Riggs, Missouri Weather Forecast Discussion

047
FXUS63 KLSX 210344
AFDLSX
Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1044 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 06z TAFs through 06z Sunday Night)
Issued at 1043 PM CDT Sat Sep 20 2025
Showers and thunderstorms have diminished in both coverage and
intensity from earlier this evening. Aside from a stray shower,
dry conditions are expected with VFR conditions expected through
Sunday morning.
Chances of showers and thunderstorms will increase Sunday
afternoon between 20z-00z, but activity will remain isolated to
scattered until more widespread activity moves in Sunday evening.
VFR conditions will continue up to that point, when conditions
become more favorable for MVFR ceilings/visibilities that
accompany this activity late evening Sunday into Sunday night.
The best potential for widespread MVFR conditions, potentially
localized IFR, will be late in the period as more persistent
rainfall and embedded thunderstorms move west to east.
Maples
&&
.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&
$$
WFO LSX

NWS LSX Office Area Forecast Discussion