Louis, Missouri Weather Forecast Discussion

973
FXUS63 KLSX 171901
AFDLSX
Area Forecast Discussion
National Weather Service Saint Louis MO
201 PM CDT Wed Sep 17 2025
.KEY MESSAGES...
- While cooler weather will begin on Friday and continue into early
  next week, temperatures are expected to remain several degrees
  above normal into next week.
- There are multiple chances of thunderstorms starting on Thursday
  afternoon through early next week. The best chances (40-70%) are
  across central and northeast Missouri Thursday evening.
&&
.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 159 PM CDT Wed Sep 17 2025
Similar to yesterday, a few showers and thunderstorms have
developed across western Missouri on the eastern edge of the
instability gradient. Much of this activity should miss our area
just to the west, but far western sections (mainly western
Moniteau/Boone counties) may see an isolated shower/weak
thunderstorm through early this evening.
Overall, a near persistence forecast is expected across much of the
area with a mostly clear sky, light winds, and seasonably low
dewpoints. This should allow temperatures to drop back into the
upper 50s to low 60s, nearly identical to this morning. Some spotty
mid 50s are once again possible in favored river valleys across east
central and southeast Missouri as well as portions of south-central
Illinois. Further west, there are some changes. For one, we are
expecting a bit more in the way of cloud cover for most of central
and northeast Missouri. As a result, lows should be more on the
milder side, only reaching the mid 60s. There also may be another
window for some spotty shower activity, mainly between 0300 and 0600
UTC in parts of central/northeast Missouri.
There should be some ongoing shower/weak thunderstorms across
northwest Missouri early Thursday morning, but those are expected to
weaken/dissipate with time. I cannot completely rule out a few
showers sneaking into parts of central/northeast Missouri, but left
the forecast dry given the more stable environment across our area
and the weak low-level moisture convergence staying to our west.
Isolated to widely scattered showers and thunderstorms should
develop later on Thursday afternoon across central Missouri due to
increasing diurnal instability with little/no convective
inhibition. These should stay fairly weak, with even the 75th
percentile of the HREF only showing about 1000-1250 J/kg of SBCAPE
with very weak effective shear (10-15 knots) on the western
periphery of the mid/upper level ridge axis.
Thursday should be the last 90+ degree day for most, if not all, of
the area during this current stretch of warmth. Given the date, it
is also certainly not out of the realm of possibility that tomorrow
is the warmest day the rest of the calendar year 2025. Regardless,
most of the area should top out again in the low 90s. Parts of
central/northeast Missouri should have more persistent cloud cover.
Therefore, temperatures may struggle a bit more out that way and did
cool highs a few degrees more into the upper 80s.
Gosselin
&&
.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 159 PM CDT Wed Sep 17 2025
(Thursday Night - Friday)
The Thursday night time period looks to be the best chance (40-70%)
for showers and thunderstorms. More specifically, Thursday evening
across central and northeast Missouri. This is when a midlevel
shortwave trough will move through western Missouri in conjunction
with increasing low-level moisture convergence. This should provide
some beneficial rainfall across much of central/northeast Missouri.
Rainfall amounts do not look too heavy though, mainly on the order
of 0.20-0.40". Locally higher amounts (0.50-1.00") of course may
occur associated with thunderstorms.
Overnight, the midlevel shortwave trough departs for the Upper
Midwest. We also lose nearly any semblance of low-level convergence
across the area. Finally, what little instability is available
continues to decrease nocturnally. These indicators all portend to
decreasing coverage and intensity. By late Thursday night, only a
few isolated showers are expected as activity moves eastward toward
the Mississippi River.
Uncertainty increases on Friday as there are differences in both the
timing and the track of the next midlevel shortwave trough. This in
turn influences the location of a weak surface front downstream as
well. The GFS is a bit faster/further south with the midlevel
shortwave while the ECMWF is slowest/further northwest. So while
there is confidence that the coverage of showers and thunderstorms
should tick up again Friday afternoon in a north-south axis, the
question is where.
Temperatures are expected to cool at least slightly on Friday, but
how much is an open question. There is more confidence in thicker
cloud cover west of the Mississippi River, so this is where highs
should be cooler. More specifically, highs only in the low 80s are
expected in parts of central/northeast Missouri. Furthest east,
highs near 90 degrees are expected in parts of southeast Missouri
and south-central Illinois.
(Friday Night - Next Wednesday)
There remains a signal for additional chances of showers and
thunderstorms this weekend into early next week, but confidence in
anything more specific is quite low. Deterministic model guidance
continues to diverge with the evolution of the midlevel shortwave
trough Friday night into early Saturday. The 0Z ECMWF actually
closes the low off and meanders it into the mid south. The
deterministic ECMWF does have some support from a majority of
members from the 0Z EPS, so that scenario is not a complete outlier.
The GFS and most of its members are more progressive, with the
midlevel shortwave moving north into the Great Lakes. Quasi-zonal
flow is then left behind this feature with additional disturbances
moving across the mid-Mississippi Valley. These disturbances could
then help produce additional rounds of showers and thunderstorms
across the region. The ECMWF/EPS solution would lead to mostly dry
weather over the weekend as chances of showers and thunderstorms
move off to the south of the area with the departure of the closed
low. Given the high degree of uncertainty, PoPs are generally in the
low chance range, or slightly above climatology. Over the next 24-36
hours, spread in guidance hopefully will decrease so the weekend
becomes a bit clearer.
Looking into next week, there is actually a pretty good signal for a
true cutoff mid/upper level low to form somewhere across the central
part of the country. You can see evidence of this in 3 of the 4
clusters of the 500-hPa height pattern (~70% of members from the
grand ensemble). This is a strong signal this far out, so the
likelihood of a cutoff forming does appear high. The question of
course will be where as there is a lot of spread in ensemble
guidance, mainly it is position latitude wise. More members develop
the cutoff low to our south, but about 1 in 3 are much further to
the north (Upper Midwest). Regardless of where this occurs, the
pattern does not look particularly hot (like this week), as there
should be plenty of cloud cover not to mention the possibility of
showers and thunderstorms. Highs are mostly likely to be mainly in
the low to mid 80s each day, with more potential for cooler
temperatures during the day if there is any more widespread
rainfall. The spread between the 25th/75th percentiles of the NBM is
still higher with respect to high temperatures compared to low
temperatures and peaks in the 7-10 degree range early/mid next week.
Gosselin
&&
.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1215 PM CDT Wed Sep 17 2025
Dry, VFR conditions with light winds are expected through the
period. More mid/high level clouds will move into
central/northeast Missouri this evening. Further east, a mostly
clear sky will continue. There will be some river valley fog once
again tonight, mainly in eastern Missouri. Added a TEMPO group
for a brief period of MVFR conditions at KSUS between 0800 and
1200 UTC Thursday, somewhat similar to what occurred there earlier
this morning.
Gosselin
&&
.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&
$$
WFO LSX

NWS LSX Office Area Forecast Discussion