609
FXUS63 KLSX 040846
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
246 AM CST Tue Nov 4 2025

.KEY MESSAGES...

- Mild late fall temperatures are expected most of this week. The
  only significant chance of rain comes Thursday night.

- A cold front late this weekend will plunge temperatures 25
  degrees or more making it feel more like early winter for the
  first part of next week.


&&

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 244 AM CST Tue Nov 4 2025

Ridging to our west this morning expands eastward today into more of
a zonal flow across the central US. Surface high pressure shifts
eastward into the Southeast US while lee troughing develops east of
the Northern Rockies over the Northern Plains. The pressure gradient
between the high to our east and the trough to our west will set up
a southerly flow across our region today leading to a warm late fall
day. Yesterday`s cold front has become hard to find in the wind
fields this morning, but it`s still displaying a sharp dewpoint
gradient across our region: from the mid 20s at Quincy to the mid
40s in the Ozarks. This remnant front pushes quickly northward today
with the developing southerly flow. With the wind trajectory across
the Ozarks today, the warmest temperatures should be in central
Missouri, likely near Jefferson City by the Missouri River, where
the southerly winds have the greatest downslope component to aid in
the warming. It is these return flow patterns that we often see
periods of elevated fire danger in the cool season. However, in this
case there is clear moist advection taking place today with upstream
dewpoints in the 40s and low 50s with this moisture deep enough to
not be scoured out by mixing. In fact, high resolution ensemble
guidance is indicating high confidence in the dewpoint forecast with
only a 1 or 2 degree spread in the Interquartile Range for the
dewpoint this afternoon in central Missouri. So although this would
be the area for greatest potential for elevated fire danger this
afternoon, current expectations are that our humidity levels stay
high enough to negate this concern.

As low pressure pushes eastward across the Upper Midwest, driven by
a shortwave trough aloft, a cold front will push southeastward
behind it, moving through our region on Wednesday. This does bring a
shift form southwesterly winds to northwesterly winds and some
cooler temperatures. Current forecast high temperatures on Wednesday
show a temperature gradient based on the arrival time of that front,
with southern areas warming into the 70s again while northern areas
only make it into the 60s and the greatest uncertainty in between.
With the mid level synoptic scale lift staying well north of us over
the northern Great Lakes and a lack of moist convergence along the
front we have strong confidence that this will be a dry frontal
passage for our region.

Kimble

&&

.LONG TERM...  (Wednesday Night through Monday)
Issued at 244 AM CST Tue Nov 4 2025

Surface high pressure behind Wednesday`s front moves into the region
fairly quickly, arriving at the Mississippi River Wednesday night.
This will lead to a clear, calm night with radiational cooling
producing a much colder night than the one before, with most areas
falling into the 30s. Leaned on the MAV and 10th percentile of NBM
for low temperatures in this period. The surface high pushes
eastward on Thursday into the Ohio Valley and toward the
Appalachians leading to a return southerly flow across our region
and the initiation of a warm up. After the chilly morning,
temperatures warm some 30 degrees back into the mid 60s. Again,
though, there`s substantial moist advection on this flow with
dewpoints rising into the 50s, limiting the fire danger potential.

The next shortwave trough drives a surface low east through the
northern Great Lakes Thursday night with an accompanying cold front
sliding southeastward through our region Thursday night into Friday.
This one has a better chance of rain with it as the shortwave trough
aloft extends further south leading to greater lift available for
precipitation ahead of and along the front Thursday night into
Friday morning. Among the 12Z long range guidance, 60 to 80 percent
of ensemble members produce measurable rain across our region in the
24 hours ending midday Friday, but less than 40 percent of those
members produce 0.1 inch or more. Thus while this represents our
best rain chances in the forecast, amounts will be light.

Friday`s wave begins to carve out broader troughing across the
middle of the country, and a secondary cold front drops down from
the north with even colder air behind it. This stronger cold front
remains just to our north on Saturday as the next shortwave trough
pushes eastward through the Central Plains sending surface low
pressure eastward along the front through Iowa or northern Missouri.
The bulk of the precipitation associated with this wave is likely to
fall along and to the north of the surface low track. At the moment
this is likely to stay just north of our area in Iowa and points
east toward the southern Great Lakes, however there is still some
variability on the track of this wave. Guidance with a more
southerly track does bring more of this precipitation into northern
Missouri and central Illinois, although even with these southerly
tracks the temperature is expected to remain warm enough for rain.
Among the 12Z long range guidance, up to 50 percent of members
produce measurable rain in the vicinity of Quincy in the 24 hours
ending Sunday morning. With most of our area remaining south of the
front as the low passes by on Saturday, we`ll remain mild in the 60s
to near 70 before a sharp cool down once that front surges southward
Saturday afternoon and evening.

Saturday`s cold front is the most potent of all of this week`s
fronts and we`re really going to feel the cold punch that it brings.
Over the past several days, there has been some uncertainty on the
trajectory of the cold air push, with some guidance pushing the core
of this cold more to the east of our area with us getting just a
glancing blow. However, this is becoming a minority among the
guidance. Among the 12Z long range guidance ensemble clusters, only
Cluster 4 representing just 11% of ensemble members maintained the
more easterly trajectory, while the clear majority favored a much
stronger push of cold air directly through our region. This trend
seems to be continuing with the new 00Z guidance as the
deterministic GFS, ECMWF, and CMC all now have the surface high
sliding south through the Plains with strong cold advection in the
northerly flow across Missouri and Illinois. Among each of these
major ensemble families, the average 850MB temperature arriving in
our area late Sunday into early Monday has fallen 5C or more just in
the last 2 major model cycles, with the ensemble means now in
the -5C to -9C range. This indicates that even with strong mixing
we`ll only see surface temperatures in the low to mid 40s. The
latest deterministic NBM is reflecting this colder trend with
highs not rising out of the 40s on Monday.

Dewpoints drop into the 20s or even into the teens suggesting a very
low floor for nighttime lows if we are able to get a clear, calm
night. This would be less likely within the initial cold advection,
but becomes more likely by Monday night into Tuesday morning when
the surface high is more likely to nudge eastward through our
region. The probabilistic NBM now has subfreezing lows very likely
(70 to 100 percent) through our entire region. In fact, most areas
now have greater than a 50 percent chance of falling to 25F or
colder.

Guidance remains in good agreement that although this cold air mass
will be the strongest of the season so far, it also won`t last very
long. The broader upper trough heads eastward as ridging spills out
into the Plains next week. With the surface high pressure axis
crossing our area around Monday night into Tuesday morning, our warm
up begins on Tuesday. This return flow will stand a greater chance
of leading to elevated fire danger conditions considering the much
drier initial air mass and lack of substantial moisture return
during the initial stages of the warm up.

Kimble

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1038 PM CST Mon Nov 3 2025

Dry and VFR flight conditions continue through the end of the TAF
period. Winds will remain calm/light and variable overnight,
becoming southerly Tuesday morning. Gusts will develop quickly
during the late morning/early afternoon, gusting into the low 20s
(mph). This will continue into early evening before dissipating.

Ahead of an approaching cold front that will move through the
region after the current TAF period, a low-level jet will develop.
Pointed northeast into central and northeast Missouri and west-
central Illinois this will cause some gradual speed shear below
2,000 across this area. However, because the transition to the
highest speeds (40-50kts) is gradual and there is very little
directional component, I`ve held off on including a mention of
LLWS in the TAFs for now.

Delia

&&

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

$$

WFO LSX