920
FXUS63 KLSX 071105
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
505 AM CST Wed Jan 7 2026

.KEY MESSAGES...

- Widespread showers are expected on Thursday, with the greatest
  chance (80%) for 0.5" or more of rainfall being confined to
  northeastern Missouri.

- Cooler temperatures around seasonable normals are expected this
  weekend.


&&

.SHORT TERM... (Through Thursday Night)
Issued at 256 AM CST Wed Jan 7 2026

Upper-level ridging is building back into the Midwest in the wake of
yesterday`s shortwave, with surface riding nosing into the Middle
Mississippi Valley. Ample low-level moisture converging around a
surface low over the Great Lakes has a sizable portion of the
Midwest beneath low stratus and fog. Some of this fog has formed
across portions of northeastern Missouri, west-central Illinois, and
south-central Illinois. Most observations have been reporting patchy
fog, but KUIN has been around 1/4 to 1/2 mile through much of the
early morning. With temperatures in this area just below freezing,
deposition will produce isolated slick spots, especially on elevated
surfaces. A lack of cloud cover and increasingly southerly winds
will cause this fog to dissipate shortly after sunrise.

Through the day, an upper-level trough digging into the
Intermountain West will begin kicking a cutoff over the California
Baja toward the Southern Plains. Deep southerly to southwesterly
flow ahead of the cutoff and over the Middle Mississippi Valley will
advect warm air into the region and continue our trend of well above
normal temperatures today.

Through tonight into tomorrow morning, the cutoff will becoming
increasingly absorbed into southwesterly flow as it enters the
Plains ahead of the trough. This will correspond with a surface low
deepening over the Central Plains and moving northeastward through
the day Thursday. Robust warm air advection ahead of the low will
feed widespread rainfall across the Midwest, though recent guidance
trends have shifted the axis of heaviest rainfall just north of the
CWA. Ensemble-based probabilities now have the greatest chance for
0.5" of rainfall (80%) focused across northeastern Missouri, with
the rest of the CWA having a 40-60% chance of seeing such
accumulations.

The northerly track of the low will allow for the warm sector to
encapsulate much of the CWA on Thursday. A minority of CAMs produce
scattered to widespread robust showers along a prefrontal trough
within the warm sector during the afternoon and evening. Shear will
be ample, with low-level hodographs showing more than enough
curvature to produce rotating updrafts. However, much uncertainty
still abounds in regards to instability and lift. Much of the upper-
level lift via the cutoff/shortwave will be quickly moving off to
the northeast in the late afternoon and evening, with the prefrontal
trough outpacing the already low instability. What instability
present will be confined to the lowest levels of the atmosphere,
leading to very shallow updrafts that will unlikely be able to even
produce lightning. The 00z HREF reaches out to 00z Friday, when the
prefrontal trough will be roughly over the St. Louis metro. At that
lead time, the guidance has a 15% probability of 100 J/kg of SBCAPE.
This may be artificially too low, as not the entire HREF membership
reaches out to that lead time, but it sill highlights the
uncertainty behind the forecasted instability. If even 200-300 J/kg
of SBCAPE can be realized, there will be enough stretching potential
for a low tornado threat in the strongest showers - mainly south of
I-70. Given the conditionality of this threat, we will continue to
not publicly message the SPC Day 2 Marginal.

Elmore

&&

.LONG TERM... (Friday through Tuesday)
Issued at 256 AM CST Wed Jan 7 2026

Friday morning, guidance consensus is that the shortwave and surface
low responsible for Thursday`s weather will be quickly moving
through the Great Lakes. The attendant cold front`s positioning
still varies slightly among guidance, with some showing it just
eastward of the CWA and others show it still moving through the CWA.
The former solution supports drier conditions to start Friday, while
the latter supports lingering low rain chances into the afternoon. A
majority of guidance shows a brief dry period ahead of the trough
axis approaching the region. This will provide lift among lingering
low-level moisture to force light precipitation Friday night and
Saturday morning (50% chance). Guidance consensus is that this light
precipitation would start as rain before transitioning to snow
thanks to a majority of the atmosphere dropping to or below
freezing. However, deterministic soundings vary in the degree of
saturation in the dendritic growth zone, so confidence is low in
when the transition would take place or if it would stay a rain/snow
mix. Even with a quicker transition to snow, surface temperatures
right at freezing and warm ground temperatures are expected to limit
accumulation and impacts.

Ensemble clusters diverge notably on Sunday in how quickly the
trough shifts eastward, with faster solutions showing upper-level
ridging building into the Middle Mississippi Valley before the day
is done, while slower solutions show the trough`s axis being
directly over or just eastward of the region. Despite this spread in
solutions, the spread in low-level temperatures is minor (2-3
degrees C), leading to low spread in surface temperatures and high
confidence in Sunday being the coolest day of the period as highs
top out generally in the mid 30s.

While guidance consensus is that at least weak northwesterly flow
persists over the Middle Mississippi Valley on Monday, the main
thrust of the trough and post-frontal air mass shifts eastward,
allowing low-level temperatures to moderate and surface temperatures
to return to above normal through Tuesday. A series of shortwaves
digging into the Midwest Monday and Tuesday will deepen the upper-
level trough over the eastern CONUS and push temperatures cooler
starting mid next week.

Elmore

&&

.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 459 AM CST Wed Jan 7 2026

An expansive area of fog is blanketing the region, with KUIN
having been impacted through the early morning hours. With
southerly winds gradually ramping up through the morning and
temperatures warming, this fog is expected to dissipate here
within the first few hours of the TAF period. After fog clears the
terminal, VFR flight conditions are expected at the local
terminals through at least this evening.

Elmore

&&

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

$$

WFO LSX