Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

172
FXUS63 KLSX 170743
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
243 AM CDT Thu Sep 17 2026

.KEY MESSAGES...

- A late season heat wave continues through at least Saturday
  with peak heat index values of 100-105F+ along/south of I-70.

- Scattered showers with an isolated sub-severe thunderstorm are
  possible (20-30%) across portions of the area this
  afternoon/evening.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 236 AM CDT Thu Sep 17 2026

A broad and robust mid/upper-level ridge continues to encompass the
eastern half of the CONUS with 500mb heights at or exceeding
climatological records across the region. A slight northwesterly
nudge of the ridge axis will happen today, which will help to push
the low/mid-level baroclinic zone across IA slightly further to the
north. The result for the area will be an extremely hot and
unseasonably humid day with temperatures along/south of I-70
forecast to approach record values (KSTL 97 ; KCOU 98). According to
the latest HREF, KSTL has about a 40-50% chance of exceeding that
record high of 97F today, which was set back in 1931. While Columbia
has lower chances (10%) of exceeding the record high of 98F from
1895. Regardless of records or not, these incredibly hot ambient air
temperatures will combine with near record high humidity (dewpoints
in the lower 70s) to result in heat index values of 100-105F+ for
nearly all locations south of US route 36, with values near 100F
along and to the north of there. A Heat Advisory is in effect for
the southern two thirds of the area through Friday and through
Saturday for locations along/south of the I-70 corridor.

Similar to yesterday afternoon, forecast soundings indicate there
will be some areas across the warm sector that become uncapped. An
area of higher moisture convergence, in a similar location to
Wednesday, generally along and south of I-44 may lead to the
initiation of scattered showers and isolated thunderstorms. A few
pockets of marginal deep layer shear (20kts) may be enough to
briefly support a few updrafts. However, with the record strong
mid/upper-level ridge aloft leading to rising heights and
subsidence, coverage should be limited with relatively short lived
updrafts. No severe weather is expected with the stronger storms
potentially capable of small hail, gusty winds, abundant lightning,
and brief heavy rain.

By Friday, a surface high will be sliding southeastward into the
Upper Great Lakes, resulting in northeasterly flow into the
baroclinic zone that should be across the Ohio River Valley. At the
same time, the mid/upper-level ridge slowly begins to weaken leading
to slight height falls aloft. The evolution of these features may
allow the baroclinic zone to nudge southward closer to the area. As
a result, additional low rain chances (20-30%) return to northeast
MO/west-central IL Thursday night into Friday. For most of the area
south of US route 36, hot weather will persist as 850mb temperatures
remain near 20C across the area. As a result, highs should be
largely in the low/mid 90s Friday afternoon with heat index values
once again 100-105F+ where a Heat Advisory is in effect.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 236 AM CDT Thu Sep 17 2026

While the robust mid/upper-level ridge will continue to weaken into
Saturday, 850mb temperatures will be slower to cool, allowing for
continued heat into the weekend. In fact, confidence in hot ambient
air temperatures is actually the highest Saturday with LREF IQR
spreads around 3-5 degrees centered on near record values for some.
This higher confidence forecast is likely a result of stronger low-
level southwesterly flow that will result in two things. The first
being stronger WAA, mixing, and downslope warming off the Ozark
Plateau helping to boost ambient air temperatures downstream of the
Ozarks. Secondly, this wind direction should advect slightly lower
moisture (dewpoints in the 60s) into the region that will allow for
a quicker warmup thanks to a `drier` heat. Therefore, confidence is
high that dangerous and unseasonably hot conditions will continue
for most along/south of I-70 through Saturday.

A much anticipated upper-level longwave trough slowly traverses the
Intermountain West on Sunday. This feature is broad and has
continued to trend slower among long-range guidance, which for us
means continued heat. If slowing trends continue with each forecast,
there is potential that the Heat Advisory may need to be extended
for portions of the area on Sunday. This upper-level feature will
eventually aid in leeside cyclogenesis which will work to push a
cold front across the area. It seems increasingly likely that
unseasonably hot weather (highs upper 80s-mid 90s) will continue to
envelop at least parts of the area through Sunday, as this potential
cold front trends slower. Guidance is in good agreement that the
front may finally reach the area on Monday with cooler temperatures
to follow. It is still unclear just how cool the airmass advected in
will be but it appears that moisture will be a little slower to move
out, which might make it seem warmer than it is into Tuesday.
Analyzing LREF high temperature statistics for next week in the post-
frontal airmass reveal that an IQR spread of 10 degrees and
10th/90th values ranging 20 degrees indicates that confidence is
lower in the source region and identity of the airmass. With LREF
temperature IQRs ranging from highs in the 70s to 80s, it is at
least a signal that cooler weather and a return to near normal
temperatures are on the horizon by the middle of next week.

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1108 PM CDT Wed Sep 16 2026

VFR flight conditions are expected to prevail through the
period. Light and variable surface flow will turn to the west-
southwest Thursday morning across much of the area, then a weak
cold front will dip into northern Missouri and central Illinois.
There are some indications that widely scattered thunderstorms
will develop in the afternoon again south of the front...mainly
along and south of I-70. Confidence in development and coverage
is low at this time so have left mention of storms out of the
terminal forecasts.

&&

.CLIMATE...
Issued at 231 PM CDT Mon Sep 14 2026

Daily record high temperatures may be threatened for both St. Louis
(KSTL) and Columbia (KCOU) this week. NOTE: Records at Quincy (KUIN)
are not shown below due to cooler temperatures forecast:

          9/15        9/16        9/17        9/18        9/19
KSTL   101/1939     98/1954     97/1931     97/1925     96/1955
KCOU   101/1893     99/1954     98/1895     98/1893     98/2022

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 8 PM CDT Friday for MOZ041-050>052.
     Heat Advisory until 8 PM CDT Saturday for MOZ047>049-059>065-
     072>075-084-085-099.
IL...Heat Advisory until 8 PM CDT Friday for ILZ058>060-065-098-099.
     Heat Advisory until 8 PM CDT Saturday for ILZ064-069-070-074-
     079-100>102.

&&

$$

SHORT TERM...Peine
LONG TERM...Peine
AVIATION...Carney
CLIMATE...Gosselin