054
FXUS63 KLSX 201757
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1157 AM CST Thu Nov 20 2025

.KEY MESSAGES...

- Showers and thunderstorms are expected today, mainly along and
  south of I-70. Locally heavy rain is possible where
  thunderstorms occur, resulting in ponding or nuisance flooding
  in areas with poor drainage.

- More widespread rainfall is expected tonight through early
  Saturday morning. Mean rainfall totals of 0.50-1.00 will be
  common by Saturday morning with locally higher amounts in/near
  thunderstorms.

- Temperatures will largely be above normal through early next
  week. Much colder air arrives for the Thanksgiving Holiday.



&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 331 AM CST Thu Nov 20 2025

(Synopsis)

Water vapor imagery shows the Mid-Mississippi Valley currently
positioned at the northern fringe of a broad ridge that encompasses
the Gulf Region. Upper flow turns southwesterly over the Plains,
ahead of a close upper low centered over Arizona. At the surface, a
stalled boundary extend east to west through northern Arkansas with
surface high pressure building southeast out of Canada into the
northern Plains. Clouds continue to cover the area as moisture is
funneled north and east out of Texas and into the Midwest. Clouds
continue to stream in from the southwest with surface temperatures
and dewpoints in the 40s.

(Through 18z This Afternoon)

The morning is playing out in similar fashion as yesterday with
another cool season look and feel to it. Low clouds generally range
from broken to overcast with calm/light winds and narrow dewpoint
depressions at the surface. Any breaks in the cloud deck are
becoming harder to find as mid-level moisture is ejected northward
around the western side of the Gulf ridge, through Texas, and into
the Midwest. Clouds will be locked in over much of the area with one
exception - per RAP, dry mid-level air is pulled eastward through
northern Missouri, where a glimpse of sunshine just may be seen on
occasion late this morning into early this afternoon.

The stalled boundary near the Missouri/Arkansas border makes little
northward movement this morning, lifting into the far southern
sections of the CWA or thereabout. Mid/upper vorticity glide
eastward over the boundary as warm, moist air is weakly force over
the cooler boundary layer. RAP/HRRR simulated reflectivity favor
this area for showers and a few thunderstorms this morning with
chances (60-80%) peaking this morning through about 17-19z. A strong
inversion (thanks to WAA) will keep thunderstorms elevated and
embedded in a general east/west oriented line at the northern edge
of ~1000 J/kg of MUCAPE. This is accompanied by a pool of 1-1.5"
PWATs that cross southern Missouri, suggesting that where
thunderstorms develop, there may be some efficient rainfall
production. Mean QPF through noon today shows 0.75-1" of rainfall
along and south of Sullivan through Bonne Terre, MO and Chester, IL.
LPMM does show spots of 2-3" with a localized 4". This isn`t a slam
dunk flash flood scenario with the brief 4-6 hour window for locally
heavy rainfall and recent dry conditions. Nonetheless, nuisance
ponding/flooding could occur if potential is realized and higher
amounts fall.

(18z This Afternoon-00z Friday)

Chances (40-60%) will continue to across the south after the noon
hour with a trend toward more scattered coverage as upper lift moves
east. A sharp cutoff is expected to the north with model sounding
showing a deep layer of dry air residing between 900-500MB. Early
morning radar return over northeast CWA hasn`t produced much more
than trace amounts or low end measurable precipitation. Steadier,
persistent rainfall will hold off to the north until later this
evening. Initially, there is a small pocket of instability
associated with the system, but largely looks like better convective
potential will be further south. That`ll be something to keep an eye
on as time progresses.

Things look a little more interesting as we work our way through the
week. This first system doesn`t have too much of an impact on
temperatures as there is not much cold air immediately behind it. It
another system that become tightly wound up of the northern Plains
by midweek, which sends a strong cold front southward Wednesday.
This does have a better shot of cold air that will greet us for the
Thanksgiving holiday. It becomes much colder by next Thursday with
temperatures more indicative of mid to late December. Highs may not
even get out of the 30s north of I-70 with low to mid-40s to the
south. I`d also keep an eye on a couple of weak systems that track
across the region next weekend. While long range guidance can rarely
be taken verbatim, the pattern shift will have temperatures take a
hard dive into winter-like territory and with cold air in place,
there could (emphasized) be opportunity for flurries/light snow in
the extended stretch. It`s nothing to be too excited about, but more
of a heads-up for weekend holiday travel.


Maples

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 331 AM CST Thu Nov 20 2025

(00z Friday-12z Saturday)

The closed upper low will begin to move eastward into the Plains
tonight and begin to open as it passes overhead through the day
Friday. This will shift the boundary northward as a warm front late
this evening through Friday morning. The broad pool of 1-1.5" PWATs
push far enough north to flirt with the Missouri/Iowa border
overnight into Saturday. The upper low will provide a broader region
of ascent with increasing rain chances tonight. All of the area will
get in on some kind of rainfall tonight into Friday. The lingering
question will be where the best forcing sets up in relation to the
mid-level low.

Hi-res guidance grasps this potential better, showing a east/west
oriented corridor of forcing at the northern side of the 850MB low.
Where this ends up exactly may shift slightly, but the favored
region runs generally along the I-70 stretch and locations to the
north. Unlike this morning`s round, however, convective potential
shifts east with the pool of MUCAPE as rainfall is driven by upper
ascent that slants westward (with height) from the surface low track
( central Kansas to near the Missouri/Arkansas border). HREF spread
shows there is still some variability with ~0.75" spreads in the
Interquartile Range (IQR) over northern Missouri/west-central
Illinois. This isn`t too uncommon with these forcing scenarios, so
placement of higher QPF will depend on where exactly this lines up
and/or how quickly the upper system begin to open. Should it open
quicker, precipitation coverage will likely take on more of a
scattered/numerous theme, as opposed to the broader shield of
persistent precipitation that might have been expected only a few
days ago. These trends are more evident in the 1+" probabilities that
have dropped in the last couple of cycles. While 50-70% of the
ensemble members paint areas south of Jeff City through Festus and
Chester with 1+ inch or more, chances quickly taper to about 15%
near Quincy.

Rainfall will begin to wind down Saturday morning as the upper wave
moves to the east-northeast. While not everyone will see the higher
totals previously mentioned, much of the CWA will benefit from mean
QPF of around 0.50" over northern Missouri and west-central Illinois
to just above an inch over our southern counties by early Saturday
morning, as a cold front drifts north to south behind the departing
low.

(12z Saturday-00z Monday)

Surface high pressure builds eastward from the central Plains with
mostly clear skies. Despite clearing, cold air advects into the
region behind the cold front, returning high temperatures closer to
their seasonal norms (50s). The surface high sets up somewhere
overhead late Saturday night into Sunday morning, providing an
opportunity for more efficient cooling. NBM guidance places the
coldest air (30s) east of the Mississippi River Sunday morning, but
there are subtle difference in the medium/long range guidance to
suggest low temperatures could shift slightly, depending on the
position of the surface ridge. The ridge then moves east during the
days Sunday with a slight uptick in temperatures on return flow. The
stint of near normal is brief with highs returning to the low/mid-60s
Sunday. Expect high clouds to begin to increase later in the day, as
the next system is poised to arrive early next week.

(00z Monday-Friday)

NBM is tightly clustered through next Friday (Nov. 28th), which
provides higher confidence than normal. We`ll see another chance
of rain early in the week with a cooling trend through the
remainder of the week.

Another closed upper low will swing out of the southwestern U.S. and
into the Plains over the weekend. This will bring another shot as
rainfall as early as late Sunday night and into early next week. The
pattern and evolution of the system in global deterministic guidance
is fairly similar to how this current system plays out late this
week. The closed low begin to open as it crosses the central section
of the country and eventually gets consumed by the northern stream.


Maples

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1139 AM CST Thu Nov 20 2025

IFR flight conditions are expected to prevail through the period.
A warm front will drift north from southern Missouri to near the
I-70 corridor by late tonight. The front will produce more
widespread rain and possibly a few thunderstorms tonight into
Friday. Rain and fog will reduce the visibility to 3-5SM and
occasionally below through much of the night. There may be some
improvement to ceilings this afternoon, but not likely to get
above 900ft, and then ceilings will drop again with the rain
tonight.

Carney

&&

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

$$

WFO LSX