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FXUS63 KLSX 121112
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
612 AM CDT Wed Mar 12 2025

.KEY MESSAGES...

-There is an enhanced risk for severe thunderstorms Friday afternoon
and evening. The main threat will be damaging winds and tornadoes
the main threats.

-Weekend temperatures will cool behind Friday`s system with
 Sunday`s highs 20-30 degrees cooler than Friday.


&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 307 AM CDT Wed Mar 12 2025

Tuesday`s surface front remains stalled along the I-70 corridor in
Missouri and the I-64 corridor in Illinois. To the north of the
front winds are largely east to northeasterly, which will keep this
area slightly cooler today than areas south of the boundary. Highs
north of the front will be in the low to mid 70s, but south of the
front, southwesterly flow will bring another day of high
temperatures in the upper 70s to low 80s.

By Thursday the front will weaken and flow will become southeasterly
across the region. This flow regime is not as favorable for the
robust warming we`ve seen in recent days. However a warm start and
abundant sunshine will help temperatures peak in the mid 70s to low
80s area wide. During the day a mid-level trough, the focus of
Friday`s storm system, will come onshore over southern California
and push eastward into the Four Corners Region.

Delia

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 307 AM CDT Wed Mar 12 2025

By Friday morning the mid-level trough will be scooting across the
Southern Plains, becoming negatively tilted as it does. A strong
surface low will develop in response. As it strengthens and shifts
eastward the surface pressure gradient will become increasingly
tight and coupled with deep mixing will result in strong winds at
the surface. There is a 50-80% chance for sustained winds above 20
mph and gusts above 35 mph. These winds will persist through Friday
and Saturday as this system pushes across the mid-Mississippi Valley.

The mid-level trough will begin to swing northeast through the
forecast area during the afternoon and evening while at the surface,
the first cold front will bring widespread showers and thunderstorms
to the region. Recent model trends have shown a faster progression
of the front, leading to higher instability across the forecast area
within at least the deterministic models. While the entire ensemble
envelope depicts that at least 500 J/kg of MUCAPE will be available
during the afternoon and evening, there is still a great deal of
spread with SBCAPE (200-1300 J/kg) and potential for much higher
MUCAPE than mentioned above. The strong dynamics with this system,
including a 50-60 kt LLJ and strong upper level jet, will spawn
convection during the afternoon and early evening which will quickly
organize into a QLCS. Despite the potential for relatively weaker
instability, the strong synoptic forcing is expected to balance the
QLCS under the 60+ kts of 0-6 km shear. Strong low-level winds and
the development of a QLCS mean that damaging winds, some
significant, will be the main threat with this system, though QLCS
tornadoes are also a concern. If instability on the upper end of the
guidance spread develops, more significant and widespread impacts
will be expected.

Thunderstorms are expected to exit the forecast area overnight and
into the early morning hours. Saturday will be 10-20F cooler than
Friday thanks to increased cloud cover and westerly surface flow. An
additional cold front will move through the area late Saturday,
driven aloft by a second, weaker mid-level shortwave that will
follow on the heels of Friday`s system. This front will bring a
surge of cold air advection into the region and result in high
temperatures on Sunday an additional 10-20F cooler than Saturday.
This cool down won`t last, a mid-level ridge and southwesterly
surface flow will return to the central CONUS by Monday and warm air
advection will resume. This will key the region up for the next
storm system that could move through the region during the second
half of next week.

Delia

&&

.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 610 AM CDT Wed Mar 12 2025

Dry and VFR flight conditions will continue through the end of the
period. A stationary front is currently stalled between KCOU/KJEF
and KSTL/KSUS/KCPS. The waivering of the front through the day
will cause variable surface winds, however speeds will remain
around 5 kts or less. North of the boundary winds will prevail
from the east, south of the boundary they will prevail from the
southwest to southeast.

Delia

&&

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

$$

WFO LSX