Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

153
FXUS63 KLSX 081845
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
145 PM CDT Sat Aug 8 2026

.KEY MESSAGES...

- It will be hot and humid Sunday through Wednesday next week.
  Heat index readings between 100-104 are expected for parts of
  the area Sunday. Monday through Wednesday will see the heat
  index increase to 105+ for much of the area Monday through
  Wednesday.

- The chance for showers and thunderstorms increases late
  Wednesday into Thursday, with the possibility of scattered
  showers and storms continuing through the end of the week.
  This should bring an end to this latest round of hot and
  humid weather.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 136 PM CDT Sat Aug 8 2026

An upper level ridge over the western CONUS will control our weather
for the next few days.  This will result in warm and
humid weather through the short term, with only very slight chances
for precipitation across northeast Missouri and west central
Illinois through Sunday night.  That chance for precipitation will
mostly be late tonight into Sunday morning as low level warm
advection increases east of a low pressure system over the Great
Plains.  Guidance is pretty insistent that most of the precip will
stay over Iowa, but the tail end may scrape our far northern
counties.

Otherwise, warm and humid southerly flow increases Sunday
with 850mb temperatures increasing to 20-22C.  This mixes down to 91-
95F at the surface which agrees well with high temperature guidance
for tomorrow.  Humidity looks marginal to get heat index values
above 104 tomorrow afternoon, which would be needed to make 4
days (Sunday through Wednesday) for a durration-critieria Extreme
Heat Warning.  Will therefore not issue a warning this afternoon and
continue the watch in case subsequent data points to higher heat
index values tomorrow.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Issued at 136 PM CDT Sat Aug 8 2026

The primary concern heading into next week continues to be
potentially dangerous heat, at least through Wednesday.  The
eastern edge of the upper level ridge remains over the eastern
Plains and Mid Mississippi Valley through the first half of the
week. Deterministic guidance keeps the low level thermal ridge over
Missouri and the southern 1/2 of Illinois through Wednesday with
850mb temperatures in the 22-25C range.  This agrees well with the
LREF mean through midweek, and forecast sounding show mixing at
least that high in most locations.  This mixes down to to mid and
upper 90s for highs, perhaps kissing 100-102 in some locations. The
NBM`s forecast highs have a good handle on this for the most part,
though It may be a tad cool in northeast Missouri into west
central Illinois.  With the high humidity in place, HI values are
100-105+ area-wide at least on Monday and Tuesday.  Confidence in
100+ HI values decreases Wednesday as a short wave rounds the
eastern edge of the ridge and moves through the Mississippi Valley.
The deterministic ECMWF and GFS print out QPF Wednesday afternoon
between 18-00Z as this wave moves through.  Afternoon convection
would almost certainly put the kibosh on any heat
headlines...however, the timing of that wave and potential
convection is very uncertain at this time.  If the wave is slower
and Wednesday is dry, it will almost certainly be at least as warm
and humid as Monday and Tuesday.

The deterministic guidance diverges for the latter half of the week
with the GFS digging a strong trough into the Ohio Valley which
produces an amplified upper level pattern and pronounced northwest
flow over the Mid Mississippi Valley.  This pushes a cold front
through Missouri and Illinois, and our area remains on the cool side
of the boundary through Saturday.  The ECMWF remains more zonal with
a couple of much weaker short waves rippling by Thursday through
Saturday.  While both solutions show cooler conditions for Thursday
through Saturday, there`s a lot of uncertainty in how much cooler it
will actually be.  High temperature IQRs in the LREF increase from
around 5-6 degrees on Wednesday to 8-12 degrees Thursday.  By Friday
and Saturday the high temperature IQR increases to 10-15 degrees in
many locations, and 15+ for some.  The NBM looks to be about the
middle of the temperature range with highs mainly in the low to mid
80s.  These temperatures are entirely plausible, and I see no reason
to deviate at this time given the uncertainties.

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1235 PM CDT Sat Aug 8 2026

A band of cumulus/strato-cumulus with bases around 2200ft has
developed over northeast Missouri into west central Illinois
early this afternoon. Expect these MVFR ceilings to lift and
scatter over the next couple of hours as daytime heating
continues. VFR flight conditions are expected to prevail across
the area thereafter. There is a chance that an are of showers
with a few thunderstorms may move into northeast Missouri and
west central Illinois between 10-12Z Sunday morning. However,
chances are low (only 20-30%) at this time, so have not included
any mention of precip in the KUIN terminal forecast for this
update.

&&

.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...Carney
LONG TERM...Carney
AVIATION...Carney