Area Forecast Discussion
Issued by NWS St. Louis, MO

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

000
FXUS63 KLSX 121913
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
213 PM CDT Tue Mar 12 2024

.KEY MESSAGES...

-There is a 40-50% chance of showers and thunderstorms Wednesday
night into Thursday. A few storms could become strong to severe and
produce large hail.

-Showers and thunderstorms will move through the area Thursday
 night into Friday, several could be strong to severe and will be
 capable of all hazards (large hail, damaging winds, and
 tornadoes).


&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 212 PM CDT Tue Mar 12 2024

Conditions have warmed well above average once again today under a
mostly clear sky and strong southwesterly flow. A mid-level
shortwave will slide through the quasi-zonal mid-level flow into the
forecast area this evening, and combined with WAA and the LLJ, will
produce showers and a few thunderstorms across portions of the
forecast area. Instability is somewhat limited tonight. The best
probabilities for MUCAPE greater than 250 J/kg are west of our
forecast area (70%+ chance via the SREF), but the SREF shows a 50%
chance of >250 J/kg MUCAPE along and west of the Mississippi River.
While a few thunderstorms are possible across the forecast area
tonight, strong to severe thunderstorms are not expected. The
progressive shortwave and veering of the LLJ will push precipitation
out of the area before sunrise.

A mid-level longwave trough will come onshore in the western CONUS
late Tuesday and deepen into Wednesday. By mid-day Wednesday the
trough will expand across the western CONUS into the Central Plains
as it slowly slides east. At the surface a low will bloom across the
Central Plains and move eastward into Kansas, and continued
southwesterly flow will advect the warm front northward through the
forecast area Wednesday evening. Another round of showers and
thunderstorms will slide across the forecast area Thursday night as
the LLJ ramps up and interacts with a warm front that will lift
north during the evening. The chance for showers and thunderstorms
will be limited to the evening hours before the warm front lifts
north of the forecast area. More bountiful instability (70%+ chance
>500 J/kg MUCAPE via the SREF), 30-50 kts of 0-6 km shear, and 7-8
degree lapse rates would support strong to a few severe
thunderstorms with hail as the main hazard. Areas closer to the
surface low will have the better chance of seeing severe
thunderstorms.

MRM

&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 212 PM CDT Tue Mar 12 2024

Focus then turns to Thursday afternoon and overnight as a
subtle shortwave slides across the region at the same time the
surface low moves northeast of the area. WAA in the vicinity of the
low and warm front north of the area has a chance (20-50%) of
producing showers and thunderstorms Thursday morning, mainly across
northeast MO and west-central IL. The best chance (60-70%) for
showers and thunderstorms will be along the cold front as it moves
through, Thursday evening into the overnight hours. The best
location for precipitation will be across southeast Missouri and
southern Illinois where moisture return will be unaffected by
downslope drying off the Ozark Plateau.

Southwesterly flow will continue to advect warm, moist air into the
region and help with destabilization. Some uncertainty remains in
how unstable the region will become, particularly across central MO
where the shadow of the Ozark Plateau can produce a local drying
effect. Deterministic and SREF ensemble guidance indicate 1000-2000
J/kg of MUCAPE will be available across the area during the
afternoon and evening, though lower values are expected in central
Missouri from the influence of the Ozarks. A weak cap will be in
place, but convergence along the front, coupled with the ramp up of
the LLJ in the evening, will be enough to generate showers and
thunderstorms. Where convection is able to form it will be able to
take advantage of the robust instability and 40-50 kts of 0-6 km
westerly shear to become severe. These storms will be discrete, at
least initially and all hazards are possible with storms that are
able to root in the surface layer, including tornadoes. Uncertainty
remains with this system and the evolution of the thunderstorms, the
0-1 km bulk shear varies across deterministic models as they handle
the sfc winds differently. More southerly surface winds would
promote a more favorable tornadic environment compared to
southwesterly winds. The cold front will exit the forecast area
early Friday morning, leaving the region cool and dry in it`s wake.

The strong mid-level longwave trough will remain stalled over the
Four Corners region through the weekend while a northern stream mid-
level trough over southeast Canada will keep the forecast area in
westerly flow. Largely dry conditions will continue into the first
half of the work week under this flow regime. Another cold front
will drop through the mid-Mississippi Valley Saturday into Sunday,
bringing another shot of cooler air to the region. This will result
in low temperatures during the first portion of the work week
dipping below freezing. Over eager gardeners should continue to hold
off on outdoor planting.

MRM

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1148 AM CDT Tue Mar 12 2024

VFR flight conditions will prevail ahead of a chance (40%) for
showers and thunderstorms that will move through during the
evening tonight. Confidence is low in where exactly showers and
thunderstorms will form, so have left mention as VCSH/VCTS for
now. Winds will remain largely southerly, diminishing in speed
overnight.

MRM

&&

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

$$

WFO LSX


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.