424
FXUS63 KLSX 070344
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1044 PM CDT Sun Sep 6 2026

.KEY MESSAGES...

- Less humid air is in store for Labor Day before a (brief)
  return to well-above normal temperatures Tuesday through
  Wednesday.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 224 PM CDT Sun Sep 6 2026

The backdoor cold front has lost some definition, but is
located somewhere across west/southwest Missouri. Light
east/northeast winds continue to gradually advect in lower
dewpoints into the region. This drier air will overspread the
entire region tonight and last through the Labor Day holiday.
There likely will be at least some thin cirrus overhead tonight,
but this may not be thick enough to really prevent temperatures
from dropping into the low to mid 60s for most locations. These
lows would be near to slightly above normal for the date, but
the coolest in nearly 10 days across the region. Highs on Labor
Day are forecast to top out in the mid 80s to near 90 degrees,
still about 5 degrees above normal, but much cooler than much of
the last week especially factoring in the much lower humidity
levels.

&&

.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
Issued at 224 PM CDT Sun Sep 6 2026

After a break reprieve from the well-above normal temperatures, the
summerlike heat returns on Tuesday as a near-record breaking
midlevel ridge builds back into the mid-Mississippi Valley. Highs in
the mid 90s are expected, though heat index are not forecast to be
much above actual air temperatures due to lingering lower humidity
levels.

A strong mid/upper level trough is forecast to cross the Great
Lakes/southeastern Canada on Wednesday, with a surface low crossing
Hudson Bay. A cold front draped to its southwest will move through
our region late Wednesday/Wednesday night. This front has sped up a
bit compared to how it looked yesterday, which does induce more
temperature uncertainty for much of the area. In addition, ongoing
showers and thunderstorms are forecast near the boundary with cloud
cover debris also potentially keeping temperatures a bit lower. We
are now looking at highs most likely in the low to mid 90s for most
locations, though the exact frontal timing is key. A slower (faster)
front would likely yield higher (lower) temperatures than forecast.
Dewpoints will also be coming up ahead of the front, so peak heat
index values around 105 degrees (magnitude heat advisory criteria)
are possible in parts of central and east-central Missouri as well
as southwest Illinois ahead of the boundary.

The slow-moving cold front will also be the best chance of
widespread rainfall in at least 2 weeks, if not longer. PoPs
continue to increase with each successive forecast cycle, with the
best chances (70-80%) Wednesday afternoon through early Thursday
morning. Probabilities for at least 0.25" is above 50% areawide,
with 0.50"+ topping out around 50% along the I-70 corridor in
Missouri. There may also be a low threat for a few strong to severe
showers and thunderstorms given a semi-favorable environment (30+
knots of deep-layer shear and at least moderate instability) for
organized convection along the frontal boundary itself Wednesday
afternoon/evening.

Forecast uncertainty heading toward next weekend remains high due to
a lot of spread with where the anomalous mid/upper level ridging
will be centered at the center of the omega block. While only one of
the four clusters of the 500-hPa height pattern show the ridge
moving back into the mid-Mississippi Valley, that cluster is
comprised of a plurality (35%) of LREF members. If this occurs, a
return to a hotter and dry pattern could transpire. The other three
clusters have quasi-zonal flow with ridging to the south or west.
This type of pattern should yield at least slightly cooler
temperatures (but still likely above normal) as well as additional
chances of showers and thunderstorms.

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1040 PM CDT Sun Sep 6 2026

VFR flight conditions and light easterly winds are expected to
continue to prevail through the end of the period. There`s still
a small chance (<30%) that valley fog will form over parts of
the eastern Ozarks. If fog does develop, it could locally reduce
the visibility to 1SM or less.

&&

.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...Gosselin
LONG TERM...Gosselin
AVIATION...Carney
CLIMATE...Pfahler