671
FXUS63 KLSX 232313
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
513 PM CST Sat Nov 23 2024

.KEY MESSAGES...

- Temperatures warm back into the 60s on Sunday before a cold
  front brings colder air back in Sunday night into Monday.

- The next weather system tracks across the southern US during the
  middle of the week, spreading precipitation into our region
  Wednesday into early Thursday. Even colder air arrives after
  Thanksgiving.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 225 PM CST Sat Nov 23 2024

This afternoon the last gasps of our recent cool air mass are
exiting to the east. To the west, a westerly mid level flow across
the Rockies has initiated lee troughing over the western High Plains
and a broad southerly return flow at the surface in response. This
is initiating a sharp warm up with 60 degree temperatures already
nudging into southwest Missouri this afternoon. While we will lose
the sun before that warm air makes it into our area, warm advection
continues through the night, mainly aloft. Temperatures tonight
won`t be as cold as last night due to an increased southerly breeze
preventing ideal cooling. The biggest change will be in central
Missouri where lows are up to 15 degrees warmer tonight compared to
last night`s freeze.

The surface lee trough strengthens on Sunday and begins to drift
eastward in response to a shortwave trough emerging out of the
northern Rockies. This sets up a stronger south to southwest low
level flow over our area out ahead of a sharpening cold front. Warm
advection will be strongest aloft where 850mb temperatures increase
to +12C or higher, setting the stage for a much warmer day on
Sunday. We`ll see the 60 degree temperatures overspread the entire
region. Increased moisture on the southwesterly low level jet will
eventually bring in cloud cover first to central Missouri and
spreading north and east from there. This will limit the full mixing
potential later in the day and bring some sprinkles or light showers
mainly overnight Sunday night ahead of the approaching cold front.

Kimble

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 225 PM CST Sat Nov 23 2024

As the shortwave moves east across the Upper Midwest, the surface
low will track northeast into the Great Lakes with the cold front
moving southeast behind it. There`s still some variation in the
guidance on how quickly the front moves through our region on
Monday, though this variance continues to decrease relative to
earlier forecasts as shown by a decreasing NBM Inter-Quartile Range
(IQR) for highs on Monday. The consensus has the front in the
central to northern portion of the forecast area at daybreak on
Monday and exiting the southeast CWA by early Monday afternoon.
Ahead of the front, temperatures warm back into the 60s, but behind
it they will be dropping into the 40s. The sharp gradient in the
high temperature forecast on Monday is due to the timing of that
front. Southern and central parts of the area will see their highs
around midday before watching the temperature begin to tumble in the
afternoon. Northern areas will hold more steady in the 40s after the
early morning frontal passage. A stiff northwest breeze will also
make it feel noticeably colder on Monday.

The surface high builds quickly southward through the Plains on
Monday and nudges eastward Monday night. This sets up what will
likely be the coldest night of the season so far. Winds die off
under the influence of the high, and so long as any post frontal
clouds clear out we will set up ideal cooling conditions. Dewpoints
in this air mass will be in the teens to low 20s, setting a low
floor for temperatures for those areas where those ideal cooling
conditions can occur. This represents the next best opportunity for
STL to finally reach the freezing mark for the first time this
season. The surface high moves east across the region on Tuesday.
Despite a fair amount of sun, temperatures only warm into the 40s
with fairly high confidence as represented by a very low NBM IQR.
That`s some 5 to 10 degrees below normal, and 20+ degrees colder
than Sunday. Despite the cold temperatures, this may be the best
travel day near the Thanksgiving Holiday.

The next trough moves quickly through the western US on Tuesday and
out into the Southern Plains on Wednesday. With Monday`s cold front
having made it all the way to the Gulf Coast, the return of warm,
moist air ahead of this next trough won`t have time to make it all
the way north to our area, at least not at the surface. The surface
warm front moves quickly north, but only to about Arkansas and
western Tennessee before the surface low tracks eastward along it.
Moisture spreading up and over this front will be the primary
catalyst for precipitation Wednesday into Thursday. Wednesday is
likely to start out dry, but precipitation chances increase later in
the day with the best focus for precipitation Wednesday night.
Temperatures will be in the 40s as rain begins on Wednesday, but
with wet bulb cooling we will drop back into the 30s. Temperatures
aloft are cold enough for snow, but surface temperatures are likely
to remain above freezing with enough depth to melt snow to rain
before reaching the ground.

In addition to the wave tracking to our south, another related wave
tracks simultaneously eastward through the Great Lakes, sending a
reinforcing shot of colder air southward behind it. Where this
incoming cold air surge meets with the northward moving moisture
represents the greatest potential for a change over to snowfall.
First, it brings the potential for surface temperatures to cool a
few degrees, to the point where snow no longer melts before reaching
the ground. In this situation, snowfall accumulation remains
unlikely due to warm ground and light precipitation rates. So while
30 to 60 percent of ensemble members show this switch to snowfall
for much of our forecast area (greatest in the north), it doesn`t
necessarily represent any accumulation. However, the incoming surge
of cold air has the potential to increase the frontogenetical
forcing up at about 850MB and bring an increase in precipitation
rates, aiding in the dynamic cooling of the column and causing a
switch to snow. This represents the best potential for meaningful
snowfall, although it remains a low probability scenario at this
point. Among the 12Z ensemble members, only 10 to 30 percent produce
1 inch of snowfall or greater, roughly correlating to what can
reasonably be assumed to be an impactful ground accumulation.

While there remains uncertainty on the track, timing, and strength
of this system, the general consensus view is well agreed upon among
the guidance. Precipitation begins on Wednesday, ending on Thursday
morning, with the surface low tracking south of our forecast area,
meaning we are in the cooler air for this one. The greater
uncertainty is the arrival of the secondary surge of colder air and
where it can initiate a change from rain to snow before
precipitation ends.

After this wave passes by on Thursday, even colder air moves into
the region. There`s some uncertainty on just how far south the core
of the cold air gets, so just how cold we get is a little uncertain.
NBM probability of daytime highs not rising above freezing increases
to as high as 60 percent for parts of our area as early as
Saturday.

Kimble

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 509 PM CST Sat Nov 23 2024

Dry/VFR conditions are forecast overnight through the day on
Sunday. Light winds will turn out of the southeast and then south
late tonight into Sunday morning. Some gusts to around 20 knots
are possible at the central Missouri sites as well as KUIN.
Ceilings on Sunday will gradually lower, but chances for MVFR
conditions and any light rain shower activity look to hold off
until Monday evening, if not later.

Gosselin

&&

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

$$

WFO LSX