911
FXUS63 KLSX 061121
AFDLSX

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

.KEY MESSAGES...

- Another day of heat index values around 100 F is forecast for
  central and southeastern MO, but cooler temperatures and lower
  humidity will arrive elsewhere today, becoming areawide by
  Monday.

- Heat and near-record breaking temperatures will briefly return
  Tuesday into Wednesday.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 258 AM CDT Sun Sep 6 2026

A weak backdoor cold front is tracking the CWA early this morning,
expected to finally clear central and southeastern MO through
sunrise. A few showers or weak thunderstorms cannot be ruled out
with the front, but more importantly it marks the onset of weak, low-
level CAA. A relative maximum in dewpoints (70 to 78 F) exists in a
corridor extending 150 miles behind the front which will linger
across central and southeastern MO into early afternoon, supporting
heat index values near or just above 100 F with high temperatures in
the mid-90s F. Elsewhere to the east, high temperatures will be in
the upper 80s to low-90s F with dewpoints falling into the 60s F.
Through Monday (Labor Day), the airmass will cool and dry further as
a surface high pressure center builds across the Great Lakes.
Therefore, high temperatures on Monday will reach the mid-80s to
around 90 F and dewpoints will lower to between the mid-50s and low-
60 F with the aid of diurnal mixing. Heat index values on Monday
will be well below 100 F CWA-wide, closer to air temperatures, and
20+ F lower than than what was seen Saturday. It is worth noting,
however, that these temperatures are still above average for early
September--a testament to just how anomalous the heat has been.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Issued at 258 AM CDT Sun Sep 6 2026

By Tuesday and Wednesday, the upper-level ridge that had been in
place across the region last week, shifts back eastward, quickly
retreating the surface high pressure and enabling low-level
southwesterly WAA that warms 850-hPa temperatures back to 21 to 23 C
(95-99th climatological percentile). As a result, high temperatures
will once again warm into the mid-90s to around 100 F, nearing daily
records. Fortunately, humidity/dewpoints will be slow to increase
through Tuesday. On Wednesday, a cold front is progged to advance
southward into the CWA, favoring a more substantial increase in
dewpoints with more time for advection and moisture pooling near the
front. The trade off is that this front makes temperatures less
clear, especially north of I-70. To the south ahead of the front,
favorable placement of the low-level thermal ridge and west-
southwesterly flow may actually support Wednesday being the hottest
day of this shorter heat period in that area. There, it will not be
difficult for heat index values to at least flirt with 100 F, but
probabilities of 105+ F heat index values are 5 percent or less in
global ensemble model guidance. Although these probabilities are
likely underdone, it is not yet clear if heat index values will
exceed 105 F outside of a sporadic/localized basis (single day Heat
Advisory Criteria).

Ensemble model 24-hour probabilities of measurable rainfall with
showers and thunderstorms increase to 60 to 90 percent Wednesday
evening with the cold front, persisting through late Thursday as
frontogenetical forcing lingers behind the front that eventually
advances south of the CWA in a quasi-zonal upper-level flow regime.
The post-frontal airmass, cloud cover, and precipitation support
Thursday being the coolest day of the week with high temperatures in
the 80s F, but it is conceivable that temperatures even remain in
the 70s F if/where clouds and precipitation are persistent.

Friday through next weekend, global model guidance is in agreement
that the upper-level wave pattern will amplify across the CONUS but
ensemble model 500-hPa height cluster analysis reveals that there is
considerable disagreement on the timing of an eastward propagating
upper-level trough or if it becomes cutoff near the Pacific
Northwest and a downstream ridge dominates much of the CONUS. This
range of scenarios leads to agreement in at least some warming of
temperatures further above average into the weekend, but the NBM
interquartile range increases to 15 F by Sunday with uncertainty if
or when there is a cold frontal passage. Similarly, ensemble model
membership shows very little cohesion with regard to rainfall,
leading to broad-brushed (in time and space) lower probabilities of
measurable rainfall that continue through the weekend.

&&

.AVIATION /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 620 AM CDT Sun Sep 6 2026

Light ENE winds are expected today. Although VFR conditions are
expected for most of the day, there is an area of IFR ceilings
pushing southwest through Illinois. This is likely to arrive in
the St Louis metro within the next 1-2 hours, but it`s progress
will likely stall as daytime heating commences. This cloud deck
is expected to lift and scatter out through the morning hours
leading to VFR conditions returning. The most likely areas
affected are UIN and CPS, with STL also seeing a good chance of
IFR this morning.

&&

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

&&

$$

SHORT TERM...Pfahler
LONG TERM...Pfahler
AVIATION...Kimble
CLIMATE...Pfahler