713
FXUS63 KLSX 061119
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
619 AM CDT Mon Oct 6 2025

.KEY MESSAGES...

- There is a 20-60% chance of rain today and tonight with the
  best chance across far northeast Missouri and west-central
  Illinois behind a cold front.

- Temperatures will begin cooling to near normal Tuesday, lasting
  through at least Thursday.

&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 330 AM CDT Mon Oct 6 2025

A cold front is currently draped from central Iowa into central
Kansas, just creeping into northeast Missouri. Through the course of
the day this front will edge eastward towards the CWA, driven aloft
by a large mid-level trough riding east along the US-Canada border.
Precipitation from this front is expected to form along and behind
the front, and so is not expected to impact the area until tonight.

Showers and thunderstorms are still possible ahead of the front
across much of the forecast area, driven by diurnal instability in
an uncapped and marginally unstable environment. Coverage will be
isolated to largely scattered, and only a lucky few will see rain
today. Warm cloud depths greater than 10kft and above normal
precipitable water values will support efficient rainfall, and in
the strongest storms rainfall amounts will approach 1 inch.

At the same time, a mid-level shortwave will flow north into the mid-
Mississippi Valley along the western edge of a departing mid-level
ridge. This shortwave will produce showers and thunderstorms,
increasing in coverage as instability builds during the day. The
bulk of the precipitation has shifted southeast out of the forecast
area compared to 24 hour ago. The exact location of the rainfall
will depend on the movement of the shortwave, and a slight shift to
the northwest would bring more rain into the forecast area. The
strongest storms in this system will be able to produce up to 2-3"
of rain given the deep warm cloud depths and the nearly 2"
precipitable water values (99th percentile).

The result of all of this is at least a 20% chance of rain across
the area through most of today and cloud cover hemming the region in
from the northwest and southeast. The cloud cover and rain chances
will knock a few degrees off today`s highs as compared to
yesterday`s toasty temperatures.

The cold front will continue to progress into the region through the
afternoon and evening, bringing a chance of rain to northeast
Missouri and west-central Illinois overnight. This will be the best
chance for rain across the CWA, yet won`t last long as the low-level
frontogenesis and surface convergence weakens as the front
progresses through the area. Coverage is expected to weaken
overnight before reaching the I-70 corridor, and pass through the
remainder of the CWA dry on Tuesday. Isolated showers and
thunderstorms are possible ahead of the front across far southeast
Missouri and southern Illinois where another disturbance will push
through the Bootheel/southern Illinois region. Once again we will be
on the northwest periphery of this rain unless the track
of the disturbance shifts.

Temperatures on Tuesday will depend on the location of the front
during the morning and early afternoon, with temperatures behind the
front near normal thanks to the incoming cooler continental air.
Areas ahead of the front will warm into the mid to upper 70s under
southerly flow, though warming will be stunted by lingering cloud
cover.

Delia

&&

.LONG TERM...  (Tuesday Night through Sunday)
Issued at 330 AM CDT Mon Oct 6 2025

The mid-level trough across the northern US border will continue to
progress east during the mid-week period, allowing a mid-level ridge
to build into the region in its wake. Guidance indicates this ridge
will not be particularly strong as it builds into the region
Wednesday, and there is the potential for shortwave disturbances to
move through the flow into the mid-Mississippi Valley. While
confidence in any one disturbance being able to squeak some rain out
of the drier post-frontal air is low, I can`t rule it out. The ridge
persists into the weekend, and will dominate our sensible weather in
one shape or form.

While yesterday`s ensemble guidance was highlighting two scenarios
for the evolution of the mid-level pattern, today`s guidance is
suspiciously under-dispersed. Confidence is low in where the axis
of the ridge will form mid-week and how it will shift over the
remainder of the week and into the weekend. This results in low
confidence in the temperature forecast beyond Thursday or so. A
more eastward ridge axis will put us back in southwesterly flow,
resulting in a return to temperatures in the 80s, all things
considered. If the ridge axis stays to the west of us,
temperatures will be cooler than the past several days, though to
what degree remains uncertain.

Delia

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 557 AM CDT Mon Oct 6 2025

VFR conditions largely prevail at most terminals, except for KSUS
and KCPS where some intermittent MVFR fog has developed. The main
threat to the terminals will be a cold front approaching from the
northwest and a surge of moisture from the southeast, both leading
to lowering CIGs and increased rain chances tonight. Before that,
a low (less than 30% chance) for showers and a few thunderstorms
will threaten the terminals. That said, with no forcing mechanism
in place to focus this convection, the threat at any one terminal
is too low to mention in the TAF.

This evening as the two systems impinge on the Mid-Mississippi
Valley, CIGs will begin to fall. While periods of MVFR conditions
look likely at the St. Louis terminals and KUIN, these conditions
probably will not be widespread or long-lasting. Conditions in
central Missouri are more uncertain, as these terminals are
furthest from the low-level moisture. Beyond the TAF period,
conditions will gradually improve through Tuesday as the cold
front passes through the region.

MRB

&&

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

$$

WFO LSX