857
FXUS63 KLSX 311745
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1245 PM CDT Sat Oct 31 2026

.KEY MESSAGES...

- A mild, largely-dry Halloween is in store with intermittent
  sprinkles/light rain being the only sensible weather.

- A more potent system arrives late Monday and brings the
  likelihood of a widespread, beneficial rainfall through at
  least Thursday. Hazardous weather associated with this system
  is very unlikely (less than 10%).

- Temperatures begin a gradual cooling trend today, but remain
  at least near early November normals.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 308 AM CDT Sat Oct 31 2026

Widespread mid-level cloud cover blankets the Mid-Mississippi
Valley this morning, forced by diffluence and very meager
isentropic ascent in advance of a dampened shortwave across the
central Plains. Any precipitation of note is far to the north of
our region along a wing of warm air advection across eastern
Iowa and northern Illinois. The combination of the weak
isentropic ascent and meager forcing boost from the approaching
trough appear sufficient to threaten some sprinkles or light
rain areawide today. Any drops that fall will be few, far-
between, and unlikely to be measurable or cause problems for
outdoor activities today. There is a reasonable (60%) chance
among high-resolution guidance that the threat for this light
rain ends by mid-afternoon, leaving trick-or-treaters with a
mild, dry Halloween night.

The main thrust of the weak mid-level wave arrives well-after
midnight, increasing rain chances mainly south of I-70 through
Sunday morning. The eastward progress of this shortwave looks slower
compared to 24 hours ago, which resulted in some low (20-30%) rain
chances lingering into Sunday afternoon. Otherwise, expect
temperatures to be noticeably cooler (by up to 10 degrees)
Sunday compared to today.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Issued at 308 AM CDT Sat Oct 31 2026

Weak ridging aloft builds into the region as the shortwave departs
late Sunday, leading to a dry and seasonable start to the next work
week. Meanwhile, a much more potent trough spanning much of the
western CONUS will track downstream and approach the Mississippi
Valley. The deep southerly flow that results from such an
amplified pattern will allow near-record moisture to transport
poleward along a cold front to force widespread showers and a
few thunderstorms. While ensemble members and deterministic
guidance agree on the broad strokes of how this system will
evolve as it tracks east, a few key details are less certain.
Regardless of these differences, the system is forecast to bring
a widespread, beneficial rain to the entire region early
Tuesday through Thursday. Exact values will vary by location,
and hinge to some extent on the less-certain aspects of the
wave`s evolution, but NBM and LREF probabilities of 1.00 inches
are now in the 50-80% range (highest across central and
northeast Missouri). While lower elsewhere, appreciable rainfall
is still forecast.

The most notable difference evident in the ensembles, according to
EOF patterns of the individual members, is the amplitude and
structure of this trough. How it evolves and interacts with a
reinforcing shortwave amidst the polar jet stream will affect the
track and strength of any surface features. Surface low tracks vary
in the guidance from South Dakota to Kansas by Tuesday morning,
impacting the system`s speed and available instability. As of now,
most guidance either 1) keeps a single amplified wave that
suppresses instability too far south or 2) shears off a second wave
from the base of the main trough that causes additional surface
cyclogenesis to pinch off instability. Either way, despite ample
deep-layer shear, severe thunderstorm chances largely evade the Mid-
Mississippi Valley. While model soundings vary a bit in how
saturated the column becomes, and for how long it stays saturated,
the factor that may inhibit heavy rain more is the lack of
instability that was already discussed. While there is still time
for shifts in the trough structure/surface low track to favor a
particular hazard, currently there is little to suggest this system
brings anything other than needed rain.

A growing number of ensemble solutions suggest the wave closes as it
tracks overhead, making it more sluggish and difficult to advance
downstream. As a result, rain chances will linger to some extent
through much of the upcoming week. Exactly when rain chances drop
off, and how much of a break we get before another system, is very
uncertain with the upper-level patten depicted in wildly-different
ways beyond Wednesday. The persistent rain chances and cloud cover,
coupled with at least some more polar-type airmasses approaching,
allow us to continue our trend cooler as we close the first week of
November. Still, with normal temperatures also cooling, we likely
won`t see temperatures too out of the ordinary for this time of year.

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1240 PM CDT Sat Oct 31 2026

VFR flight conditions with occasional sprinkles or even a few
areas of light rain will prevail this afternoon into the
evening. Rain is expected to become more persistent and
widespread this evening, and ceilings/visibilities will fall to
MVFR from south to north tonight. Areas of IFR look likely by
early Sunday morning across the eastern Ozarks, and possibly as
far north as the I-70 corridor, however confidence is not high
enough to include IFR in STL Metro or central Missouri TAFs at
this time.

&&

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

&&

$$

SHORT TERM...MRB
LONG TERM...MRB
AVIATION...Carney