Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

762
FXUS63 KLSX 202043
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
343 PM CDT Mon Jul 20 2026

.KEY MESSAGES...

- Widely scattered showers and thunderstorms this afternoon will
  diminish this evening after sunset. Another round of scattered
  thunderstorms is possible tonight. A few storms may produce
  wind gusts of 50-60 mph across parts of northeast Missouri and
  west central Illinois.

- A cold front will move through the area on Tuesday. There is a
  chance for thunderstorms with the front, mainly in the
  afternoon across southeast Missouri and southern Illinois.

- Cooler and less humid air will move into the area behind the
  front, with highs only in the low to mid 80s on Wednesday and
  Thursday.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 338 PM CDT Mon Jul 20 2026

Warm and humid conditions continue across the area this afternoon
across much of the area.  A weak cold front moved across much of
Illinois and stalled near the Mississippi River last night.  This
front is now moving slowly back to the northeast, and the warm moist
airmass is building back into Illinois behind it. Scattered
thunderstorms over the eastern Ozarks will dissipate this evening as
daytime heating wanes.  Storms arent showing much organization and
deep layer shear is only around 20kts.  However the RAP is showing
MLCAPE values in excess of 3000 J/Kg so an isolated strong wind gust
cant be ruled out.

Another much stronger cold front will move from the Upper Midwest
through the Mid Mississippi Valley on Tuesday, and this will break
our most recent stretch of hot weather.  Additional thunderstorm
development is expected ahead of the front tonight across Iowa,
though it is unclear how widespread and how strong the storms will
be by the time they reach our forecast area.  Forecast soundings are
showing a fairly deep dry layer in the low levels, and subsequently
a great deal of CINH.  Both the RAP and GFS therefore show very
little QPF with this front, and even the HRRR only shows widely
scattered convection as the front moves through.  However, CAPE
values will continue in the 2500-3000 J/Kg range ahead of the front
early Tuesday morning, with around 40kts of deep layer shear, so any
storms that can punch through that CINH could become severe, most
likely in far southeast Missouri into south central Illinois Tuesday
afternoon.

&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
Issued at 338 PM CDT Mon Jul 20 2026

A deep long wave trough over the eastern U.S. will allow a much
cooler Canadian high pressure system to dig into the Mississippi
Valley Tuesday night and Wednesday.  Temperatures are expected to be
much cooler in the low to mid 80s in most locations Wednesday and
Thursday as high pressure moves from the Midwest into the Ohio
Valley.  Medium range guidance is moving a short wave trough from
the Great Plains into the Mid Mississippi Valley Thursday into
Friday.  There are some timing and amplitude differences with this
wave, however guidance suggests the low level reflection of the wave
will generate a low level jet Thursday night into Friday which will
produce precip that persists through much of Friday into Friday
night.  Clouds and precip on Friday look to suppress temperatures ,
and highs may not even reach 80s.  Warmer weather spreads east from
the Great Plains through the weekend into early next week as the
long wave pattern shifts east and the upper ridge builds back into
the Mississippi Valley.

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1240 PM CDT Mon Jul 20 2026

Patches of 3-5SM MVFR visibility in haze will persist into this
afternoon, generally along and east of the Mississippi River.
There may be some improvement later this afternoon as a warm
front drifts northeast from near the Mississippi River into
central Illinois. Additionally, a complex of thunderstorms over
eastern Iowa/northwest Illinois will continue moving south-
southeast through the afternoon. Current thinking is that the
western end of this complex will dissipate before entering our
forecast area, although it may sideswipe Adams, Brown and
possibly Pike counties. There will also be isolated to widely
scattered showers and thunderstorms this afternoon across the
eastern Ozarks. The eastern Ozarks convection should dissipate
quickly after sunset. Another complex of thunderstorms is
expected to develop later tonight over Iowa ahead of a cold
front, and may move into northeast Missouri/west central
Illinois before sunrise Tuesday morning. However confidence in
the coverage and timing of these late-night thunderstorms is low
at this time. The cold front is expected to move into northeast
Missouri and west central Illinois between 10-12Z Tuesday, and
continue south through the day. The front will bring a band of
wildfire smoke through the area which may impact visibilities
through the day.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 7 PM CDT Tuesday for MOZ041-047>051-
     059>065-072>075-084-085-099.
     Heat Advisory until 8 PM CDT this evening for MOZ018-019-026-
     027-034>036-042-052.
IL...Air Quality Alert until midnight CDT tonight for ILZ058>060-
     064-065-069-070-074-079-095>102.
     Heat Advisory until 7 PM CDT Tuesday for ILZ069-074-079-
     100>102.
     Heat Advisory until 8 PM CDT this evening for ILZ095-097-098.

&&

$$

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