Hidden Valley

000
FXUS63 KLSX 060531
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1231 AM CDT Sun Sep 6 2026

.KEY MESSAGES...

- Cooler (and less humid) air will move into the region for the
  rest of the holiday weekend before a (brief) return to heat
  midweek.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 251 PM CDT Sat Sep 5 2026

As expected, heat index values are running a few degrees on 
average higher than the past several days, with most locations 
already above 105F. Most sites are likely nearing their peak, 
but may continue to tick up a bit more as temperatures climb. 

GOES-19 satellite imagery has shown pockets of agitated cumulus, and 
even some isolated shower/thunderstorm activity over the past
1-2+ hours. Most of this activity should focus across southeast
Missouri, but a stray shower/storm may be as far north as 
northeast Missouri/west central Illinois this afternoon. SPC 
mesoanalysis shows moderate instability (1000-1500+ J/kg MLCAPE)
and very weak shear. Therefore, most of these storms should be 
weak and brief in nature as individual updrafts are choked off 
by their own downdrafts. One storm in Lincoln county did have a 
weak MARC signature followed by a weak microburst (winds 30-40 
mph), so cannot rule out a stronger storm or two, but this 
should be the exception and not the rule. Nocturnal 
stabilization should help dissipate things, though there is 
about a 10-15% chance roughly along the backdoor cold front as 
it moves through overnight tonight. 

Speaking of this front, latest surface analysis is showing it 
located across southeast Iowa into eastern Illinois. This boundary 
will make it through the area late tonight into early Sunday 
morning. Weak low-level cold air advection will commence behind it, 
though surface dewpoints will still lag the front by approximately 
12 hours. Therefore, it will be another warm and humid day for parts 
of the area, mainly in central/southeast Missouri. Peak heat index 
values of near or just above 100 degrees are forecast for portions 
of these areas, and have a heat advisory until 7 PM Sunday evening. 
Widespread lower dewpoints (relative humidity) will move in areawide 
Sunday night into Labor Day. Highs in the upper 80s to low 90s are 
forecast. Dewpoints in the upper 50s to low 60s are also expected, 
so heat index values will not be any higher than the ambient air 
temperature. While those temperatures are still 5-10 degrees above 
normal, it will feel 20+ degrees cooler when comparing to today's 
heat index values.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Issued at 251 PM CDT Sat Sep 5 2026

(Tuesday - Wednesday)

Model guidance shifts the anomalous mid/upper back toward the mid-
Mississippi Valley for midweek. Temperatures will warm back up each 
day through Wednesday as a result, though relative humidity values 
Tuesday will still be low. The spread on the timing of the midweek 
cold front has lowered, and now looks like late Wednesday night into 
early Thursday. The hottest day will be ahead of this front, with 
highs Wednesday back in the upper 90s to near 100 degrees, again 
threatening daily records. Heat index values may approach 105 
degrees as well in some parts of the area, though the confidence in 
how widespread these values will be is low at this time range.


(Wednesday Night - Next Saturday)

The best chances (40-60%) of showers and thunderstorms is associated 
with the frontal boundary, though some weak low/mid level 
frontogenesis lags the surface front, which may prolong the threat. 
These rain chances also are likely to come up as the differences in 
the frontal timing decrease in the coming days. Chances for 
measurable rain on the LREF over a 24-hour period ending Thursday 
evening are 60-80%. There is skepticism on widespread significant 
rainfall at this time, but any rain would be a welcome sight given 
the dryness over the past 2+ weeks. Probabilities for 0.25"+ of rain 
are only in the 25-40% range along/north of I-70 for reference.

Behind the cold front, highs in the mid to upper 80s are expected 
for Thursday and Friday (Thursday potentially being aided by cloud 
cover and rain). While still about 5 degrees above normal, this may 
be the first time below 90 degrees for many areas. 

Uncertainty in the evolution of the pattern across the CONUS 
increases heading toward next weekend. While there is still going to 
be an omega block, where the trough-ridge-trough pattern sets up is 
the question mark. Three of the four clusters of the 500-hPa height 
pattern from the LREF have our area beneath zonal or northwest flow 
aloft (i.e., the center of the ridge is to our south or southwest). 
This in theory could yield either another cold frontal passage 
sometime or a "ridge-runner" shortwave and rain chances. One cluster 
has the amplified ridge right over us, which would mean hotter and 
dry weather. This cluster though only comprises 20% of the LREF 
membership, so this scenario is not favored at this time.

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 1231 AM CDT Sun Sep 6 2026

A band broken band of MVFR ceilings is expected to approach the 
Mississippi River through sunrise with confidence increasing in a 
brief period of impacts at KUIN before lifting and scattering 
occurs. Otherwise, dry and VFR flight conditions will prevail along 
with light northeasterly winds as a cold front clears the area early 
this morning.

&&

.CLIMATE...
Issued at 231 AM CDT Sat Sep 5 2026

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

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...Heat Advisory until 7 PM CDT this evening for MOZ041-047>050-
     059-072-099.
IL...None.

&&

$$

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