011 
FXUS63 KLSX 071919
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
219 PM CDT Mon Sep 7 2026

.KEY MESSAGES...

- After cooler and less humid conditions today, heat will
  briefly return Tuesday into Wednesday, but heat index are
  expected to be overall lower than with the previous heatwave.

- The next opportunity for showers and thunderstorms will come
  with a cold front Wednesday into early Thursday.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 214 PM CDT Mon Sep 7 2026

After a brief two-day reprieve from our extended heatwave today and 
yesterday, attention turns back to its return tomorrow and 
Wednesday. While this iteration should be relatively short-lived, 
we are monitoring the potential for additional heat headlines, 
particularly during the day Wednesday. Before we get there 
though, be sure to relish today's relative lack of heat and 
humidity, as low dewpoints in the 50s are taking the edge off of
highs that are on their way to reaching the mid 80s to 90 
degrees. While still several degrees above average, this is a 
far cry from the heat we saw last week, and more comfortable 
than what we will observe tomorrow and Wednesday.

Overnight tonight, surface winds will gradually turn to the south in 
response to a passing low pressure system across the Canadian 
Prairie, kicking off another round of warm air advection. When 
combined with 500 mb geopotential heights reaching near 5940m (very 
close to the climatological maximum), this should boost actual 
surface temperatures back into the mid to even upper 90s area-wide, 
with perhaps a couple spots in central Missouri touching the century 
mark yet again. Fortunately though, dewpoints will be slower to 
increase, and most areas will see a heat index nearly matching the 
actual temperature, precluding the need for another Heat Advisory 
for now (although we will need to watch those central MO counties 
closely).

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Issued at 214 PM CDT Mon Sep 7 2026

By Wednesday, the combination of continued southwesterly flow and 
moisture pooling ahead of an advancing cold front is expected to 
boost dewpoints back into the low/mid 70s, with some compressional 
warming also possible ahead of the front. This leads to actual 
temperatures once again in the mid/upper 90s but with much more 
noticeable humidity, and current ensemble mean forecasts suggest 
that areas that stay south of the front will see heat index values 
in the 105-110 degree range Wednesday afternoon. The key point of 
contention, then, is how quickly the front will push through the 
area, as this will determine how much of our area will see another 
round of headline-worthy heat. Since this is a one-day thing (or 
perhaps two for parts of central MO) and because we are still 48 
hours away, we will hold off on issuing any heat headlines (most 
likely in the form an Advisory) until we can narrow this down over 
the next 24 hours. However, we suspect that at least some areas 
along and south of I-70 are more likely than not to have some form 
of heat headline by this time tomorrow. In fact, some areas north of 
I-70 will likely be cooler Wednesday than Thursday thanks to the 
front, and perhaps some lingering showers and cloud debris from 
lingering nocturnal convection along our northern fringes. 

During afternoon, the focus shifts to the passage of a slow-moving 
cold front, which will be driven south by a shortwave moving 
through the upper Mississippi Valley and Great Lakes. As 
mentioned in the short term discussion above, the combination of
prolonged moisture advection and pooling ahead of the front 
will boost precipitable water values to at least the 95th 
percentile of climatology (2+ inches) in a narrow corridor as it
moves through Missouri and Illinois. Modest instability is also
expected to develop, with roughly 750-1500 J/kg of MU CAPE on 
either side of the boundary. Competing with this instability
will be what appears to be a strong-warm sector cap, though, 
which limits our confidence in surface-based storms developing 
in the warm sector. Not only this, but dynamic forcing remains 
relatively weak along the front, and this limits our confidence 
in storms firing anywhere but right along and behind it. Still, 
there is enough shear, moisture, and instability to support some
organized updrafts capable of locally heavy rain and occasional
downbursts, although the latter may be limited if storms remain
elevated. 

Not only this, but storm motions are likely to move nearly 
parallel to the boundary itself, and although outflow should 
keep the front moving south, some limited training of storms is 
possible and we can't rule out a few streaks of heavier rain 
(1-3 inches). Rain rates will likely be quite high in the 
stronger cores, so these amounts shouldn't be too difficult to 
achieve even if the coverage is limited. Fortunately, the 
flooding threat should be rather limited, as a lack of recent 
rain should help the soil soak most of this (much needed) rain 
up. Showers and thunderstorms should slowly move south between 
late Wednesday afternoon and early Thursday morning, with the 
most widespread coverage overnight. 

From Thursday onward, confidence in the day-to-day details drops 
considerably. On the one hand, we do expect a significant 
temperature drop from Wednesday to Thursday, with ensemble 
temperature forecasts near seasonal averages Thursday afternoon with 
minimal spread. There is also likely to be a lull in precipitation 
during the day Thursday in the wake of the activity the previous 
night. Beyond that, a slowly building ridge across the Mississippi 
Valley and eastern CONUS is more likely than not to drive a gradual 
warming trend back to at least slightly above average, with 
continued uncertainty regarding the strength/position of the ridge. 
Not only does this impact the timing and magnitude of rising 
temperatures, but also our exposure to additional shortwaves and 
rounds of showers/storms. Once such shortwave seems to have a 
bit more broad support among ensembles late Friday/early 
Saturday, with current precipitation probabilities of 30-50%. At
the moment, this represents the next best opportunity for 
additional rain beyond Wednesday.

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1146 AM CDT Mon Sep 7 2026

VFR conditions are expected throughout the period. Light east
winds will switch to southerly tomorrow morning, but will remain
generally light. No precipitation is expected.

&&

.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...19
LONG TERM...19
AVIATION...19
CLIMATE...Pfahler