534
FXUS63 KLSX 071745
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1245 PM CDT Sat Jun 7 2025

.KEY MESSAGES...

- Areas of dense fog will continue before improving within a few
  hours after sunrise.

- Scattered thunderstorms are expected this afternoon and early
  evening. A few of these storms may be strong to severe with
  damaging winds and an isolated, weak tornado possible.

- A cold front will move through the area late Sunday
  afternoon/evening providing a focus for additional showers and
  thunderstorms. Some of these storms may be strong to severe
  with damaging winds and large hail the primary threats.


&&

.SHORT TERM... (Through Sunday Night)
Issued at 327 AM CDT Sat Jun 7 2025

(Through Tonight)

GOES-16 nighttime microphysics RGB satellite imagery shows a mixture
of dense fog and low stratus across much of the area. The dense fog
is currently most prevalent in parts of central and east-central
Missouri, but has been expanding over the past couple of hours.
This expansion will likely continue leading up to sunrise.
Yesterday`s rain and light/variable winds along with any clearing
is a near-perfect recipe for dense fog. Gradual improvement in
visibilities is expected after sunrise, with the dense fog
advisory due to expire at 1400 UTC.

Meanwhile, widespread convection is ongoing across northern
Oklahoma, southeastern Kansas, and far southwest Missouri very
early this morning. It is a bit of deja vu as convection is nearly
in the same exact area as 24 hours ago. The brunt of these storms
should move eastward and stay just south of the CWA, but
widespread showers along with some embedded thunderstorms are
forecast to clip parts of southeast Missouri later this morning. I
cannot completely rule out a strong thunderstorm or two, but
timing is not ideal and short-term guidance shows little
instability (~500 J/kg) and modest deep-layer shear (~30 knots).

Of more interest for later today is a midlevel shortwave trough
currently evident in GOES-16 water vapor imagery in central
Nebraska. This feature is expected to move eastward across the
Missouri-Iowa border into north-central Illinois by early this
evening. This feature along with moderate low-level moisture
convergence should yield widespread showers and scattered
thunderstorms roughly along/north of the I-70 corridor this
afternoon and evening. Deep-layer shear is pretty impressive ahead
of the midlevel trough, generally on the order of 40-50 knots.
The main limiting factor will be instability. The combination of
some stratus (from lifting morning fog) and thick mid/upper level
convective debris clouds should limit heating/diurnal instability
today. Probabilities from the HREF for 1000+ J/kg of SBCAPE at
2100 UTC are above 50% in a very small area in central and east-
central Missouri for a couple of hours. If there are more breaks
in the cloud cover however, the instability will be higher which
would increase the severe threat. The most likely scenario is for
instability to be limited enough to keep convection largely
unorganized and on the weaker side. A few strong to severe
thunderstorms are on the table however given the very strong
shear. Damaging winds would be the main threat, though cannot rule
out a brief, weak tornado or two near the track of a weak surface
low. The environmental 0-1 km shear/helicity is pretty weak, but
sometimes surface lows bring just enough of their own vorticity to
induce tornadogenesis. In addition, LCL values will be on the low
side.

Any threat for strong to severe thunderstorms should cease by mid-
late evening as instability lessens and forcing diminishes behind
the departing midlevel shortwave trough. Most of the night will be
dry with some decrease in cloudiness. Fog will be another concern
tonight given that at least some areas will receive another round
of widespread rainfall. Any areas that manage to see partial
clearing and also are on the receiving end of today`s rainfall
will be of particular concern. Exactly where this ends up being
and how widespread is unknown right now, but this is something to
monitor going forward.


(Sunday - Sunday Night)

A cold front is forecast to move through the bi-state region late
Sunday afternoon/evening, providing a focus for additional
thunderstorms. The combination of weak mid/upper level height falls
(cooling aloft) and weak convergence along the frontal boundary
should help overcome some weak capping for the development of at
least scattered convection along the boundary. Instability
along/ahead of this boundary is forecast to be higher Sunday
afternoon, with probabilities on the HREF of at least 1500 J/kg of
50-70% across parts of central Missouri Sunday afternoon. The mid-
Mississippi Valley will be on the southeastern periphery of the
stronger deep-layer shear, with deterministic guidance showing
anywhere from 30-35+ knots along the boundary. The convective mode
should be a mixture of multicellular clusters and transient
supercells. Damaging winds and large hail are the primary threats
given deep mixing and dry midlevels (high DCAPE) as well as plenty
of hail CAPE. Any tornado threat looks minimal at best given much
higher LCL values (especially compared to this afternoon) and
very weak 0-1 km environmental shear/helicity. In addition, any
right-moving supercells would tend to move off the initiating
boundary. These may tend to weaken with time as they move into a
lower shear environment. In addition, sustained motion along the
boundary (and a source of surface vorticity) likely would be
needed to produce a tornado and right movers as alluded to above
would tend to move off of the boundary (and a source of surface
vorticity).

Chances of showers and thunderstorms will gradually diminish from
northwest to southeast Sunday night as the cold front continues to
push into the mid south. Increasing dry and cool air advection comes
in its wake, leading to a mostly dry forecast overnight. Lows
ranging from the mid 50s to low 60s are forecast, with the coolest
locations in parts of northeast Missouri.


Gosselin

&&

.LONG TERM... (Monday through Friday)
Issued at 327 AM CDT Sat Jun 7 2025

(Monday - Wednesday Night)

All signs continue to point toward a quieter and mostly dry pattern
for early/mid next week. Sunday`s boundary is expected to stall out
across the mid south on Monday, but should be far enough south to
preclude any showers or thunderstorm activity across our region.
However, if the front stalls further to the north, parts of
southeast Missouri may get clipped by some of this activity. A
secondary cold front is forecast to move through the area on
Monday, but limited moisture and very weak surface convergence
should lead to a dry frontal passage.

Slightly cooler than normal temperatures are also forecast for
early/mid week, with highs mainly in the upper 70s to low 80s and
lows in the 50s.


(Thursday - Friday)

Ensemble guidance depicts a weak midlevel shortwave trough moving
slowly out of the south-central Plains late this week. This feature
is subtle, and completely cutoff from the mean flow. Therefore, it
may take its time impacting our region. Chances of showers and
thunderstorms do increase late in the period, with the best chances
(60-70%) coming on Friday. Very weak flow aloft and likely modest
instability suggests little/no threat for organized strong/severe
convection.

Temperatures are also expected to climb back closer to normal late
next week. Uncertainty for highs on Friday increases due to the
likelihood of thicker cloud cover and chances of thunderstorms.
Any persistent shower/thunderstorm activity would help limit the
temperature rise Friday afternoon, and possibly keep readings from
reaching the 80 degree mark in some locations.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1239 PM CDT Sat Jun 7 2025

A storm system is approaching the region, sliding through this
afternoon and early evening. The greatest chance for thunderstorms
is at KCOU and KJEF, though occasional lightning strikes are
possible at the other local terminals as the system moves through
the area. The rainfall will increase low-level moisture that will
likely lead to another round of fog tonight, with the best chances
at KCOU and KJEF. Impacts are possible at other local terminals
due to fog, but confidence is low in that occurring right now.
Conditions will improve tomorrow morning after sunrise, though
another round of showers and thunderstorms are possible tomorrow
afternoon.

Elmore

&&

.CLIMATE...
Issued at 227 PM CDT Fri Jun 6 2025

St Louis has still not seen 90 degrees yet in 2025. The average
first 90 degree reading is May 20. The most recent late 90-degree
start was June 10, 2021. If St Louis doesn`t hit 90 by June 10,
it will be the latest first 90 degree day since at least 1995
(June 19). The latest date of first 90 degrees on record was July
4 in both 1961 and 1912.

Columbia has also not yet seen a 90 degree reading in 2025, however
the average first date is much later (May 31). The latest date of
first 90 degrees on record was July 14, 1904.

Quincy reached 90 degrees on May 15 of this year, about 3 weeks
earlier than their average first 90 degrees (June 2).

Kimble


&&

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

$$

WFO LSX