264
FXUS63 KLSX 081137
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
637 AM CDT Sat Aug 8 2026

.KEY MESSAGES...

- Hot and humid conditions are expected this weekend and through
  the middle of next week. Heat index readings in the 100 to
  108 range will start Sunday and persist into Wednesday.

- The heat breaks Thursday, with several rounds of showers and
  thunderstorms persisting through Friday.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 236 AM CDT Sat Aug 8 2026

Today: The radar is showing a decaying MCV from earlier
convection making its way across southern MO, and then there is
a cold front moving south from southern IA and into northern MO.
The MCV is expected to continue progressing to the east while
weakening, eventually getting sheared out by an approaching
shortwave over IL/WI. No showers or thunderstorms are expected
today due to the dry air aloft. Model guidance is also
indicating lack of forcing available in the region, with weak
subsidence caused by the convergence and negative vorticity
advection aloft. Meanwhile models are in consensus that the cold
front will stall out in central MO. Then late in the day, the
cold front will lift back to the north.

Moisture along the stalled front is expected to pool, with
dewpoints into the 70s during the day. This added moisture along
with air temperatures in the low to mid 90s will cause heat
index values to reach or exceed 100 F, particularly south of the
I-70 corridor. The retreating cold front will allow for higher
dewpoints and warmer temperatures to advance northward into the
region.

Sunday: An upper level low over the James Bay will remain quasi
stationary with several upper level disturbances heading east across
the Great Lakes region. A cold front will move south out of Canada
and then stall across southern Minnesota. Showers and thunderstorms
will likely develop over Iowa in response, but there will be a
southern extent to the precipitation as PWATs fall off from
around 2" in Iowa to 1.3" (just below the 50th percentile for
this time of year) over east/central Missouri. This means that
some storms will clip northern Missouri, but will likely not
have much of a southern push. Ahead of the cold front, dewpoints
will rapidly rise as low level moisture pools. 1000/ 850 mb
thicknesses also start to balloon due to compressional warming
with values approaching 1440 m. Given this, high temperatures
will likely be in the mid 90s with the latest run of NBM showing
only a two degree spread in the interquartile range (IQR). This
combined with dewpoints in the mid to upper 70s means that
triple digit heat index values will be widespread. Peak heat
index values will likely be located near and along the I-70
corridor where values could top out in the 105 to 110 degree F
range. These range of values could last until Wednesday. As
such, we have issued a Extreme Heat Watch starting Sunday and
persisting into Wednesday.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Issued at 236 AM CDT Sat Aug 8 2026

Monday through Wednesday: The overall upper level pattern remains
largely unchanged through Wednesday. Mid-level ridging across the
southwestern United States will remain elongated to the east while
multiple rounds of vorticity traverse east across the Dakotas and
Great Lakes region. Each day will feature highs in the mid 90s with
dewpoints in the mid to upper 70s. As such, the Extreme Heat Watch
is in effect through Wednesday. It should be noted that even though
IQRs are only a few degrees Monday and Tuesday, confidence rapidly
decreases starting Wednesday on the overall synoptic pattern. On
Wednesday, the IQR at STL from the NBM is six degrees while the IQR
by Friday and Saturday rises towards 15 degrees. Confidence is
also lower further north (e.g., Quincy, IL) due to the
possibility of showers and thunderstorms or convective cloud
debris starting Monday (IQRs 5 to 10 degrees).

Thursday and Friday: The rapidly falling confidence at the end of
the long term is due to the closed low that is currently over the
James Bay. This can be seen on the WPC Ensemble Sensitivity Analysis
(ESA) webpage which shows the variance in the extended coming
from how the closed low opens up and ejects east across Ontario
and Quebec. As the wave opens up, it is forecast to nudge south
and east shunting the ridging south. How far south is the main
question, as a multiple rounds of showers and thunderstorms
could form along a stationary boundary across the region.
However, where and when the wave ejects is still very uncertain.

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 634 AM CDT Sat Aug 8 2026

Dry and VFR conditions are forecast to persist through the TAF
period. A weak cold front will waver across northern Missouri
and west-central Illinois today, but with nearly no upper-level
ascent to force convection, it will very likely have no
operational impact.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Extreme Heat Watch from Sunday afternoon through Wednesday
     evening for MOZ041-047>051-059>065.
IL...Extreme Heat Watch from Sunday afternoon through Wednesday
     evening for ILZ069-074-079-100>102.

&&

$$

SHORT TERM...Asel/BAH
LONG TERM...BAH
AVIATION...MRB