217
FXUS63 KLSX 172308
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
608 PM CDT Fri Jul 17 2026

.KEY MESSAGES...

- Hot and humid conditions will lead to heat index values near
  or over 100 degrees for parts of the region through Tuesday.

- A cold front on Tuesday will end this stretch of hot weather,
  bringing cooler and much less humid weather next week.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 214 PM CDT Fri Jul 17 2026

This afternoon`s showers and embedded storms have formed
primarily within a very subtle remnant wave aloft which is
gradually moving east out of southeast Missouri this afternoon.
This convection will largely dissipate with the loss of heating
this evening. Storm motions have been brisk enough to limit any
threat of flash flooding, though local downpours are expected
where the storms track.

Looking at the bigger picture, we see ridging extending north
over the Rocky Mountains and a trough digging out of Central
Canada into the western Great Lakes. At the surface, high
pressure over Louisiana is keeping a warm and humid flow across
the Southern Plains and into our region. While temperatures
have not been extreme, largely right around normal for this time
of year, dewpoints in the low to mid 70s have already pushed
heat index values to near 100 degrees today, and as we go into
tomorrow we expect to add another degree or so. As the trough
digs through the Great Lakes we`ll see our upper flow turn
toward a more subsident northwest flow. This will help establish
a bit more of a cap tomorrow, preventing more widespread
afternoon convection. However, the cap isn`t all that strong, so
I wouldn`t be surprised if a cell or two were able to form.
More sun will contribute to the slightly warmer temperatures and
a few more locations will get up to 100 heat index. This is
just the beginning of several days in which heat index values
are expected to reach or exceed 100 for portions of the area.
This has prompted the issuance of a Heat Advisory for areas
along and south of the Missouri River and into portions of
southwest Illinois. Areas to the north and east are more likely
to be affected by a couple of cold fronts in the coming days,
limiting the duration of the heat, thus they were not included
in the Advisory at this time.

The first front pushes south through Iowa Saturday afternoon and
evening behind the trough digging through the Great Lakes. The
bulk of the convection along and ahead of the front is expected
to be further to the east. However, a few storms may form in the
vicinity of this front as it reaches the northeast corner of
Missouri into Central Illinois Saturday evening. Although we
can`t rule out a local threat of damaging winds with these,
confidence is too low locally to begin messaging that threat at
this time.

&&

.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
Issued at 214 PM CDT Fri Jul 17 2026

Saturday night`s cold front loses its southward push as the
trough driving it pushes eastward into New England. The result
will be a stalling front in our area on Sunday, roughly
stretched from northwest to southeast not far from the
Mississippi River. To the northeast of the front, it`ll be a
little bit cooler, but to the southwest of the front we`ll still
be in the hot and humid air mass with heat index values even a
touch higher than Saturday, in the low 100s. The eastward extent
of the current Heat Advisory is based on the expected location
of this front Sunday afternoon. There will be the potential for
afternoon showers and thunderstorms especially in the vicinity
of that front, but light winds aloft should prevent much in the
way of storm organization. Locally heavy downpours can be
expected as the primary impact.

The next trough begins to sink south out of Canada into the
Northern Plains on Monday. Flow ahead of it should push the
remnant front back to the northeast, putting our entire area
back into the hot and humid conditions. Monday is the day that
features the most widespread and some of the most intense heat
of this air mass. The entire area is likely to see temperatures
warming into the low to mid 90s with continued high humidity.
Heat Index values may reach or exceed 105 in spots. The trough
digging to our north, though, is the harbinger of a change in
the weather pattern. It`s grabbing a cooler and much less humid
air mass from further north in Canada and shoving it southward.
The cold front arrives in our area on Tuesday, pushing from
north to south across the region during the day. Uncertainty on
the timing of this front does affect the temperature forecast
Tuesday. Ahead of the front we have one more day in the hot and
humid air mass. In fact, moisture pooling ahead of the front may
contribute to the most extreme humidity with heat index values
near or greater than what was seen Monday. However, north of the
front there will be significant relief. The northern extent of
the current Heat Advisory is based on the expected frontal
location on Tuesday. The latest NBM keeps POPs on Tuesday below
mentionable levels (15 percent) locally. While ensemble guidance
does favor areas to the east closer to the trough for convection
ahead of the front, I expect the NBM is a bit too dry here. Up
to 60 percent of low resolution ensemble members produce rain in
our forecast area in the 24 hours ending Tuesday night.

The air mass behind Tuesday`s front will be cool and dry by July
standards. The air mass has its origins in the Arctic north of
Alaska where sea ice is still widespread. Latest ensemble
guidance has come into better agreement on this air mass`s
potential for our area. Just comparing ensemble forecast 850
temperature Wednesday evening, yesterday`s 16/12Z guidance had 5
to 6 degrees of IQR spread, while today`s 17/12Z has cut that
in half to 3 degrees. This gives greater confidence in 850MB
temperatures around +15C, almost 10C cooler than they are ahead
of the front. Assuming full mixing that puts highs in the low
80s, 5 to 10 degrees below normal for the time of year. The
latest NBM is still a touch warmer than that, but will likely
come down a few degrees as it catches up to the latest guidance
consensus. Dewpoints, however, will also be notably low.
Although there`s more uncertainty in guidance here, there`s a
good consensus that dewpoints at least fall into the 50s,
something we haven`t seen locally in about a month. NBM
probability of dewpoint below 60 rises above 50 percent for much
of the area Wednesday and Thursday. That also allows for some
cool nights given good cooling conditions. This air mass will
gradually moderate in the days after the front, but this break
from the heat and humidity is likely to last through the end of
the week at least.

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 607 PM CDT Fri Jul 17 2026

Confidence is high in dry and VFR flight conditions at each of
the local terminals through the TAF period.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory from noon Saturday to 7 PM CDT Tuesday for
     MOZ041-047>051-059>065-072>075-084-085-099.
IL...Heat Advisory from noon Saturday to 7 PM CDT Tuesday for
     ILZ069-074-079-100>102.

&&

$$

SHORT TERM...Kimble
LONG TERM...Kimble
AVIATION...Elmore