399
FXUS63 KLSX 251925
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
225 PM CDT Sat Jul 25 2026

.KEY MESSAGES...

- Dangerous heat and humidity will overspread the rest of the
area tonight into Sunday, and last through at least Monday.

- Mostly dry weather is expected over the next 48 hours, but there
is a low chance (~20%) of storms moving in from the north Sunday
morning and/or developing near the Mississippi River by afternoon.

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 221 PM CDT Sat Jul 25 2026

The heat and humidity has arrived early this afternoon with
temperatures climbing to near the 90 degree mark in/around
central Missouri. Dewpoints have also surged, with mid to upper
70s values areawide. Heat index readings have crested 105
degrees at KJEF/KCOU/KMYJ. Meanwhile, farther east,
temperatures still have not hit 80 degrees in parts of south-
central Illinois due to lingering thicker cloud cover. Given
slightly hotter, more humid conditions than expected yesterday
in parts of central Missouri, expanded today`s heat advisory
slightly farther east to include Audrain/Callaway/Osage
counties.

A very mild night is in store tonight due to the aforementioned
high surface dewpoints (>95th percentile for the date) and at
least some increase in high cirrus. Lows are only expected to
drop back into the low to mid 70s across the bi-state region, or
about 5-7 degrees above normal for late July.

Widespread dangerous heat is still expected everywhere on Sunday.
Peak heat index values of 105-110F are likely during the
afternoon/peak heating. This is not a certainty however everywhere,
as there are subtle signs that convection is possible that could
alter temperatures/heat index values. There is a low chance (~20%)
that a cluster of thunderstorms will be ongoing Sunday morning to
our north. Most guidance does not have much in the way of convection
across the Upper Midwest early Sunday morning, and those that do
tend to weaken it with time. However, forward-propagating Corfidi
vectors are basically due north-south. So...if anything can sustain
itself through the morning hours, it very well could move into the
area. Instability will also be high (and increasing) into the
afternoon so there is a highly conditional threat for damaging wind
gusts. Forecast soundings are also supportive of wind gusts being a
threat in any robust thunderstorms due to steep low-level lapse
rates and high DCAPE values. Another possibility (again, about
20%) is for afternoon diurnal development of convection near the
Mississippi River. Deterministic guidance does show a subtle
midlevel shortwave trough approaching from the northwest, with
weak surface convergence near a trough axis. CAMs are highly
variable, but do show at least some signal for isolated
thunderstorm development in that region. Given the environment
mentioned above, damaging wind gusts via microbursts would be a
concern with any stronger updrafts.

&&

.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
Issued at 221 PM CDT Sat Jul 25 2026

(Sunday Night - Tuesday)

There is little/no change in the ambient air mass through Monday
night, so we are still expecting very mild nights (lows in mid to
upper 70s) along with at least one more dangerously hot/humid day on
Monday. Similar to Sunday though, I am not sure that it will stay
dry areawide. There may be thunderstorms across the mid Missouri
Valley and/or Upper Midwest that could at least try and spill
into the region with north/northwest flow aloft. Some guidance
(such as the ECMWF) also shows afternoon development along/east
of the Mississippi River (again, near that same surface trough
axis). For now, kept this period dry given the low confidence,
but we may need to add PoPs on Monday. The chances of storms
being widespread and early enough to really negatively impact
heat index values is very low though, so no changes to heat
headlines for Monday. It is possible though that some counties
may need to be upgraded to a warning if confidence in at least 2
days of 110+ HI values increases.

Forecast confidence in temperatures/heat index values on Tuesday
remains low due to differences in the timing of the synoptic cold
front. The ECMWF remains 12-18 hours faster than the GFS. These
differences should get ironed out over the next 36-48 hours. The
advisory though may need to be extended another day for at least
portions of the area, with better chances the farther south you go.


(Tuesday Night - Next Saturday)

Confidence is fairly high that the dangerous heat and humidity will
end across the entire area by Wednesday, with even the slower GFS
progressing the front through the region by late Tuesday night.
Temperatures (and humidity levels) are expected to drop back closer
to normal values for late July/early August. The chances for more
widespread showers/thunderstorms may also tend to increase as the
mid/upper level ridge shifts more toward the desert southwest. This
may allow for "ridge-running" shortwaves to produce rounds of
showers and thunderstorms that could traverse the region.

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1159 AM CDT Sat Jul 25 2026

Stratocumulus is lifting early this afternoon, and should be
above 3 kft AGL areawide within the next couple of hours. Some
fog is possible late tonight, but held off on adding to any
terminals as there may be enough high clouds to prevent fog
development. Finally, there is a low chance (~20% or less) of
thunderstorms Sunday. The "best" chances seem to be pointed
toward the afternoon roughly along the Mississippi River and
after this TAF package. Therefore, kept TAFs dry.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory from 11 AM Sunday to 9 PM CDT Monday for MOZ018-
     019-026-027-034>036-051-052-059>065-072>075-084-085-099.
     Heat Advisory until 9 PM CDT Monday for MOZ041-042-047>050.
IL...Heat Advisory from 11 AM Sunday to 9 PM CDT Monday for
     ILZ058>060-064-065-069-070-074-079-095>102.

&&

$$

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