

Due northerly flow in the lowest 100 hPa will also bring in another
round of low stratus in tonight. This round is forecast to make it
back into most of eastern Missouri and all of our Illinois counties.
Parts of central and southeast Missouri may be far enough
west/southwest to stay more clear. The clearer sky and lighter
surface winds should allow for more cooling in these locations, with
lows slightly below the freezing mark. Further east/northeast, lows
in the mid to upper 30s should be more the rule due to the
aforementioned clouds and breezier conditions.

Friday will be a very similar day as today, with cool and cloudy
conditions especially in the morning hours. One big difference will
be the weaker northwest winds. As a surface ridge moves eastward
into Missouri early Friday, northwest winds will really slacken off
from west to east. So while high temperatures will once again be in
the mid to upper 40s Friday afternoon, it won`t feel as cold as
today.

Chillier conditions are forecast Friday night as the region should
sit beneath a mostly clear sky with light winds in proximity to the
surface ridge of high pressure. Lows are forecast to range from the
mid 20s to low 30s. Nearly the entire area should drop at or below
the freezing mark, with possible exceptions of parts of
southwest/south central Illinois and/or the urban heat island of
metropolitan St. Louis.


Gosselin

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 233 PM CST Thu Nov 21 2024

(Saturday - Sunday Night)

After a frosty start Saturday morning, there is high confidence in
warming temperatures and dry weather through the weekend. Low-level
warm air advection strengthens behind the departing ridge axis with
rising mid/upper level heights. Saturday`s highs are forecast to be
in the low to mid 50s (near normal), with readings reaching into the
60s for most on Sunday.


(Monday - Tuesday Night)

Uncertainty with respect to temperatures ramps up on Monday as model
guidance shows a midlevel shortwave trough moving across the Upper
Midwest. There are still large track and timing differences with
this feature, along with its associated surface low and attendant
cold front. This front will be moving through the bi-state area, but
the timing of its passage will be critical for Monday`s high
temperatures. A faster (slower) passage would yield colder (warmer)
temperatures than currently forecast. The spread between the
25th/75th percentiles of the NBM is quite large, generally on the
order of 10-15 degrees. There will also be a sharp gradient from
northwest to southeast, so it is quite possible highs in northeast
Missouri are in the low 40s while parts of southeast Missouri and
southwest Illinois are in the low to mid 60s.

Cooler weather will infiltrate the whole region behind this cold
front, with lows mostly in the 20s expected night and highs on
Tuesday afternoon in the 40s.

In terms of rain chances, there is not too strong of a signal for
measurable rain, let alone anything significant. Low-level moisture
advection on deterministic guidance looks weak, with stronger
mid/upper level forcing for ascent staying well to our north.
Probabilities over the 24-hour period ending Monday evening from the
LREF range from 30-60%, with the highest probabilities in southeast
Missouri and southwest Illinois. Looking at chances of even a tenth
of an inch though, probabilities drop to near or below 10%.


(Wednesday - Thanksgiving)

Forecast uncertainty increases further for mid/late next week,
including in/around the Thanksgiving holiday. Three of the four
clusters of the 500-hPa height pattern (85% of total membership of
the LREF) show zonal flow, though one cluster is much more
pronounced with a midlevel shortwave trough moving west to east
across the CONUS. This feature is actually a piece of energy from
the deep, closed low currently near the Pacific northwest coast that
gets "left behind" over the next couple of days. There are big
differences with when this feature makes landfall on the west coast
and how much it can deepen east of the Rockies. This is a system we
will have to watch for the local area, especially given its timing
in/around the Thanksgiving holiday. For anyone traveling away from
the region, this is a system that could produce a variety of
impactful weather depending on where you are heading so keep up to
date for future forecast changes. At this early juncture, a
preponderance of ensemble members suggests widespread light rain to
impact the region sometime between Wednesday and Thanksgiving night.
Odds for any snow mixing in increase the further north you go, but
chances for any impactful accumulation of snow with this system at
look very low in our area. Chances for 0.10" of more of liquid
falling as snow on the LREF only peak out at about 20-25% in parts
of northeast Missouri and west-central Illinois, with odds tailing
off farther to the south. For at least 0.25" of liquid falling as
snow, chances are 10% or less areawide. Climatology also strongly
would favor at least predominantly rain given the time of year and
that these systems that make landfall in California and move west-
east tend to favor the I-80 corridor for accumulating snow, not I-70.


Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 510 PM CST Thu Nov 21 2024

Stratus has moved back into KUIN and will gradually overspread the
area from north to south tonight including all of the terminals.
Flight conditions will initially be VFR with the onset of the
stratus, however ceilings should drop to MVFR within several
hours. MVFR flight conditions will prevail on Friday morning with
clouds becoming more cumuliform and ceilings improving to VFR on
Friday afternoon. Gusty northwest winds will slowly diminish
tonight.

Glass

&&

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

$$

WFO LSX