Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

278
FXUS63 KLSX 110927
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
427 AM CDT Tue Aug 11 2026

.KEY MESSAGES...
- Dangerous heat and humidity is expected to continue through
  Wednesday. The Extreme Heat Warning and Heat Advisory remains
  in effect.

- A cold front is expected to bring rain chances and cloud
  cover Wednesday through Friday, possibly bringing relief from
  the heat and humidity for some. A few thunderstorms may
  threaten damaging wind and a few weak tornadoes Wednesday
  afternoon, mainly in Illinois.

- Confidence is high (~80%) that dangerous heat and humidity
  will return late in the week and into the weekend followed by
  some relief early next week.

&&

.SHORT TERM /THROUGH WEDNESDAY/...
Issued at 423 AM CDT Tue Aug 11 2026

Observable in mid-level water vapor imagery from GOES East, a
ridge-runner shortwave and a low-level jet are providing
enough lift for some robust convection in eastern NE and western
IA. These showers and thunderstorms will track east giving a
chance (20-30%) of some remnant showers in northeastern MO into
west central IL later this afternoon. The ridge, which has been
the primary reason for the continued heat and humidity, remains
over most of the CONUS. Tuesday is expected to be the worst of
the heat and humidity for this round. Model guidance continues
to show 850mb temperatures between 23-24 C (above 95th
climatological percentile), translating to mid to upper 90s
surface temperatures. Although, if convective remnant clouds
remain, we could see slightly lower temperatures, but slightly
higher dewpoints due to lack of vertical mixing. These high
temperatures combined with the expected mid-70s dewpoints will
result in heat index values between 105-110 F. Wednesdays
temperatures wont be nearly as bad as Tuesday, with guidance
indicating lower to mid-90s for surface temperatures and low 70s
dewpoints resulting in slightly lower heat index values closer
to 95-105 F. The lower temperatures are attributed to the chance
of cloud cover and rain chances through the day on Wednesday.

Wednesday morning there is a chance (30-40%) of rainfall that
would impact northeastern MO and west central IL. Model guidance
indicates another ridge-runner along with a low-level jet
will provide lift for another round of showers and thunderstorms
after sunrise over northeastern MO and west central IL. There
is some upper level subsidence during this time that will
prevent this round causing heavy rainfall and severe weather.
Expected rainfall totals from the morning rain could range from
0.1 to 0.3 inches primarily in northeastern MO with locally-
heavier amounts possible up to 0.5 inches.

During the afternoon on Wednesday, there is a possibility of a
convective system that will translate through northern and
central IL into IN that may push a boundary into southwestern
IL. Combined with the heat and low-level moisture present in the
region, it can provide buoyancy in the atmosphere allowing for
convection to spark. As a result, SPC has issued a Marginal Risk
for northern and eastern MO as well as a Slight Risk for
central and southern IL. Their primary threats are for damaging
winds and isolated hail. During the late afternoon or early
evening, the possibility of a tornado can not be ruled out.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Issued at 423 AM CDT Tue Aug 11 2026

Thursday and Friday, guidance is indicating rounds of showers
and thunderstorms will impact IA and northern IL, which will
send convection into northeastern MO and west central MO. There
is support for development throughout the depth of the
atmosphere, with upper level divergence to the north of our
region and low-level warm air advection providing lift. NBM
guidance gives a 60-80% chance of southeastern IA and extreme
northeastern MO of receiving at least 1 inch of rain in this
system. The gradient associated with the possible rainfall
totals is very tight across the region. For example, just to the
south of the max swath, NBM guidance indicates that there is a
50-80% of northeastern MO and west central IL receiving at least
0.25 inches of rain. To further illustrate this, the St. Louis
metropolitan area only has around a 30% chance of receiving 0.1
during the same time. With the convective nature of this event
and the coarse resolution of the model data, we cant rule out
the possibilities of locally higher totals depending on where
the swath of rain ends up.

Between the cold front and the ongoing rain/clouds by Thursday,
northeastern MO and west central IL have a higher potential of
seeing slight relief from the heat by a few degrees. Although,
it is still uncertain with maximum temperature IQRs of around 10
degrees. This wide range in values is the result of the
uncertainty surrounding how far south the front will progress
along with the exact amount of rain and cloud cover. Regardless
if such relief does occur, there is high confidence (80-90%)
that temperatures will rebound to the levels that they have been
the last couple days. This is a result of model guidance
consensus that the ridge will build back north, bringing
continued heat for the entire region through Sunday. There are
signs of another cooldown in the guidance starting Monday. While
exact values are not certain, the entire range of the IQRs for
Mondays high temperatures is lower than the low end of the IQRs
we currently see for temperatures in the short term.

&&

.AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1038 PM CDT Mon Aug 10 2026

VFR conditions are expected for the majority of the TAF period.
Light south winds will persist overnight, with some low level
wind shear possible at JEF where surface winds are more likely
to become calm underneath a 40kt southwesterly low level jet. At
other terminals, this is less likely due to either slightly
higher surface winds, or slightly weaker winds aloft. At UIN,
there is a growing potential for a few showers and thunderstorms
tomorrow morning, with probabilities increasing to near 30%. As
such, a PROB30 has been introduced to the TAF. Probabilities are
also growing at St. Louis terminals at the same time, but remain
too low to include at this time (20%). Additional showers and
thunderstorms will be possible near the end of the period in the
same areas, but chances for this later round remain too low to
include at this time (20%) also.

Otherwise, breezy southwest winds and VFR cumulus clouds are
expected in most areas throughout the day tomorrow.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 8 PM CDT Wednesday for MOZ018-019-026-027-
     034>036-042-052-072>075-084-085-099.
     Extreme Heat Warning until 8 PM CDT Wednesday for MOZ041-
     047>051-059>065.
IL...Heat Advisory until 8 PM CDT Wednesday for ILZ058>060-064-065-
     070-095>099.
     Extreme Heat Warning until 8 PM CDT Wednesday for ILZ069-074-
     079-100>102.

&&

$$

SHORT TERM...Asel/MRB
LONG TERM...Asel/MRB
AVIATION...19