923
FXUS63 KLSX 172328
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
628 PM CDT Fri Oct 17 2025

.KEY MESSAGES...

- The first widespread rain in weeks is headed our way on
  Saturday. A few embedded thunderstorms may be strong to severe
  Saturday afternoon.

- A cold front Saturday night brings a shift toward cooler
  temperatures that lasts through next week.


&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 233 PM CDT Fri Oct 17 2025

Our area has been under the influence of some degree of upper level
ridging for quite some time now. In fact, it`s been about 3 weeks
since we`ve seen significant rainfall in St Louis, compounding a dry
trend that began two months ago in mid August. We finally have an
opportunity to change that very soon. The upper ridge axis has begun
shifting eastward as noted by mid and high level clouds which have
been streaming northward through our region today, a sign of that
southerly flow on the western side of the ridge. That deep southerly
flow ultimately comes from the Gulf and is transporting moisture
northward. That moisture is step one in getting some rain in the
forecast.

Step two in the quest for rain is a source of lift. Looking to our
west we see a surface trough axis extending from the Texas Panhandle
northeast to Minnesota. This is a reflection in part of a more
subtle trough axis aloft extending from two more significant
embedded troughs over Winnipeg, Manitoba and Flagstaff, Arizona.
This entire system shifts eastward over the coming days, colliding
with that surge of Gulf moisture to produce more widespread rain.

The rain chances begin late tonight into Saturday morning as the
surface trough enters our region from the northwest. Another even
weaker embedded wave aloft tracking in the vicinity of the surface
boundary will aid in the development of showers and a few
thunderstorms mainly during the morning hours across northern
Missouri into central Illinois. However, this looks more like widely
scattered showers at the moment, so perhaps less widespread and less
rainfall overall for this area.

The trough will push southeast through the day on Saturday and
although widely scattered showers or a few thunderstorms are likely
to continue as it moves south, the more concentrated and focused
lift will come Saturday afternoon as the trough moves out of Arizona
into Oklahoma and increases the low level moisture surge northward
ahead of it into the boundary. This moisture surge coinciding with
peak heating will create the best opportunity for surface based
convection, with initiation expected over the Ozarks spreading
quickly ENE while the overall convective axis shifts southeastward
through the evening. With precipitable water maximizing near 1.5
inches, near the 95th percentile of climatology for this time of
year, there is a lot of moisture to work with. So with thunderstorms
that form we do expect heavy rainfall rates, but with the overall
convective axis continuing to shift southeast the more intense rates
won`t last long enough at any one location to produce much concern
for flash flooding. The greatest potential for this convective line
to pause or linger in one location is across the Ozarks where
southerly low level flow has the effect of more effectively
counteracting the southward push of the boundary. Some CAMs do show
this trend, while others keep the broader line more progressive.
HREF LPMM indicates a swath of 2 to 4 inches of rain is possible
where the convective line lingers longest. Our recent dryness works
in our favor in terms of limiting the flash flood threat from this
rainfall, although intense rainfall rates can counteract this.
Latest WPC Excessive Rainfall Outlook indicating a Slight Risk of
excessive rainfall is appropriate in this area, but the threat is
not great enough to warrant a Flood Watch or broader public
messaging of this threat. For most this will just be a beneficial
rainfall after the recent dryness. Most of our area will be in the
more stratiform rain areas with lower rainfall rates away from the
deeper convection on the southern periphery of the broader
developing complex.

As far as severe weather concerns go, we are looking at a dynamic
system with strong wind flow aloft and sufficient deep layer shear
to produce more organized thunderstorms where deep convection
develops. However, latest trends have shifted this threat a bit
southeast relative to prior forecasts, with this threat being
greatest on the southern periphery of the broader developing
convective complex. Although initial cells may attain some
organization, relatively weaker instability should limit the
potential for hail and relatively unidirectional shear should limit
the potential for tornadoes. The better chance for severe weather
will come from stronger, deeper cores collapsing or developing into
broader consolidated outflows with damaging winds the primary
threat. This threat is mainly for the afternoon hours with the
threat shifting southeast quickly during the evening.

Broad stratiform rain aided and enhanced by broader lift associated
with the trough aloft will push northeast and out of our forecast
area overnight Saturday night. Although some embedded lightning
remains possible, the severe weather threat ends as the low level
stability increases this evening. A broad swath of 0.5 to 1.5 inches
of rain is most likely, with embedded swaths of 2 to 4 inches where
that deeper convection persists a bit longer. Northern parts of the
area, though, are likely to get missed by this more widespread rain
Saturday evening associated with the convective complex to the
south, but a cold front entering from the north brings enough lift
to produce one final round of showers before things end Sunday
morning.

Kimble

&&

.LONG TERM...  (Sunday through Next Friday)
Issued at 233 PM CDT Fri Oct 17 2025

Sunday morning`s cold front does bring a shift toward cooler
temperatures and much drier air. Sunday`s highs, largely in the 60s,
are similar to tonight`s lows, some 20 degrees cooler than we`ve
been lately. Dewpoints falling into the 30s set a low floor for
overnight lows, but this air mass is moving through quickly so we
are not likely to get a solid night of clear, calm conditions just
yet. Our coolest temperatures Sunday night are only forecast in the
low to mid 40s.

Later in the week, though, another trough moving into the Upper
Midwest pushes another cold front through our area Monday night into
early Tuesday. We don`t get much of a return flow ahead of this
front, though, so temperatures only warm back into the 70s Monday
before getting knocked back down by the front. Similarly we don`t
have enough moisture return for a significant chance of rain with
this front. Instead, it just reinforces the cooler air with another
cool and even drier air mass. This one lingers a bit longer than the
last one, with guidance honing in on surface high pressure settling
over our area Tuesday night into Wednesday. This will be when our
coldest temperatures will be observed. Dewpoints in the low 30s or
even upper 20s suggest an even lower floor for temperatures under
ideal radiational cooling conditions. Even deterministic NBM low
temperature forecasts are in the range of a potential frost. The
probabilistic NBM has probability of low temperature falling into
the 30s at 30 to 80 percent across most of our forecast area
Wednesday morning with the exception of the St Louis urban area.
There is even some potential (up to 20 percent) in some areas of
dropping to 32 degrees. These probabilities have been increasing
over the last few model runs as long range guidance has come into
better agreement on the timing of the surface high moving through
our region.

Later in the week we see the surface high shifting eastward and a
split flow setting up over the western to central US. A ridge in the
northern stream allows for a warming trend while a tough passing
through the southern stream interacts with the return of Gulf
moisture to bring rain chances back into the region. There`s still a
good deal of variability in the guidance on the timing and track of
this trough, so there`s greater uncertainty on precipitation chances
than there is on the gradual warming trend. On the earlier end, this
trough (and rain chances) could arrive as early as Thursday, while
some guidance holds it off until the weekend or potentially misses
us altogether.

Kimble

&&

.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 622 PM CDT Fri Oct 17 2025

A cold front will bring impacts to the local terminals through the
day tomorrow, particularly during the late afternoon and evening.
An initial round of scattered showers and thunderstorms are
expected during the morning, and these will be primarily capable
of low visibilities due to heavy rainfall if they directly impact
terminals. The greater chance for rain will come late in the
period, with widespread rainfall and embedded thunderstorms
expected. The rainfall paired with an increase in low-level
moisture and cooler air with the front will gradually decrease
ceilings across the area. While MVFR ceilings are most likely with
this low stratus, confidence in the exact height is low at this
lead time.

Elmore

&&

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

$$

WFO LSX