Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

450
FXUS63 KLSX 041951
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
251 PM CDT Fri Sep 4 2026

.KEY MESSAGES...

- Late summer heat will continue on Saturday before temperatures cool
  on Sunday and Labor Day. The relative humidity will also be
  much lower on Labor Day compared to earlier in the holiday
  weekend.

- Another period of heat and near-record breaking temperatures
  is forecast Tuesday into Wednesday.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 250 PM CDT Fri Sep 4 2026

Surface observations so far today, not surprisingly, are very
similar to yesterday at this time. On average, heat index values
are roughly ~1F higher, but by and large most sites will top
out in the 100-105F range. A few more locations today probably
will crest 105F, mainly in the St. Louis area southeast into
southwest and south- central Illinois. GOES-19 visible satellite
imagery shows scattered diurnal cumulus over a greater portion
of the CWA early this afternoon. A few brief, spot showers
cannot be ruled out, but by and large dry weather is forecast
through tonight.

Persistence continues to be an excellent first guess in terms of a
forecast for Saturday as the late summer heat wave continues.
Temperature at 850 hPa do warm about 0.5-1.0C, which suggests most
locations may top out a degree or so warmer than today. That also
should entail more widespread 105F+ peak heat index values, likely
making it the hottest of this first stretch of September heat. There
is a bit better of a signal for isolated showers, and perhaps a few
rumbles of thunder, Saturday afternoon across southeast Missouri and
southwest Illinois.

&&

.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
Issued at 250 PM CDT Fri Sep 4 2026

(Saturday Night - Labor Day)

Deterministic model guidance is now showing a quicker passage of the
backdoor front, moving through much of the area Saturday night
before clearing central Missouri sometime Sunday morning. The
dewpoint (relative humidity) decrease however lags behind the front
about 9-12 hours so areas along/west of the Mississippi River will
stay humid into Sunday afternoon. Another day of 100-105F peak heat
index values are possible for parts of central, east central, and
southeast Missouri. Due to some lingering uncertainty on the timing
of the front, we have elected to hold off on extending any heat
headlines at this time. However, a conversion to an advisory at
least for parts of central/southeast Missouri seem more likely than
not, but this will be further evaluated over the next 12-24 hours.

Labor Day continues to look like the "pick" day of the holiday
weekend, with cooler temperatures moving in, but more importantly,
much lower dewpoints (humidity levels). The signal for these lower
humidity values have increased substantially compared to yesterday
from the LREF. Probabilities for dewpoints below 65F are 70+% now
areawide, with values in the mid to upper 50s likely (60-80%)
roughly along/east of the Mississippi River. While high temperatures
are still expected to be quite warm for early September (upper 80s
to low 90s), the much lower humidity levels means a definitive
(albeit fairly short) break from the ongoing heat.


(Tuesday - Next Friday)

After a brief reprieve from the heat, the record-breaking mid/upper
level ridge shifts back east into the mid-Mississippi Valley by
Tuesday. Temperatures will increase back into the mid to upper 90s,
though the good news is humidity levels will be slower to increase.
Therefore, at least a majority of the area should see heat index
values stay below the century mark.

Wednesday continues to have the best more potential for possible
record-breaking heat ahead of an approaching cold front. Speaking of
which, there has been a notable shift in both the GEFS/EPS toward a
stronger midlevel trough moving across the Great Lakes. This has led
to increased confidence that the midweek cold front will move
through the area, the question is exactly when? Guidance has about a
12 hour spread, with late Tuesday night/Wednesday morning on the
earlier end of the spectrum with Wednesday afternoon/night on the
later end. This spread with respect to the frontal timing is why
there is low confidence with specific high temperatures for
Wednesday. However, IF the front is slow enough to not impact
temperatures, I still think that Wednesday may be the hottest day of
the month with widespread upper 90s to low 100s possible. In terms
of precipitation chances, the frontal passage is probably are best
chance (20-30%) for seeing showers and thunderstorms.

Cooler temperatures are likely behind the midweek cold front, but
how much cooler is uncertain. Given recent trends in ensemble
guidance, it appears more likely that we should see highs in the 80s
for the first time since late August for Thursday and next Friday.
There is some bad news though for anyone hoping for Fall. The large-
scale mid/upper air pattern across the CONUS characterized by a
large omega block does not look to change. Furthermore, ensemble
guidance suggests the ridge may move back into the lower to mid
Mississippi Valley. If this indeed were to happen, another bout of
well-above normal temperatures would likely occur. However, before
that may occur, our area may be in a more favorable spot (on the
northeast periphery of the ridge) late next week for a possible
"ridge-runner" or two. Given how dry it has been the past couple of
weeks, any rain would be a welcome sight for most.

&&

.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1219 PM CDT Fri Sep 4 2026

Dry/VFR conditions with diurnal cumulus dissipating this evening
along with winds becoming light/variable are expected.

&&

.CLIMATE...
Issued at 455 AM CDT Fri Sep 4 2026

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

September 4
KSTL: 104/1954...81/1925
KCOU: 103/1913...73/1947
KUIN: 105/1913...75/1947

September 5
KSTL: 103/1954...80/1899
KCOU: 103/1899...75/1983
KUIN: 100/1925...76/1912

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...Extreme Heat Warning until 8 PM CDT Saturday for MOZ018-019-
     026-027-034>036-041-042-047>052-059>065-072>075-084-085-
     099.
IL...Extreme Heat Warning until 8 PM CDT Saturday for ILZ058>060-
     064-065-069-070-074-079-095>102.

&&

$$

SHORT TERM...Gosselin
LONG TERM...Gosselin
AVIATION...Gosselin
CLIMATE...Pfahler