Severe Thunderstorm Warning 220, NWS St. Louis, MO

Area Forecast Discussion
National Weather Service St Louis MO
229 PM CDT Sun Jul 26 2026

.KEY MESSAGES...

- Dangerous heat index values of 110-115+ are forecast on Monday.

- The dangerous heat may continue on Tuesday along/south of I-70,
  but relief is expected areawide beginning Wednesday.

- Isolated to scattered thunderstorms are forecast through late
  this evening, mainly in south-central Illinois. A few of
  these storms may be severe with damaging winds.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 222 PM CDT Sun Jul 26 2026

Dangerous heat and humidity exists areawide this afternoon.
Dewpoints (humidity) levels have neared or broken daily records
at KSTL, KCOU, and KUIN. Dewpoints near 80 degrees along with
temperatures climbing into the mid 90s has yielded heat index
values near 115 so far today for most areas, with some spots
topping 120 degrees. It truly does not get much worse in terms
of heat index in this part of the country, with each climate site
likely to crest into the top 10 lists for highest heat index
values. This is at least the worst heat index values for an
individual day since August of 2023. Unfortunately, the area is
in store for another one on Monday, though likely not quite as
bad. Peak heat index values of 110-115+ are forecast across the
area on Monday.

The other focus in the near term is chances for showers and
thunderstorms. There is a subtle, weak surface trough
axis/convergence zone oriented northwest to southeast across central
Illinois. This boundary is being reinforced by differential heating,
with a mostly sunny sky to its southwest and some
stratus/stratocumulus to its northeast. The boundary itself has
gotten a bit more diffuse with time as it heads slowly
northeast. Surface winds are now southerly well into even north-
central Illinois, tough some light southeasterlies still exist
farther to the south/southeast. The longer storms take to fire,
the farther northeast the boundary will get. Storms should head
from north/northwest to south/southeast, which means far
eastern counties in south-central Illinois have the best chance
(~30%). Farther west, I cannot rule out some isolated
showers/thunderstorms, but the better threat is to the east.
Damaging winds are the main threat with any robust updraft given
steep low-level lapse rates and DCAPE values well over 1000
J/kg. Some CAMs suggest storms may congeal and organize a cold
pool, which would greatly increase the damaging wind threat. As
alluded to above, exactly when and where this occurs is not
highly certain, but better odds may be just to our east. Another
area to keep an eye on is across the mid-Missouri Valley. There
are some showers up there now, but a couple of CAMs suggest
thunderstorms may develop and then head toward northeast
Missouri/west central Illinois by mid-late evening. Effective
shear is not as strong in this area, and the later timing may
also prove to be a limiting factor. That being said, if anything
were to congeal and get an organized cold pool, that also could
then survive longer and may yield damaging wind gusts all the
way southeast into south-central Illinois. This isn't a likely
scenario by any means, but another area to monitor this evening.

&&

.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
Issued at 222 PM CDT Sun Jul 26 2026

(Monday Night - Tuesday)

Spread has decreased within model guidance with respect to the timing
of the cold front late Monday night - Tuesday. The GFS has trended
faster, and now is more in line with the ECMWF. The front is
expected to move south of the area now closer to midday Tuesday. The
air mass behind this front is not terribly cool though by any
stretch, and similar to many fronts in mid/late summer, the
dewpoints don't drop precipitously behind the boundary. That means
another day of heat and humidity, though it should be nothing like
today (or even Monday for that matter). Peak heat index values
of 100-105F are more likely along/south of I-70, but exactly how
widespread these values will be will depend on the exact timing
of the front (as well as clouds, etc). Given there is still
this uncertainty, held off on any extensions to heat products as
both the area and the product type itself - warning or advisory
are not clear.

(Tuesday Night - Next Sunday)

Confidence remains high in a definitive end to the oppressive heat
and humidity areawide by Wednesday with the cold front expected to
be well to our south. The incoming air mass though is one
characterized by 850-hPa temperatures and surface dewpoints closer
to normal. Not surprisingly, high and low temperatures also should
be close to seasonal normals. While not exactly "cool", it will feel
about 20+ degrees cooler than peak heat index values the past 48
hours. Relief indeed.

High temperatures are also expected to further cool by Friday due to
an increase in shower and thunderstorm chances. For ~5 days out,
there is a pretty good signal for a possible MCS to impact portions
of the area late Thursday night into Friday. This definitely looks
like the best chance (40-60%) for a widespread rain over the next 7
days.

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1201 PM CDT Sun Jul 26 2026

Isolated to scattered thunderstorms are forecast to develop over
the next few hours, mainly across southwest and south-central
Illinois. While I cannot rule out a stray storm impacting the
metro terminals, left TAFs dry with light winds through Monday
morning.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Extreme Heat Warning until 9 PM CDT Monday for MOZ018-019-026-
     027-034>036-041-042-047>052-059>065-072>075-084-085-099.
IL...Extreme Heat Warning until 9 PM CDT Monday for ILZ058>060-064-
     065-069-070-074-079-095>102.

&&

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