Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

422
FXUS63 KLSX 092004
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
304 PM CDT Tue Jun 9 2026

.KEY MESSAGES...

- There is a low chance of an isolated severe thunderstorm with
  isolated damaging winds; a brief, weak tornado; and/or locally
  heavy rainfall early this evening.

- Severe thunderstorms are possible Wednesday across northeastern
  MO/west-central IL, then again across a larger portion of the
  area late Thursday afternoon into night.

- Heat index values could reach 100 F in the warmest locations
  Wednesday and Thursday afternoons, but it is uncertain if
  additional Heat Advisories will be needed.


&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 304 PM CDT Tue Jun 9 2026

A remnant MCV is currently tracking along I-70 into the St. Louis
metro early this afternoon, but a strong capping inversion sampled
by latest KSTL ACARS soundings explain why the feature is currently
precipitation free and nearby cumulus has little vertical growth. As
the MCV continues eastward into IL, it should encounter an
increasingly unstable airmass and gradually less of a capping
inversion away from an incoming upper-level ridge, increasing its
chances of developing showers and thunderstorms. CAMs are varied on
how quickly this development will occur, with some solutions
delaying development until the MCV exits the CWA to the east.
Overall, deep-layer wind shear is modest, but near the MCV the
latest SPC Mesoanalysis has up to 30 kt of deep-layer wind shear
which suggests that a transient supercell or bowing segment is not
out of the question if thunderstorms can be sustained. Around 20 kt
of 0-1 km wind shear and ambient vorticity near the circulation may
support a brief, weak tornado in addition to isolated damaging
winds. With precipitable water near 2" and slow storm motions,
locally heavy rainfall is also possible. But, again, it is uncertain
if development will occur in the CWA. Additionally, heat index
values may fall short of Heat Advisory criteria (105+ F) this
afternoon/evening due to more cloud cover and airmass recovery from
this morning`s dissipating MCS, but some spots could still exceed
values of 100 F.

Most of the showers and thunderstorms this evening into overnight
are expected to shift to the northeast of the CWA as the upper-level
ridge builds overhead and forcing associated with a nocturnal LLJ is
focused across central IL into southern IN.

Model guidance has trend toward the upper-level ridge shifting to
the east of the CWA on Wednesday, opening the Mid-Mississippi River
Valley to southwesterly flow ahead of a longwave trough across the
Northern Rockies and Northern Plains. In this southwesterly flow, a
shortwave trough is progged to migrate northeastward from the
Central Plain to Great Lakes, glancing the CWA. There is variability
in the timing and track of the shortwave trough but around 30 to 50
percent of HREF membership has a capping inversion overcome with
showers and thunderstorms developing across northeastern MO and west-
central IL during late morning through afternoon. However,
probabilities are higher in those areas during the evening with
thunderstorms developing along a nearby cold front and mainly
relying on an MCS with some component of eastward propagation being
established. With 1500 to 3000 J/kg MLCAPE and around 20 to 30 kt of
effective wind shear, a brief supercell cannot be ruled out, but mid-
level dry air suggests upscale growth into clusters or an MCS could
occur if a cold pool is generated. All of these factors point to a
few strong to severe thunderstorms capable of damaging winds and
large hail, but a lack of strong low-level wind shear lowers the
tornado treat. Subtle shortwave ridging as well as decreasing
instability and wind shear decreases confidence in showers and
thunderstorms lingering in the CWA overnight.

The overall setup on Wednesday looks to support warmer high
temperatures further into the 90s F with low-level southwesterly
flow off the Ozark Plateau but also some diurnal drop in dewpoints
with deeper BL mixing limiting heat index values. A further
complicating factor is for potential precipitation and clouds to
temper temperatures some in the northwestern half of the CWA.
Therefore, the Heat Advisory was not extended beyond today, but heat
index values could still flirt with 100 F during the afternoon and
evening. If model guidance trends toward higher dewpoints during
peak heating, then another advisory will need to be considered.

Pfahler

&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 304 PM CDT Tue Jun 9 2026

On Thursday, a more prominent upper-level trough is forecast to
round the longwave trough, providing a more significant
southeastward push of the cold front into the CWA and accordingly
much higher confidence in the development of more widespread showers
and thunderstorms. There is still variability in the timing of the
front in model guidance but a general consensus that it will be
moving into northeastern MO and west-central IL during the evening,
closest to peak heating with MLCAPE of 2000 to 3000 J/kg indicated
by most deterministic model guidance. This portion of the CWA will
also favored to see the strongest deep-layer wind shear of 30 to 35
kt, favoring strong to severe thunderstorms with an initial mixed
mode of supercells and line segments quickly growing upscale into a
QLCS with deep-layer shear nearly paralleling the front and linear
forcing. Therefore, damaging winds and a few tornadoes, as well as
large hail initially, are all reasonable hazards with severe
thunderstorms. Thunderstorms will generally encounter gradually
lower instability and wind shear as they progress eastward Thursday
night, but exactly how quickly thunderstorms weaken is uncertain.
Similar to Wednesday, it is uncertain how warm it will get ahead of
the front with some possible impact from clouds, but ensemble model
guidance probabilities of 100+ F are 40 to 60 percent in the St.
Louis metro, higher than Wednesday with higher dewpoints expected.

Behind the front, global model guidance is in agreement that the
upper-level flow pattern across the Mid-Mississippi River Valley
will be quasi-zonal, allowing subsequent wavering northward and
southward of the front as a series of shortwave troughs pass.
Ensemble models have a growing consensus that the front will lift
northward toward or into the CWA sometime Saturday through Sunday,
providing the highest probabilities of showers and thunderstorms.
Otherwise, conditions should be mainly dry. With a cooler post-front
airmass in place through the majority of the period along with
clouds and precipitation over the weekend, temperatures will cooler
to near and even potentially below average by early next week.

Pfahler

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1237 PM CDT Tue Jun 9 2026

Largely dry and VFR flight conditions appear most likely through the
TAF period, but there are low chances of showers and thunderstorms
at St. Louis metro terminals this afternoon and then late Wednesday
morning through afternoon at KCOU, KJEF, and KUIN. However,
confidence is too low in impacts at this time for explicit inclusion
in the TAF. Southwesterly winds could occasionally gust to 18 to 25
kt this afternoon and again on Wednesday.

Pfahler

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 10 PM CDT this evening for Audrain MO-Boone
     MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln MO-Madison MO-
     Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike
     MO-Ralls MO-Reynolds MO-Saint Charles MO-Saint Francois MO-
     Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO-
     Shelby MO-Warren MO-Washington MO.

IL...Heat Advisory until 10 PM CDT this evening for Adams IL-Brown IL-
     Calhoun IL-Clinton IL-Jersey IL-Madison IL-Monroe IL-Pike
     IL-Randolph IL-Saint Clair IL-Washington IL.

&&

$$

WFO LSX