261
FXUS63 KLSX 171716
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1216 PM CDT Mon Aug 17 2026

.KEY MESSAGES...

- Temperatures more typical of late summer are forecast this week.

- Multiple rounds of showers and thunderstorms are possible
  Tuesday night through Thursday.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 317 AM CDT Mon Aug 17 2026

The latest surface analysis shows that the cold front has
pushed well off to the southeast, with winds veering to the
north. Scattered showers and thunderstorms lag behind the
surface front, and seem tied to the 850-hPa frontal zone. This
boundary should also push off to the south and east over the
next 1-2 hours, taking with it any lingering chance of rain. The
remainder of the day will be dry with partial sunshine. Strong
low-level cold air advection will continue through the day,
helping keep this afternoon`s highs to be mainly in the low to
mid 80s. These readings would be a few degrees below normal for
the date. In addition, more typical dewpoints (humidity) levels
will advect southeastward behind last night`s cold frontal
passage. After a largely hot and humid past 7+ days, you will
certainly notice the air mass change if you go outdoors today.

Tonight should be the coolest night in at least 8-9 days (and
possibly the entire month of August at some locations!), with a weak
ridge of surface high pressure across the region. The combination of
light/variable winds near this anticyclone and at least a partially
clear sky should lead to favorable conditions for radiational
cooling. Lows are forecast to bottom out in the low to mid 60s
across the bi-state region, with the exception of the urban corridor
of St. Louis where upper 60s are expected. A few spots may even
flirt with the upper 50s within favored valley terrain across
portions of east central and southeast Missouri.

Increasing mid/high level clouds are forecast on Tuesday beneath
northwest flow aloft. There may even be a few isolated rain showers
late in the afternoon across parts of northeast Missouri/west
central Illinois, but largely dry weather is expected. Temperatures
across central/southeast Missouri should rebound back into the upper
80s to low 90s as surface winds turn out of the southwest to the
west of the surface high moving into the Ohio Valley.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Issued at 317 AM CDT Mon Aug 17 2026

(Tuesday Night - Thursday)

The coverage of showers and thunderstorms should increase across the
area quickly Tuesday night as the nocturnal low-level jet
strengthens and a fairly potent midlevel shortwave trough approaches
from the northwest. Storms look pretty progressive along/north of a
weak cold frontal passage, which should limit rainfall totals. There
is a conditional threat of a few strong to severe thunderstorms, but
there is low confidence that the area will be unstable enough to
sustain severe convection. Moisture return ahead of this boundary is
the big question mark, and therefore, instability. Recent trends
have been more toward limited moisture return, with SBCAPE
probabilities of at least 1000 J/kg late Tuesday night from the
GEFS/GEPS dropping into the ~10% range. The machine-learning CSU
guidance also has trended downward with its severe probabilities
across the region Tuesday night. However, individual deterministic
models (e.g., NAM/ECMWF) show at least the potential for higher
instability, so this will be a period we will need to watch over the
next 12-24+ hours. What is not a question mark is the anomalously
strong deep-layer shear, which should be in the 35-45+ knot range.
Therefore, if there is sufficient instability, thunderstorms would
likely organize into a line capable of producing damaging wind
gusts.

Another (slower) moving midlevel shortwave trough is forecast to
cross the Mississippi Valley between Wednesday night and Thursday.
This feature should also help yield another round of showers and
thunderstorms, with the best chances (50-70%) along/south of I-70
near the 850-hPa frontal zone. This frontal zone is nearly quasi-
stationary, only oozing southward with time. Given the potential for
a long(er) duration of forcing for ascent and the potential for
showers and thunderstorms, there may be a low threat for heavy
rainfall and flash flooding across the eastern Ozarks during this
time period.


(Thursday Night - Sunday)

A surface ridge is likely to move into the mid-Mississippi Valley to
end the work week heading into the weekend. Near to below normal
temperatures and mostly dry weather is forecast Thursday night
through Saturday. Ensemble guidance shows a broad mid/upper level
trough digging into the Great Lakes/northeast by Sunday, though
there are differences with the strength of the trough. Two of the
four clusters of the 500-hPa pattern on the LREF are pretty sharp
with this feature, which would likely bring a shot of much cooler
temperatures into the region. These two clusters comprise about half
of the total LREF members. Meanwhile, the other half shows a much
weaker trough. What is interesting is that a pretty large majority
of both the EPS/GEPS favor the stronger trough and cooler air mass
making into our region while the GEFS has an even larger percentage
in the weaker trough/cold front scenario. Most typically, when 1 of
the 3 ensemble suites does not match the other two, it is not
correct. I would therefore at least slightly favor the cooler
temperature scenario to end the weekend heading into early the
following week.

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1214 PM CDT Mon Aug 17 2026

Dry and VFR flight conditions are expected to prevail through
the period at all local terminals. Fog is forecast tonight well
southeast of the local terminals, and there is a very low
chance that KCPS and KSUS see impacts early tomorrow morning.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.

&&

$$

SHORT TERM...Gosselin
LONG TERM...Gosselin
AVIATION...Elmore