254
FXUS63 KLSX 041741
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1141 AM CST Sun Jan 4 2026

.KEY MESSAGES...

- Well-above average temperatures and dry conditions are expected
  Monday through at least Wednesday.

- The next opportunity for substantial rainfall will be in the
  early Thursday morning-through-Friday timeframe, but exact
  timing and amounts are still uncertain.


&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 248 AM CST Sun Jan 4 2026

A surface high pressure center near the CWA and only thin passing
upper-level clouds has allowed rather efficient radiational cooling
of temperatures into the low to mid-20s F. However, the southern
edge of stratus is also sneaking into parts of west-central/south-
central IL, temporarily interrupting cooling beneath it. The high
pressure center will shift to the east by this afternoon, allowing
low-level WAA to ensue. As a result, high temperatures will warm
into the upper 30s to upper-40s F today, but a wave of thick upper-
level clouds accompanying a passing shortwave trough will preclude
temperatures warming further.

Low-level south-southwesterly flow and WAA will intensify tonight as
upper-level flow becomes more zonal and the first in a series of
shortwave troughs pass to the north of the Mid-Mississippi River
Valley. Some low-stratus or patchy fog is also possible across
central and northeastern MO as low-level moisture advection as
increases. This pattern altogether will favor much warmer high
temperatures on Monday in the mid-50s to low-60s F.

Pfahler

&&

.LONG TERM...  (Monday Night through Saturday)
Issued at 248 AM CST Sun Jan 4 2026

Quasi-zonal upper-level flow and passing shortwave troughs just to
the north of the region will remain at large through Wednesday with
multiple bouts of low-level WAA and a round of much weaker CAA
behind a weak cold front on Tuesday. The net result will be 850-hPa
temperatures wavering above the 90th climatological percentile with
the NBM interquartile ranges (IQRs) of high temperatures 5 F or less
and in the mid-50s to mid-60s F (warmest south, coolest north).
Since moisture and large-scale ascent is not progged to be very
deep, over 90 percent of ensemble model membership has entirely dry
conditions prevailing through Wednesday.

The CONUS upper-level flow pattern will undergo amplification late
Wednesday through Thursday as longwave troughing takes place over
the Intermountain West. In deep, downstream southwesterly flow,
multiple shortwave troughs will be ejected to the Mid-Mississippi
River Valley Thursday through Friday providing the next opportunity
for substantial rainfall, but model guidance has varied on the
timing and amplitude of these features along with how they
interact/phase with one another. The track and strength of the
surface reflection is uncertain at this point, ranging from a
stronger surface low tracking across western and northern MO to a
much weaker low tracking near the MO Bootheel, as indicated by the
latest 500-hPa height cluster analyses. These characteristics will
influence the timing and amount of rainfall and if there is any
instability for thunderstorms available. Considering all possible
scenarios, NBM probabilities of total rainfall exceeding 0.5" is 40
to 60 percent across the southeastern half of the CWA sometime in
the Thursday morning-through-Friday timeframe. Accordingly, NBM
temperature IQRs also expand to nearly 15 F by Friday, essentially
ranging from it being the warmest day of the workweek to the coolest.

Another point of variability in global model guidance is how quickly
longwave troughing advances eastward into the Mississippi River
Valley over the next weekend, impacting if precipitation lingers
into Saturday morning and how soon stronger low-level CAA and
northwesterly flow arrives. NBM temperature distributions remain
spread over the weekend, but advertise temperatures cooling toward
average.

Pfahler

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1140 AM CST Sun Jan 4 2026

VFR flight conditions are expected through the period. Light south
to southeast flow will turn to the south-southwest Monday morning.

Carney

&&

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

$$

WFO LSX