Area Forecast Discussion

841
FXUS63 KLSX 081120
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
520 AM CST Thu Jan 8 2026

.KEY MESSAGES...

- Periods of widespread showers are expected through the day. A
  shower or two may become strong enough to produce lightning, a
  brief tornado, and/or damaging wind gusts.

- More seasonable temperatures are expected this weekend ahead of
  a warming trend early next week.


&&

.SHORT TERM... (Through Friday Night)
Issued at 248 AM CST Thu Jan 8 2026

A shortwave can be seen in water vapor imagery over the Desert
Southwest moving toward the Plains ahead of an upper-level trough
digging into the Intermountain West. These features have flow
becoming increasingly south-southwesterly over the Middle
Mississippi Valley, with the thickening of mid to upper-level cloud
cover indicative of deepening moisture.

As a surface low deepens over the Central Plains early today, warm,
moist air will continue to surge into the region. Isentropic ascent
ahead of the low will yield a wing of showers that moves southwest
to northeast across the CWA through the late morning into the
afternoon. At that point, a semi-organized band of showers will
materialize over central and northeastern Missouri along a pre-
frontal trough. Initially, surface-based instability will be very
limited if non-existent. However, guidance has been converging on a
solution that brings 250-500 J/kg of SBCAPE among the pre-frontal
trough during the mid to late afternoon. Deep-layer shear will be
ample (50-60kts of 0-6km), with long, curved low-level hodographs.
This environment will allow for more robust updrafts to rotate,
leading to a threat of brief, weak tornadoes and an isolated
instance of damaging wind gusts. The limiting factor to this severe
threat is the thermal profile. Poor mid-level lapse rates noted in
several soundings may limit convective depth to the point that
updrafts fail to adequately stretch low-level vorticity for
tornadogenesis. Some of the higher SBCAPE/MLCAPE values noted in
guidance are taking instability above the mid-level warm nose into
account in addition to beneath this layer. This may be causing the
local environment to look more ideal for severe weather than in
reality. However, given that guidance consensus has been trending
toward more instability and a relatively increasingly favorable
thermal profile, our confidence is high enough to publicly message
the SPC Day 1 Marginal across the area.

In the wake of the pre-frontal trough, the local environment remains
marginally unstable ahead of the cold front slowly sinking
southeastward through the CWA. However, shear will decrease slightly
as the core of the low-level jet shifts eastward, convergence will
be weaker, and the departing shortwave will reduce lift. Therefore, a
lower chance (20-40%) for showers is expected along the front
tonight, and confidence is very low that these will be strong to
severe.

Even though the front clears the CWA to the southeast Friday
morning, flow through the mid to upper levels of the atmosphere will
still be southwesterly. This will limit cold air advection, with
temperatures on Friday running cooler than as of late, but still
well above seasonal normals. The approaching trough axis will
utilize what limited moisture remains over the area to spawn light
rainfall (40% chance) starting after midnight.

Elmore

&&

.LONG TERM... (Saturday through Wednesday)
Issued at 248 AM CST Thu Jan 8 2026

Guidance consensus is that at daybreak on Saturday, the axis of the
upper-level trough will be just east of the CWA, placing the optimal
associated lift directly over the CWA. This will keep the chance (30-
40%) for light precipitation going through the morning hours, ending
my mid-day. As temperatures bottom out near freezing across portions
of northeastern Missouri and west-central Illinois early in the
morning, light snow may mix in with the rain. Given air temperatures
right around freezing and warm ground temperatures, accumulation and
impacts are not anticipated. Once precipitation ends, the main focus
will be temperatures. As discussed prior, the post-frontal air mass
will lack punch initially due to the slow nature of the trough and
delayed onset of northwesterly winds, with temperatures on Saturday
being relatively cooler albeit still warmer than normal. However, a
potent shortwave digging equatorward along the backside of the
trough through the day will send a reinforcing shot of cold air into
the Middle Mississippi Valley, pushing temperatures to seasonal
normals Saturday night through Sunday night.

Confidence is high that this round of cold will be short-lived, as
all ensemble clusters and deterministic guidance have the trough
quickly departing the region eastward causing northwesterly winds to
slacken over the Midwest on Monday. This will allow for high
pressure at the surface to nose into the Middle Mississippi Valley
and temperatures to rise once again above normal through Tuesday. A
series of shortwaves within the northwesterly flow aloft will pass
through the Midwest early in the week, reinforcing the trough and
strengthening northwesterly flow over the CWA. Temperatures as a
result will cool Wednesday onward per ensemble means, though large
spread within the IQR of global ensemble guidance leads to low
confidence in by how much.

Elmore

&&

.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 502 AM CST Thu Jan 8 2026

A weather system will bring multiple rounds of rain and impacts
to the local terminals through the TAF period. The first round
will occur this morning into the early afternoon as a band of
light rain moves across the area. This first round will increase
low- level moisture, aiding in dropping ceilings into MVFR
territory.

The second round of rain will be this afternoon and early evening
as a band of showers moves eastward across the area. There is a
low chance of embedded thunderstorms within these showers, and if
this occurred, KSUS, KSTL, and KCPS would be the most likely sites
for impacts. This may include sporadic and gusty winds greater
than what is shown in the TAFs. These storms will also be capable
of visibilities lower than what is currently forecast. After this
round, ceilings will briefly improve as drier low-level air moves
in ahead of a cold front.

This cold front will slowly move across the terminals tonight and
has the greatest chance of producing additional rainfall at KUIN,
KSUS, KSTL, and KCPS. As the front approaches, ceilings will lower
again, with widespread MVFR expected. While IFR ceilings are only
forecast at KUIN, there is a low chance that these conditions
could be realized late in the period at the other local terminals.
Flight conditions wont improve and winds won`t die down until
beyond the current TAF period.

Elmore

&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion