Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

624
FXUS63 KLSX 181116
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
616 AM CDT Fri Sep 18 2026

.KEY MESSAGES...

- Summer-like heat and humidity continue at least through
  Saturday.

- A slow moving cold front brings rain chances Sunday and Monday
  along with a shift toward cooler temperatures next week.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 254 AM CDT Fri Sep 18 2026

A weak cold front is draped roughly west to east across the
center of our forecast area this morning, near interstate 70.
Winds to the north of the front are very light out of the NNE
while to the south winds are very light out of the south. The
surface high which is driving this front is rolling east from
Ontario into Quebec today, so this front has already lost its
push southward and will begin to slowly ooze back northward
today. Moisture pooling near the boundary will lead to
exceptionally high dewpoints in the mid 70s, near record levels
for this time of year. Another day of highs in the mid to upper
90s will combine with this humidity to drive heat index values
up to around 105 degrees across much of the Heat Advisory area.
Temperatures will be a few degrees cooler to the north,
especially where early morning low clouds take a while to burn
off.

The more widespread synoptically driven showers and
thunderstorms remain focused to our northwest where a low level
jet is streaming up and over the front. This will continue to
shift east across Iowa this morning but is largely expected to
bypass our area to the north. We will have the potential for a
few afternoon thunderstorms to pop up especially near the
remnant frontal boundary. Considering the weak shear available,
these will be of the pulse variety with quick updrafts and
downdrafts leading to very localized heavy rain or gusty winds,
but most areas will remain unaffected. The front lifts back
north tonight and we may again see a few showers or storms form
along and to the north of that boundary as it lifts back north.
By Saturday all areas will be south of the front again and in
the heat. Mercifully, though, the more extreme dewpoints will
also lift north with the front leading to overall less humid
conditions. Heat index values will still be near 100 degrees for
much of the area though. For the southern half of the area where
the Heat Advisory continues into Saturday this will be day 5 of
this heat. Shower and thunderstorm chances will be focused
largely along and north of the front where the better moisture
will be, thus largely north of our area on Saturday.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Issued at 254 AM CDT Fri Sep 18 2026

A slow moving trough currently over eastern Oregon will very
slowly push east this weekend, eventually nudging the ridge (and
the accompanying heat) south. The trough emerges out into the
Northern Plains on Sunday with a cold front sagging southward
through Iowa. This is a heat ending cold front, but the heat
will already be waning ahead of it. The ridge will be flattening
out and temperatures on the warm side of the front will top out
a few degrees cooler than Saturday, only in the low to mid 90s.
With dewpoints continued modest, in the 60s, the likelihood of
another day of 100+ heat index values is lower. Thus we`ll keep
the Heat Advisory ending on Saturday for now. The front
gradually pushes southward across the forecast area Sunday into
Monday with daytime highs maxing out in the 70s north of the
front compared to the 90s ahead of it. There`s more point based
uncertainty on the temperature forecast during this time period
due to timing uncertainty with that front. Subtle shortwave
troughs tracking east ahead of the main wave will cause subtle
movements in the surface front as well. Bursts of convection
along and behind it could also help to push the front southward.

Shower and thunderstorm chances increase along and behind the
front as it makes its way slowly southward. The timing of each
round will be influenced by those subtle waves tracking along it
and the corresponding moisture fluxes near the boundary. We
don`t expect it to be a nonstop rain, but it will be an overall
wetter period. While the timing and location uncertainty limits
the current precipitation chances to 70 percent or so in any one
forecast period, confidence is higher that just about the entire
region will get rain at some point as the front moves through,
with the best chances Sunday and Monday. For example, among the
long range ensemble guidance, nearly 100 percent of members
produce measurable rainfall in the 24 hours ending Monday
afternoon indicating that when it`s all said and done just about
everyone should have gotten wet. The average QPF from the same
ensemble group is a modest 0.3 to 0.5 inches, though some
locally higher amounts over 1 inch are possible where
thunderstorms linger.

Long range guidance is in generally good agreement that the
trough shifts east and the front pushes far enough south that
precipitation chances end by Tuesday morning. While the coolest
air takes aim further east toward New England, we still do get a
noticeably cool push locally as well. Ensemble mean 925mb
temperatures around +15C translates to about 70 degrees in a
fully mixed environment. If clouds are slow to clear then it may
be even cooler than that. The surface high bringing this air
mass lingers over Wisconsin for several days, keeping winds
locally out of the north or northeast for several days. While
the air mass will gradually modify over time, we don`t see a
return of the heat and humidity of recent weeks.

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 616 AM CDT Fri Sep 18 2026

A weak cold front remains draped near I-70 from central Missouri
through the St Louis metro at this time. Winds along the front
are very light and variable. To the north of the front, a low
IFR deck of clouds has been advancing slowly southward this
morning. It`s already made it through Quincy and has a chance of
reaching Columbia this morning before stalling and lifting.
Expect the ceiling at Quincy to gradually lift through the
morning before scattering out in the afternoon. There`s a low
chance of a pop up shower or thunderstorm this afternoon, mainly
in the vicinity of that front. However, confidence is too low in
this impacting any TAF site to include a mention at this time.

&&

.CLIMATE...
Issued at 254 AM CDT Fri Sep 18 2026

Near record heat and very warm overnight lows are expected the
next couple of days.

Record High Temperatures:

September 18:
KSTL: 97/1925

September 19:
KSTL: 96/1955

Record High Minimum Temperatures:

September 19:
KSTL: 76/1925
KCOU: 75/2022

September 20:
KSTL: 75/2024
KCOU: 73/2018

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 8 PM CDT this evening for MOZ041-050>052.
     Heat Advisory until 8 PM CDT Saturday for MOZ047>049-059>065-
     072>075-084-085-099.
     Air Quality Alert from 10 AM this morning to 8 PM CDT this
     evening for MOZ061>065.
IL...Heat Advisory until 8 PM CDT this evening for ILZ058>060-065-
     098-099.
     Heat Advisory until 8 PM CDT Saturday for ILZ064-069-070-074-
     079-100>102.
     Air Quality Alert from 10 AM this morning to 8 PM CDT this
     evening for ILZ100>102.

&&

$$

SHORT TERM...Kimble
LONG TERM...Kimble
AVIATION...Kimble
CLIMATE...Kimble