Lemay, Missouri Weather Forecast Discussion

124
FXUS63 KLSX 202014
AFDLSX
Area Forecast Discussion
National Weather Service Saint Louis MO
314 PM CDT Sat Sep 20 2025
.KEY MESSAGES...
- Showers and thunderstorms, a few strong at times, will continue
  through the afternoon today. Scattered showers and thunderstorms
  are expected overnight and tomorrow afternoon as well.
- Increasing chances for rain and cooling to near normal
  temperatures will extend into next week.
&&
.SHORT TERM...  (Through Late Sunday Night)
Issued at 308 PM CDT Sat Sep 20 2025
Scattered showers and thunderstorms will continue this afternoon as
a weak boundary is stretched along the I-70 corridor with support
from a subtle mid-level shortwave. With early clearing of the
high clouds from morning convection, instability has ramped up
across the region; ACARS sounds show around 1100 J/kg of MUCAPE at
KSTL. A few storms have already taken advantage of the strong
low- level lapse rates and 25-30 kts of 0-6km shear to grow
quickly upscale and put down near severe-level winds. A few more
storms may be able to do this in the remaining 3 hours of daytime
heating. The strongest thunderstorms will be capable of up to 50
mph gusts as the cores drop through the low- level dry air.
The low-level jet will re-strengthen overnight, with the nose
positioned into southeast Missouri. Additional shower and
thunderstorm development is expected here and across southern
Illinois overnight into early tomorrow morning. The lack of surface
focus and weaker overnight 0-6km shear (10-15 kts), combined with a
lack of diurnal instability will result in showers and weak
thunderstorms.
Another mid-level shortwave will slide through the mid-Mississippi
Valley Sunday afternoon and coupled with a weak southwesterly low-
level jet will renew isolated to scattered showers and thunderstorms
across the forecast area. Tomorrow`s strong to severe thunderstorm
parameters are similar to today`s: sufficient instability up to 1000
J/kg of MLCAPE, strong low-level lapse rates, poor mid-level lapse
rates, and 20-25 of 0-6km shear. This will result in an environment
similar to today and Friday: increased chances for thunderstorms
during the afternoon, with the strongest storms capable of 50 mph
wind gusts. While thunderstorms will weaken into the evening hours,
showers and thunderstorms will continue overnight as the low-
level jet strengthens and the shortwave deepens over the forecast
area.
Delia
&&
.LONG TERM...  (Monday through Next Saturday)
Issued at 308 PM CDT Sat Sep 20 2025
By Monday morning the shortwave responsible for the overnight
convection will still hang over the region and the associated
broader mid-level trough will reach across most of the US-Canada
border. The trough will persist overhead into the end of the work
week, keeping the region open to multiple shortwaves and chances for
rain.
The best timing for rainfall Monday will be during the afternoon as
diurnal instability combines with the mid-level shortwaves to
produce convection similar to the past few days. Isolated to
scattered showers and thunderstorms are most likely given the regime.
By late Monday one of the shortwaves within the broader trough will
strengthen and move eastward, forming a surface low over Oklahoma.
The low approaches the mid-Mississippi Valley Tuesday, increasing
chances for scattered to widespread showers and thunderstorms
across region. The mid-level shortwave will strengthen to become
a closed low by Wednesday and slide into the mid-Mississippi
Valley, and nearly 80% of ensemble guidance keep it lingering over
the mid- Mississippi Valley as the eastern low in a building
Omega Block through the end of the work week. If this were to
occur, confidence would be higher in beneficial rainfall area
wide. If the overall pattern ends up weaker, the blocking pattern
will not be able to set up. This would result in a progressive
mid-level low, ultimately resulting in lower rainfall amounts
through the end of the work week.
Highlighting the uncertainty: there is a two inch interquartile
spread in total rainfall through the end of the work week. North of
I-70 the global ensemble interquartile range is 1-3", while south of
the corridor, closer to the track of the low, the range is 2-4".
Daily chances for showers and thunderstorms, and growing
confidence in a low system moving through the area mid week
increase confidence in a "cool down" to at least near normal
temperatures next week.
Delia
&&
.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1157 AM CDT Sat Sep 20 2025
A weak frontal boundary is draped from central Missouri to the St.
Louis metro and is contributing to isolated showers and
thunderstorms popping up across the St. Louis metro. The best
chance for showers and thunderstorms will be this afternoon in the
St. Louis metro when upper level support combines with the
boundary. Brief periods of MVFR visibility are expected underneath
thunderstorms, but storms are expected to be progressive enough to
quickly move out of the terminal space. 30-40 mph gusts are
possible if a storm moves directly over the terminal.
Elsewhere (KUIN, KCOU, KJEF) confidence in thunderstorm
development is lower so I have left a mention out of the TAF for
now.
Another round of showers and thunderstorms is expected tomorrow
afternoon with similar conditions to today. Isolated to scattered
thunderstorms will be capable of brief MVFR visibilities and
30-40mph gusts. Confidence in the exact timing and location of
storms is still uncertain, so have kept a mention of this out of
the TAF for now.
Winds are northeasterly north of the boundary (KUIN), and south to
southwesterly south of the boundary. The boundary will lift
northward through the overnight hours causing all sites to become
southwesterly by tomorrow morning.
SPECIFICS FOR KSTL:
Isolated chances for showers and thunderstorms through the course
of the day with the best chances during the afternoon when the
TEMPO group is. Confidence is high that thunderstorms will develop
along a boundary near the St. Louis metro today, but exactly
where and when they form is still uncertain. Uncertainty remains
in the timing and location of afternoon thunderstorms tomorrow, so
mention has been left out of the TAF for now.
Delia
&&
.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&
$$
WFO LSX

NWS LSX Office Area Forecast Discussion