288
FXUS63 KLSX 091136
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
636 AM CDT Tue Jun 9 2026

.KEY MESSAGES...

- High temperatures will be in the lower to middle 90s through Thursday
  with the highest heat indexes expected to reach around 105
  degrees in some locations.

- Severe thunderstorms are possible late Wednesday and Wednesday evening
  over northeast Missouri and west central Illinois and again on
  Thursday afternoon and evening across much of the area.


&&

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 259 AM CDT Tue Jun 9 2026

The MCS that is currently over central Missouri will continue to
move east early this morning.  I do expect there to be a gradual
decrease in intensity as mesoanalysis fields are showing a decrease
in MUCAPE/MLCAPE across the state.  Even with this said, the MCV
should still bring some showers and thunderstorms to the area this
morning with the CAMS showing a 30-60% chance along and south of I-
70 through 8 am.  While there will be the potential for some locally
heavy rainfall with the MCS, the threat for flash flooding will be
diminishing quickly as it continues to move east. Given the linear
nature of the storms, I currently plan to let the flood watch to
expire at 7 am.   There will be some scattered showers and
thunderstorms that will persist across the southern and eastern part
of the CWA into the afternoon on the eastern periphery of the ridge
that will be building over Missouri and Illinois today. While there
should not be any organized severe thunderstorms later today, it is
possible that one or two of these storms could produce strong wind
gusts and heavy downpours given the forecast soundings that show
large CAPE and weak shear.

Then mainly dry weather is expected tonight through Wednesday as the
ridge builds over the area.  There will be an increasing chance for
showers and thunderstorms late tomorrow over northeast Missouri as a
front approaches the area in a very unstable environment.

I have expanded the Heat Advisory for this afternoon and this
evening to include northeast Missouri and west central Illinois as
well as Washington and Clinton counties in south central Illinois.
Showers and thunderstorms look like they will clear out of the area
early enough this morning allowing for enough sun this afternoon for
highs to climb into the lower 90s this afternoon.  Heat indexes in
the newly expanded advisory are expected to range from 100 to 108
degrees this afternoon with dewpoints in the middle to upper 70s.
Dewpoints on Wednesday afternoon are not expected to be quite as
high (lower to middle 70s vs. middle to upper 70s), but the highs
temperatures are forecast to be 1-3 degrees warmer causing heat
indexes to climb to around 105 in the metro area again.  The heat
advisory may need to be extended into Wednesday if there is no
convective debris cloudiness from storms over the Plains.

Britt

&&

.LONG TERM...  (Wednesday Night through Monday)
Issued at 259 AM CDT Tue Jun 9 2026

There will be two additional chances for strong to severe
thunderstorms as we go into the extended part of the forecast  The
first will be late Wednesday into Wednesday night as the atmosphere
will become very unstable across the area ahead of cold front that
will extend from the Upper Midwest into the Middle Missouri Valley.
The CAMS are showing a line of thunderstorms developing across
southern Iowa and northwest Missouri during the afternoon that will
likely move into northeast Missouri and west central Illinois during
the evening.  The NAM/GFS is showing a shortwave trough moving
quickly northeast through the upper Midwest on Wednesday night with
heights rising behind it which will hold up the front, keeping the
thunderstorms from moving too far into the CWA.  The main threats on
Wednesday look to be large hail and damaging winds.  A better chance
for severe weather will be late Thursday and Thursday evening when a
deeper trough will move across the Midwest bringing the cold front
across Missouri and Illinois.  Here again, the instability-shear
environment will be favorable to severe thunderstorms with alignment
of the front favoring mixed mode initially growing upscale into a
line.  Damaging winds, large hail, and a few tornadoes are all
possible Thursday afternoon and evening in addition to some locally
heavy rainfall.

After Thursday`s system, model guidance is showing quasi-zonal flow
with additional chances for showers and thunderstorms over the
weekend.

The heat will continue into Thursday with high temperatures in the
low to mid 90s and another day where heat indexes where parts of the
area including the metro area may reach 105 degrees.  After the cold
front moves through, high temperatures are currently forecast to
drop back into the 80s beginning on Friday.

Britt

&&

.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 630 AM CDT Tue Jun 9 2026

Overnight thunderstorms have all but died across the area, leaving
a mesoscale convective vortex in its wave over north-central MO.
The potential for additional thunderstorm activity will be the
primary aviation concern through the TAF period, though at this
point, storms chances appear to be low. One focus for storms could
be the MCV mentioned above, but it will likely be east of the
region by the time the atmosphere becomes uncapped enough to
support free convection. Additionally, a few of the CAMs run from
00z last night highlight a chance for storms along the Mississippi
River this afternoon, but it is unclear what forcing mechanism
would initiate those storms. Indeed, CAMs runs from 06z and later
have much lower storm chances across the area this afternoon, so
I`ve kept the TAFs dry.

BSH

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory from noon today to 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 from noon today to 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