070
FXUS63 KLSX 250950
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
450 AM CDT Sat Apr 25 2026

.KEY MESSAGES...

- Although there is a low threat of thunderstorms with marginally
  severe hail Sunday night/Monday morning, there is a much
  greater severe threat of all hazards Monday afternoon and
  evening.

- Above average temperatures will give way to cooler temperatures
  behind a cold front Monday night.


&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 332 AM CDT Sat Apr 25 2026

Friday`s cold front will continue to wash out near southeastern MO
and southwestern IL early this morning, which could support patchy
fog with light/calm winds and pooled BL moisture as upper-level
clouds clear. A period of dry and tranquil conditions is expected
into Sunday as an upper-level split flow regime around a ridge
across the Great Lakes/Ohio Valley leads to a dearth of large-scale
forcing. At low levels, easterly to southeasterly flow around will
facilitate gradual WAA today and Sunday. However, since there will
be increasing clouds on Sunday, high temperatures will be similar
both days and in the mid to upper 70s F--still above average for
late April.

Pfahler

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 332 AM CDT Sat Apr 25 2026

Late Sunday, an upper-level trough will begin to cross the Rocky
Mountains, facilitating surface cyclogenesis in the lee of the Rocky
Mountains with a warm front lifting northeastward into western and
southwestern MO overnight. This evolution will permit a round of
potentially severe thunderstorms across KS and western MO Sunday
evening, but as these thunderstorms continue eastward into the CWA
sometime late Sunday night or Monday morning they are expected to be
weakening and increasingly elevated as they cross the warm front and
become detached from richer moisture and surface-based instability
in the warm sector. It is not completely clear what the convective
mode of these thunderstorms will be (linear versus discrete), but if
thunderstorms are discrete, a threat of marginally severe hail is
favored as deterministic MUCAPE projections increase to between 1000
to 2000 J/kg from west to east through the night at the leading edge
of an EML plume.

On Monday, the upper-level trough will become negatively tilted
while continuing northeastward across the Mid-Mississippi River
Valley and Midwest, fostering a deepening surface cyclone tracking
near northwestern MO into IA which favorably places the CWA in the
warm sector as the warm front progresses northeastward. In this warm
sector, rich Gulf of Mexico moisture will be in place with dewpoints
in the mid to upper 60s F, exceeding the 90th percentile of
climatology. There are lingering concerns for the morning showers,
thunderstorms, and associated clouds to impact instability, but the
10th percentile of SBCAPE in ensemble model guidance has increased
(now >/= 1000 J/kg) in recent runs. This trend increases confidence
that instability will be able to recover during the afternoon even
if that is the case in part due to an incoming EML. Although this
EML will induce a capping inversion, the inversion should erode
through the afternoon and evening as the trough lifts/cools the
layer, inevitably allowing initiation and increasing coverage of
thunderstorms with time. There are several potential locations for
initiation including an arriving Pacific cold front/dryline and any
remnant outflow boundaries. Strong deep-layer shear of 45 to 55 kt
will favor supercells, at least initially, but an approximately 45
degree orientation with the front/dryline and strong large-scale
forcing suggests mergers and upscale growth into broken lines are
possible. Therefore, thunderstorms will be capable of all severe
hazards, including tornadoes with strong low-level wind shear and
hodograph curvature. If instability verifies near the ensemble model
75/90th percentile of SBCAPE (2500 to 3500 J/kg), the resulting
parameter space would be supportive of a more significant severe
weather event, including very large hail and a strong tornado or two
if a supercellular thunderstorm mode also remains dominant. The
details of Monday`s severe threat will come into better focus over
the coming days, including the exact timing, locally greater threat
areas in the CWA, and dominant severe hazards and their magnitudes.

A secondary cold front will pass Monday night, marking the onset of
prolonged, weak northerly, low-level CAA that persists through
Wednesday with high temperatures cooling closer to average on
Tuesday and even slightly below average on Wednesday. In the wake of
Monday`s trough, upper-level flow will become quasi-zonal through
mid-week. There is a signal in global model guidance for another
trough to navigate this flow and provide another opportunity for
showers and a few thunderstorms late Tuesday, depending on how far
south the cold front settles. Currently, the highest probabilities
of measurable rainfall are 50 to 70 percent along/south of I-44 in
MO and I-70 in IL Tuesday evening and overnight. Thereafter, there
is a loose consensus on a cold front to pass in the Thursday-early
Friday timeframe, leading to another round of lower measurable
rainfall probabilities (20 to 30 percent) given that moisture will
be limited. Instead, the front will likely have a greater impact on
temperatures cooling further below average.

Pfahler

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 450 AM CDT Sat Apr 25 2026

Patchy river valley fog will linger through sunrise this morning,
primarily at KSUS. Otherwise, dry and VFR flight conditions will
persist through Sunday morning with easterly winds.

Pfahler

&&

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

$$

WFO LSX