Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

513
FXUS63 KLSX 311844
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
144 PM CDT Mon Aug 31 2026

.KEY MESSAGES...

- A late summer heat wave will continue through at least Friday.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 142 PM CDT Mon Aug 31 2026

Milder temperatures are forecast overnight tonight due to a bit
more of a persistent southerly breeze, as well as air mass
modification. Even with a clear sky, almost everyone should
experience lows in the 70s. There may be a few favored river
valleys however that drop into the upper 60s across southeast
Missouri. On the other end of the spectrum, lows near 80 degrees
are favored within the urban heat island of metropolitan St.
Louis.

There is very little/no change in the overall pattern across the
region on Tuesday. Highs probably will be on average 1-2F warmer
than this afternoon due to a warmer start to the day. Temperatures
are forecast to top out in the mid to upper 90s, though a few spots
in central Missouri and/or near St. Louis may hit the century mark.
Similar to today (and yesterday), dewpoints (relative humidity) look
relatively modest. In fact, those values are expected to be closer
to average values for the first of September. That helps keep heat
index readings closer or just above the actual air temperature.

&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
Issued at 142 PM CDT Mon Aug 31 2026

(Tuesday Night - Friday Night)

The stagnant weather pattern across the CONUS should continue
through at least the end of the work week, with very anomalous (500-
hPa heights >99th percentile of climatology) mid/upper level ridging
across the mid-Mississippi Valley. Temperatures at 850 hPa also
remain > +20C, or above the 95th percentile of climatology. We
should see a continuation of high temperatures in the mid to upper
90s each day, with lows in the 70s to near 80 degrees. There remains
some skepticism at least of the dewpoint increase forecast by the
NBM. With little/no change in the ambient air mass and any frontal
boundary north of the CWA (where dewpoints may pool), it is
difficult to see much of an increase, if any at all.
Evapotranspiration rates are quite a bit less than 4-6 weeks ago. In
addition, if anything, the soils will continue to dry out with each
successive hot and dry day that transpires. Some increase may be
warranted due to a bit better moisture return from the Gulf, but it
is taking the long path across the northern Gulf into Texas and then
north/northeast through the south-central Plains into the mid-
Mississippi Valley. However, southwest surface winds are also
downsloping off of the Ozark Plateau so that may counteract the
slightly higher dewpoints at least attempting to advect into the
region. Long story short, peak heat index values may tend to
increase slightly, with more widespread 100-105 degree values toward
the end of the week. Thursday and Friday afternoon are more likely
to see more pockets of 105+ values as well. Even if these higher
heat index values verify, they would be quite a bit less than
earlier heat waves this summer, due to lower humidity. In addition,
the lower sun angle of early September is a far cry from June/July
(similar to early/mid April), days are shorter, and there has been a
whole summer of acclimation to the heat.


(Saturday - Labor Day)

Forecast uncertainty does increase a bit for the Labor Day holiday
weekend. There remains a signal for the mid/upper level ridge to
shift slightly westward more into the Plains, with a trough digging
into the northeast. This is a favorable pattern for a backdoor cold
front to pass through the Great Lakes and much of the Ohio Valley,
but there is less confidence in how far southwest the boundary will
reach. Backdoor fronts are often difficult to pin down this far out,
and this is no exception. How far that front is able to make it into
our area, if at all, likely will come down to how far west the ridge
axis retrogrades, as well as how strong the trough is coming into
the northeast. Two of the four clusters from the LREF of the 500-hPa
height pattern would be favorable for the boundary to make it to
near the Mississippi River, while the other two barely make it into
south-central Illinois. This means that the late summer heat wave
probably will last into, if not through (and possibly beyond), the
holiday weekend for central/southeast Missouri. There is less
confidence the farther northeast you head, but at least some cooling
seems likely especially as you get into west central and south-
central Illinois. An increase in cloud cover and afternoon/early
evening thunderstorm chances (20-30%) could also help take the edge
off of the heat. While the specific temperatures are not certain,
even the potential for a backdoor cold front would probably only get
readings back within 5-8 degrees of normal. Dewpoints also likely
will increase near this front, so heat index values may stay near
100 degrees. It is looking quite likely that this first full week of
September will be one of the warmest, if not the warmest, at our
official climate stations (KSTL, KCOU, KUIN). Taking a peek toward
week 2, the anomalously warm pattern is likely to continue, and if
so, could mark the warmest first half of September on record. In
other words, at this point in time there is no DEFINITIVE end in
sight for this ongoing heat wave.

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1118 AM CDT Mon Aug 31 2026

Dry/VFR conditions with light south/southwest winds are expected
to continue through Tuesday morning.

&&

.CLIMATE...
Issued at 1118 AM CDT Mon Aug 31 2026

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

September 1
KSTL: 104/2011...82/2011
KCOU: 102/1913...76/1913
KUIN: 104/2011...78/2011

September 2
KSTL: 101/2011...80/2011
KCOU: 100/1913...77/1913
KUIN: 103/1913...80/1913

September 3
KSTL: 101/2011...80/1913
KCOU: 103/1939...75/1939
KUIN: 103/1913...78/1913

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

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 8 PM CDT Friday for MOZ018-019-026-027-
     034>036-041-042-047>052-059>065-072>075-084-085-099.
IL...Heat Advisory until 8 PM CDT Friday for ILZ058>060-064-065-069-
     070-074-079-095>102.

&&

$$

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