Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

210
FXUS63 KLSX 071923 CCA
AFDLSX

Area Forecast Discussion...CORRECTED
National Weather Service St Louis MO
223 PM CDT Mon Sep 7 2026

.KEY MESSAGES...

- Hot conditions resume tomorrow and peak in some areas Wednesday
  as humidity increases, but a cold front may limit how much of
  the area sees truly impactful heat and humidity.

- Confidence is growing in a round of widespread showers and
  thunderstorms from late Wednesday afternoon through early
  Thursday morning. Locally heavy rain and occasional strong
  downburst winds are the most likely hazards.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 214 PM CDT Mon Sep 7 2026

After a brief two-day reprieve from our extended heatwave today and
yesterday, attention turns back to its return tomorrow and
Wednesday. While this iteration should be relatively short-lived,
we are monitoring the potential for additional heat headlines,
particularly during the day Wednesday. Before we get there
though, be sure to relish today`s relative lack of heat and
humidity, as low dewpoints in the 50s are taking the edge off of
highs that are on their way to reaching the mid 80s to 90
degrees. While still several degrees above average, this is a
far cry from the heat we saw last week, and more comfortable
than what we will observe tomorrow and Wednesday.

Overnight tonight, surface winds will gradually turn to the south in
response to a passing low pressure system across the Canadian
Prairie, kicking off another round of warm air advection. When
combined with 500 mb geopotential heights reaching near 5940m (very
close to the climatological maximum), this should boost actual
surface temperatures back into the mid to even upper 90s area-wide,
with perhaps a couple spots in central Missouri touching the century
mark yet again. Fortunately though, dewpoints will be slower to
increase, and most areas will see a heat index nearly matching the
actual temperature, precluding the need for another Heat Advisory
for now (although we will need to watch those central MO counties
closely).

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Issued at 214 PM CDT Mon Sep 7 2026

By Wednesday, the combination of continued southwesterly flow and
moisture pooling ahead of an advancing cold front is expected to
boost dewpoints back into the low/mid 70s, with some compressional
warming also possible ahead of the front. This leads to actual
temperatures once again in the mid/upper 90s but with much more
noticeable humidity, and current ensemble mean forecasts suggest
that areas that stay south of the front will see heat index values
in the 105-110 degree range Wednesday afternoon. The key point of
contention, then, is how quickly the front will push through the
area, as this will determine how much of our area will see another
round of headline-worthy heat. Since this is a one-day thing (or
perhaps two for parts of central MO) and because we are still 48
hours away, we will hold off on issuing any heat headlines (most
likely in the form an Advisory) until we can narrow this down over
the next 24 hours. However, we suspect that at least some areas
along and south of I-70 are more likely than not to have some form
of heat headline by this time tomorrow. In fact, some areas north of
I-70 will likely be cooler Wednesday than Thursday thanks to the
front, and perhaps some lingering showers and cloud debris from
lingering nocturnal convection along our northern fringes.

During afternoon, the focus shifts to the passage of a slow-moving
cold front, which will be driven south by a shortwave moving
through the upper Mississippi Valley and Great Lakes. As
mentioned in the short term discussion above, the combination of
prolonged moisture advection and pooling ahead of the front
will boost precipitable water values to at least the 95th
percentile of climatology (2+ inches) in a narrow corridor as it
moves through Missouri and Illinois. Modest instability is also
expected to develop, with roughly 750-1500 J/kg of MU CAPE on
either side of the boundary. Competing with this instability
will be what appears to be a strong-warm sector cap, though,
which limits our confidence in surface-based storms developing
in the warm sector. Not only this, but dynamic forcing remains
relatively weak along the front, and this limits our confidence
in storms firing anywhere but right along and behind it. Still,
there is enough shear, moisture, and instability to support some
organized updrafts capable of locally heavy rain and occasional
downbursts, although the latter may be limited if storms remain
elevated.

Not only this, but storm motions are likely to move nearly
parallel to the boundary itself, and although outflow should
keep the front moving south, some limited training of storms is
possible and we can`t rule out a few streaks of heavier rain
(1-3 inches). Rain rates will likely be quite high in the
stronger cores, so these amounts shouldn`t be too difficult to
achieve even if the coverage is limited. Fortunately, the
flooding threat should be rather limited, as a lack of recent
rain should help the soil soak most of this (much needed) rain
up. Showers and thunderstorms should slowly move south between
late Wednesday afternoon and early Thursday morning, with the
most widespread coverage overnight.

From Thursday onward, confidence in the day-to-day details drops
considerably. On the one hand, we do expect a significant
temperature drop from Wednesday to Thursday, with ensemble
temperature forecasts near seasonal averages Thursday afternoon with
minimal spread. There is also likely to be a lull in precipitation
during the day Thursday in the wake of the activity the previous
night. Beyond that, a slowly building ridge across the Mississippi
Valley and eastern CONUS is more likely than not to drive a gradual
warming trend back to at least slightly above average, with
continued uncertainty regarding the strength/position of the ridge.
Not only does this impact the timing and magnitude of rising
temperatures, but also our exposure to additional shortwaves and
rounds of showers/storms. Once such shortwave seems to have a
bit more broad support among ensembles late Friday/early
Saturday, with current precipitation probabilities of 30-50%. At
the moment, this represents the next best opportunity for
additional rain beyond Wednesday.

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1146 AM CDT Mon Sep 7 2026

VFR conditions are expected throughout the period. Light east
winds will switch to southerly tomorrow morning, but will remain
generally light. No precipitation is expected.

&&

.CLIMATE...
Issued at 258 AM CDT Sun Sep 6 2026

Record Highs...Record Max Lows (value/year):

September 8:
KSTL: 101/1947...77/1939
KCOU: 103/1947...75/1922
KUIN: 102/1947...79/1922

September 9:
KSTL: 101/1925...79/1925
KCOU:  99/1925...76/1947
KUIN: 100/2013...75/1912

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.

&&

$$

SHORT TERM...19
LONG TERM...19
AVIATION...19
CLIMATE...Pfahler