Area Forecast Discussion

222
FXUS63 KLSX 131957
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
257 PM CDT Tue Oct 13 2026

.KEY MESSAGES...

- Unseasonable, record-breaking heat ends today.

- Two main rounds of showers and thunderstorms are expected
  through Thursday: one tonight into Wednesday morning and
  another late Wednesday night into Thursday morning. There is a
  low chance of an isolated severe thunderstorm late this
  evening as well as locally heavy rainfall with both rounds.

- Behind a strong cold front Thursday night, more seasonable
temperatures and dry conditions will arrive Friday into the weekend.

&&

.SHORT TERM /THROUGH WEDNESDAY/...
Issued at 256 PM CDT Tue Oct 13 2026

To the south of a wavering cold front across far northern MO,
another day of unseasonable heat and humidity, including heat index
values of 95 to 100 F, is underway with temperatures warming around
90 or the low-90s F on top of dewpoints rising in the mid-60s to
around 70 F. With weak convergence along a front across northern MO,
a lack of large-scale ascent, and a weak capping inversion; it is
uncertain if any shower or thunderstorm development will occur
across northeastern MO before sunset with CWA-wide dry conditions
continuing instead.

Large-scale ascent will increase from west to east later this
evening as upper-level perturbations arrive and a LLJ gradually
strengthens, favoring initial development of showers and
thunderstorms across eastern KS into western and northern MO this
evening. These showers and thunderstorms will increase in coverage
and propagate eastward into the CWA overnight into Wednesday
morning. This late timing suggests that the threat of severe
thunderstorms will be decreasing as nocturnal stabilization of the
boundary layer occurs and effective wind shear decreases slightly
from west to east, limiting some of their organization. In the CWA,
northeastern and central MO would have the greatest threat of an
isolated damaging wind gust and/or marginally severe hail late this
evening. Since PW will be nearing 2", record-breaking for mid-
October, and warm cloud depths will be deep, thunderstorms will
contain efficient, high rainfall rates. However, a quickly veering
LLJ and southeastward-advancing cold front will encourage
progressive motions of showers and thunderstorms, decreasing
confidence in flash flooding, especially considering the antecedent
soil conditions are very dry. The latest HREF and REFS QPF LPMM,
although in disagreement on the location, has localized pockets of 1
to 3" which would be more beneficial to the dry soils.

As the LLJ veers further east and the effective cold front is
shunted southward, morning showers and thunderstorms will end
steadily from west to east. That being said, widespread clouds and
areas of drizzle or light showers may linger at least into the early
afternoon. These conditions will diminish warming through the day
with temperatures reaching the 70s to around 80 F--much cooler than
today but more akin to May or June cloudy day than October.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
Issued at 256 PM CDT Tue Oct 13 2026

An upper-level trough over the Four-Corners Region will finally
eject northeastward Wednesday night into Thursday, resulting in a
lift back north of the front through the CWA as a warm front in
response to surface cyclogenesis in the Southern/Central Plains. As
low to mid-level isentropic ascent/WAA increases in this process,
another round of showers and thunderstorms is forecast late
Wednesday night into Thursday morning with HREF and REFS membership
most in alignment with most widespread coverage across
central/northeastern MO into west-central IL. Since PW will again be
anomalously high, there will at least be a low threat of locally
heavy rainfall, but again the overall threat will again be limited by
mainly progressive thunderstorm motions. There has been a trend in
model guidance toward a more constructive interaction of the trough
with a potent northern stream trough, resulting in solutions with
stronger large-scale ascent and a deeper and slightly more mature
surface cyclone passing just to the west and northwest of the CWA
Thursday afternoon and evening. This evolution would result in
greater development of showers and thunderstorms along a trailing
cold front and perhaps a more favorable setup, conceptually
speaking, for severe weather. That being said, there are still
solutions with weaker ascent and moreso a frontal wave with isolated
showers, maybe a thunderstorm or two. These factors aside, the
impact of early day clouds and precip will heavily influence how
much instability is available with uncertainty reflected in the
interquartile range of SBCAPE in the REFS spanning 250-500 J/kg to
1000+ J/kg.

Behind the cold front passage Thursday night, strong and persistent
CAA will be present on Friday leading to much cooler, more
seasonable airmass being established across the Mid-Mississippi
River Valley through Saturday night with high temperatures in the
mid-50s to mid-60s F and low temperatures in the mid-30s to mid-40s
F. Exactly how much temperatures cool by Saturday morning and Sunday
morning will be determined by whether or not a surface anticyclone
is favorable positioned across the region during nocturnal
radiational cooling. The NBM has 20 to 50 percent probabilities of
sufficiently cool temperatures for frost formation (~36 F or less)
in rural and/or low-lying locations both Saturday and Sunday
mornings, but there is some spread in the temperature distribution.

Global model guidance is in agreement a warming trend will begin
Sunday as the surface anticyclone shifts to the east and low-level
southerly flow/WAA ensues beneath upper-level southwesterly flow
Sunday into early next week. In fact, the entire NBM temperature
distribution is back above average by Monday afternoon, although
nowhere near the magnitude of current above average temperatures.
There is disagreement, however, in if an upper-level trough passes
around Monday, providing the next opportunity for precipitation.

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1239 PM CDT Tue Oct 13 2026

Dry and VFR flight conditions along with southwesterly winds will
persist through sunset, but showers and thunderstorms are expected
to move eastward into the region tonight into Wednesday morning
threatening all terminals. It is worth noting that timing for
showers and thunderstorms has trended several hours later,
reflected by windows of highest confidence in the TAF.
Thunderstorms will be capable of MVFR to IFR flight conditions.
Behind showers and thunderstorms, areas of IFR ceilings and
potentially drizzle will linger into midday across much of the
area. It is uncertain how much improvement in flight conditions
there will be through the day Wednesday, but there should at
least be some lifting of ceilings during the afternoon.

&&

.CLIMATE...
Issued at 232 AM CDT Tue Oct 13 2026

Daily record highs and record warm lows continue today.
The daily records threatened are shown below.

Record High Temperatures:

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

Record High Minimum Temperatures:

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...Pfahler
CLIMATE...Maples

NWS LSX Office Area Forecast Discussion