425
FXUS63 KLSX 122316
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
616 PM CDT Mon Oct 12 2026

.KEY MESSAGES...

- Unseasonable, near-record breaking heat will continue into
  Tuesday.

- Widespread showers and thunderstorms are expected Tuesday
  night into Wednesday morning. A few strong to severe
  thunderstorms and locally heavy rainfall are possible. Another
  widespread round of showers and thunderstorms is forecast
  Wednesday night into Thursday morning.

- With a significant cooldown, temperatures will fall to below
  average at the end of the week into the weekend.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 312 PM CDT Mon Oct 12 2026

Anomalous upper-level ridging atop low-leve southwesterly flow will
remain over the Mid-Mississippi River Valley through Tuesday,
helping to maintaining an unseasonably hot and humid conditions
across the CWA. With full sunshine and some downslope warming off
the Ozark Plateau, temperatures this afternoon are anticipated to
continue warming to between the upper 80s and mid-90s F, eclipsing
and/or breaking daily and even monthly records. So far there has not
been much of a diurnal fall in dewpoints, limiting the window for
sufficiently low RH for Elevated Fire Danger. In fact, these
dewpoints are still around the 90th climatological percentile. The
warm day today and persisting southerly/southwesterly surface winds
overnight, will keep low temperatures in the mid-60s to low-70s F,
likely breaking daily record maximum low temperatures.

As an upper-level shortwave trough traverses the crest of the ridge,
a cold front is progged to advance southward and stall in the
vicinity of northeastern MO and west-central IL Tuesday morning.
With rising mid-level heights and a capping inversion it is unlikely
that this front will provide any precipitation through the
afternoon, but it will demarcate the southerly extent of cooler, albeit
still-well-above average high temperatures in the 80s F. To the
south, conditions will remain the more or less the same as today
with high temperatures in the low to mid-90s F, nearing or breaking
daily and monthly records. Dewpoints will also continue to increase
as the leading edge of more significant low-level moisture return
from the Gulf of Mexico reaches the Mid-Mississippi River Valley.
This rare October combination of temperatures and dewpoints will
even lead to some afternoon heat index values around 100 F.

&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
Issued at 312 PM CDT Mon Oct 12 2026

As upper-level perturbations arrive ahead of a trough near the Four
Corners Region and a nocturnal LLJ strengthens, showers and
thunderstorms will eventually develop Tuesday evening across western
and northern MO, increasing in coverage and expanding eastward later
in the evening into overnight. With 40 to 50 kt deep-layer wind
shear and 1000 to 2000 J/kg MLCAPE, there is some concern for a few
strong to severe thunderstorms capable of damaging winds and
marginally severe hail, especially near the front across
northeastern MO into west-central IL during the evening. However, a
potential failure point is that these thunderstorms may tend to be
elevated by that time, arriving as surface-based/mixed-layer
instability is decreasing. Overnight into Wednesday morning, there
is a threat of locally heavy rainfall as PW reaches over 2",
breaking seasonal records, with some mid-level moisture sources from
Tropical Cyclone Simon in the Eastern Tropical Pacific, supporting
high rainfall rates. Confidence is low that this rainfall will be
able to cause flash flooding, considering dry antecedent soil
conditions and a rather quick veering of the LLJ, promoting more
progressive thunderstorms that shorten the residence time of high
rainfall rates. Accordingly, both the HREF and REFS QPF LPMM contain
localized pockets of 1 to 3" through Wednesday morning.

On Wednesday as the LLJ continues to veer and focus to the east and
the effective front also shifts southward, most showers and
thunderstorms will gradually come to end from northwest to southeast
from mid-morning through the afternoon. However, widespread clouds
and even some light showers or drizzle will linger most of the day,
promoting much cooler, but still above average temperatures in the
70s to around 80 F. Of greater noteworthiness will be dewpoints in
the 70s F across most of the area, threatening daily, if not
monthly, records.

The upper-level trough over the Four Corners Region will finally
eject northeastward Wednesday night into Thursday, lifting the front
back northward Wednesday night as surface cyclogenesis also takes
place across the Central Plains into Mid-Mississippi River Valley.
Another widespread round of showers and thunderstorms will accompany
this warm front, including another threat of locally heavy rainfall
as PW once again peaks at 2"+. Although there should be some
moistening of soils from earlier rainfall and lower FFG, the overall
pattern and associated thunderstorms at this point look to be
relatively progressive. Even so, at least an isolated flash flood
threat will need to be watched going forward. With the trough
weakening/shearing out as it meets diffluent flow at the base of a
more potent trough on Thursday, weakening large-scale ascent
suggests less widespread precipitation in the warm sector and
subsequent cold front. Ensemble membership with measurable rainfall
also begins to taper off after Thursday morning, suggesting
increasingly dry conditions with more isolated showers, perhaps
thunderstorms thereafter. NBM interquartile temperature ranges are
also around 10 F, generally ranging from the 70s to 80s with values
dependent on how much of the surface cyclone`s warm sector the CWA
sees.

Behind late Thursday`s cold front, strong and persistent CAA prevail
Thursday night through Friday with temperatures cooling to below
average. There is some uncertain in exactly how cool this airmass
will be but the NBM distribution generally includes high
temperatures in the mid-50s to mid-60s F and low temperatures in the
mid-30s to mid-40s F Friday through Sunday morning. Exact low
temperatures, including the potential for frost, will be more
dependent on the position of an anomalously strong surface
anticyclone passing through the Mid-Mississippi River and if it
presence coincides with nocturnal radiational cooling. Currently,
the NBM has 20 to 40 percent probabilities of low temperatures below
36 F, a reasonable proxy for frost formation, Saturday morning and
Sunday morning in favored low-lying locations. As the anticyclone
migrates to the east ahead of another upper-level trough Sunday into
early next week, the return of low-level southerly flow will mark
the beginning of a moderating/warming trend that continues into next
week.

&&

.AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 609 PM CDT Mon Oct 12 2026

No significant changes from previous TAF package. Gusty
southwest winds will diminish this evening and increase again
late tomorrow morning. LLWS is expected to be an issue in
northeast MO and west-central IL where a southwest-northeast
oriented 35 - 40kt low-level jet will strengthen tonight. A
front is expected to stall in northeast MO early tomorrow
morning. How far south it reaches will determine whether winds
in the area stay southwesterly/south-southwesterly or veer to
northwesterly. Confidence is still low in where the front will
land.

&&

.CLIMATE...
Issued at 624 AM CDT Mon Oct 12 2026

CORRECTED FOR FORMATTING
Issued at 257 AM CDT Sun Oct 11 2026

Very warm temperatures are expected over the next several days.
This will challenge daily record highs and record warm lows on
multiple occasions. The daily records threatened are shown
below.

Record High Temperatures:

October 12:
KSTL: 89/1890
KCOU: 94/1899
KUIN: 90/2024

October 13:
KSTL: 90/1899
KCOU: 95/1899
KUIN: 87/1975

Record High Minimum Temperatures:

October 12:
KSTL: 70/1997
KCOU: 69/1928
KUIN: 70/1928

October 13:
KSTL: 69/2008
KCOU: 69/1899
KUIN: 65/1935

October 14:
KSTL: 70/1899
KCOU: 67/2017
KUIN: 67/1920

In addition, all time monthly record high temperatures are
within range of being broken. The following are the October
monthly high temperature records at each site:

October Record High:
KSTL: 94 most recently on Oct 1, 2019
KCOU: 96 on Oct 7, 1939
KUIN: 96 on Oct 7, 1939

&&

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

&&

$$

SHORT TERM...Pfahler
LONG TERM...Pfahler
AVIATION...Jaja
CLIMATE...Maples