Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

617
FXUS63 KLSX 172354
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
654 PM CDT Thu Sep 17 2026

.KEY MESSAGES...

- Unusually hot and humid conditions will persist through at
  least Saturday, with the potential to linger into Sunday in
  some areas. Heat relief is expected early next week.

- A 20-30% chance of showers and weak thunderstorms exists this
  afternoon and again tomorrow afternoon. However, more
  substantial (60-80%) rain chances arrive late Saturday
  through Monday.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 222 PM CDT Thu Sep 17 2026

As of 2 PM, temperatures area-wide have climbed into the low to mid-
90s, with dewpoints remaining in the low to mid 70s. This
combination is yielding heat index values between 100 and 108
degrees in many areas, with the highest values largely within the
footprint of the currently valid Heat Advisory. While there will be
some fluctuations over the remainder of the afternoon as deep mixing
ensues, the affect of this will be largely imperceptible as
heat index values should remain generally level for the next few
hours. While this heat isn`t generally considered to be
"extreme" by magnitude alone, it is unusually hot and humid for
this late in the season, with moisture pooling creating a very
mid-summer-like airmass. This is largely expected to continue
tomorrow as well, although a weak cold front sinking into the
area overnight may nudge the worst of the heat ever so slightly
further south. In any case, the ongoing Heat Advisory will
remain unchanged for the time being.

As for shower/thunderstorm chances, once again a lingering area of
low-level convergence is acting as an initiating mechanism for early-
afternoon convection, along with a reinforcing outflow boundary from
the north. Deep mixing appears to have already largely eroded the
cap, allowing these boundaries to drive convection in the absence of
large-scale forcing. This activity is developing very slowly with a
very weakly sheared and only moderately unstable environment, and as
a result, isolated to widely scattered pulse-type showers and weak
thunderstorms are about all we can expect from today`s environment.
At worst, steep low-level lapse rates and fairly substantial
downdraft CAPE reveal an isolated threat for brief gusty winds with
the strongest cells, likely in the 30-50 mph range. For the most
part, areas along and south of I-70 are most favored for this
activity, and is where the initial showers have formed early this
afternoon. Between now and roughly sunset, these showers should
continue to percolate, but individual storms should not last much
more than an hour or so before being cut off by outflows.

Overnight, a weak composite cold front/outflow boundary is expected
to sink into northern MO / west-central IL, with some uncertainty
regarding exactly how far south it will reach before it retreats
northward. In addition to low clouds and potentially patchy fog,
this boundary may act as a forcing mechanism for a few weak,
elevated showers and thunderstorms north of it tomorrow morning.
This should be short-lived, though, as the boundary is expected to
move back to the north, leaving behind a very similar environment as
today. While the location of outflows may be slightly different
tomorrow, a similar day of heat and humidity with isolated/widely
scattered showers and thunderstorms can be expected.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
Issued at 222 PM CDT Thu Sep 17 2026

From Saturday onward, confidence in the day-to-day details drops
considerably thanks to the very slow southward progression of an
approaching cold front. While we are confident that heat relief will
arrive at some point early next week, exactly when it does so (and
where) will be entirely dependent on this feature. On Saturday
specifically, while eroding moisture may take the edge off of our
heat index values slightly, we have maintained our currently-valid
Heat Advisory for south of I-70 until Saturday evening. We may need
to make slight adjustments to the coverage for Saturday, but at the
present time it remains representative of expected conditions. We
will also need to monitor the potential for stray pulse-type showers
again in the afternoon, but the lowering moisture content reduces
precipitation chances considerably.

Sunday and Monday are the two most contentious days in this forecast
period, as this is the most likely window for the front to move
through our area. This feature will be driven south by a combination
of convective outflows and an extension of a sprawling area of
surface high pressure across Ontario and Quebec. However, the upper
flow pattern is rather unusual, with a high amplitude ridge to
our north being undercut by weak shortwaves. While this will be
helpful for breaking out of the worst of the heat, because the
features at play are weakly forced and the synoptic pattern is
relatively nebulous, there remains quite a bit of day-to-day
spread among ensembles. In particular temperature spreads
increase dramatically Sunday and Monday, and this can be
directly attributed to the variance in frontal timing among
members. If the front continues to trend slower (which has been
the case with similar fronts recently), we will need to monitor
Sunday for a possible extension of the Heat Advisory. We can`t
rule out a limited extension into Monday, even, but this is a
much lower probability outcome. In any case, latest guidance has
trended towards a relatively sharp temperature gradient along
this front, with a 20-30 degree difference across the state of
Missouri as the boundary is moving through, and at a minimum
this should occur area-wide by Tuesday. Behind the front, much
more seasonable temperatures are expected, although there remain
relatively high (~10 degree) temperature spreads during this
period.

Finally, chances for showers and thunderstorms also increase
considerably as the front moves through the area, and while it is
likely (60-80%) that showers and thunderstorms will be observed at
times between Saturday night and Monday night, the specific "when"
and "where" will also be closely tied to the cold front timing.
There is also at least a modest potential for a few stronger
thunderstorms thanks to a slight boost in wind shear, but this
potential remains low overall. Tuesday onward, high pressure is
expected to shunt the deeper moisture south, and precipitation
probabilities drop considerably during the middle of the week.

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 639 PM CDT Thu Sep 17 2026

This afternoon`s convection has largely dissipated. The
remainder of the evening is expected to be dry after the last
shower or two finally dies. VFR flight conditions are expected
for the remainder of the evening into the early overnight
period. However, an area of stratus which is currently over Iowa
is expected to drift into parts of northern Missouri and central
Illinois after 06Z. Current thinking is that ceilings will drop
to between 1000-1500 feet, possibly lower into IFR territory. A
few models show the stratus getting as far south as Columbia in
Central Missouri, however those are outliers and do not appear
likely at this time, so VFR conditions look likely to prevail
for most locations along and south of I-70. There is a chance
for some showers over northeast Missouri and west central
Illinois Friday morning, but the probability looks low, worth
only a prob30 at KUIN. Ceilings should gradually improve Friday,
with afternoon cumulus developing across much of the area.
Another round of scattered afternoon thunderstorms also looks
possible Friday, mainly along and south of I-70. Coverage looks
too low to include mention in the terminal forecasts at this
point.

&&

.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:

           9/17        9/18        9/19
KSTL     97/1931     97/1925     96/1955
KCOU     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.
     Air Quality Alert from 10 AM to 8 PM CDT Friday for MOZ061>065.
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.
     Air Quality Alert from 10 AM to 8 PM CDT Friday for ILZ100>102.

&&

$$

SHORT TERM...19
LONG TERM...19
AVIATION...Carney
CLIMATE...Gosselin