537
FXUS63 KLSX 261120
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
620 AM CDT Sun Jul 26 2026

.KEY MESSAGES...

- Dangerous heat index values of up to around 115 degrees are
  forecast for today and portions of the area tomorrow. An
  Extreme Heat Warning is now in effect through Monday.

- There is a low chance (20%) for isolated thunderstorms this
  afternoon. A storm or two will be capable of damaging wind
  gusts.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 334 AM CDT Sun Jul 26 2026

A shortwave riding the ridge crest through the Northern Plains
spawned an MCS last evening that has since dissipated. However, CAMs
continue to resolve the remnants of this MCS poorly. Given upwind
propagation vectors, any convection that develops along outflow or
from what appears to be a weak MCV in Iowa this morning will tend to
dive southward into the CWA. With the atmosphere over the northern
CWA being capped and CAMs continuing to produce convection where
there isn`t any observed, my confidence in convection impacting the
CWA this morning is low.

With the chance of being impacted by convection this morning
remaining low, confidence is high in widespread dangerous heat. Dew
points trended greater yesterday than originally anticipated, with
most locations seeing resulting heat index values of 105-110 degrees
along and west of the Mississippi River. With a warm start to the
day this morning, the thermal ridge building further into the area,
and dew points pushed toward what was experienced yesterday,
widespread heat index values of 110-115 degrees are forecast today.
With similar conditions forecasted for Monday and considering the
compounding impact of heat, the Heat Advisory have been upgraded to
an Extreme Heat Warning through Monday.

There is a low chance that convective temperatures are reached and
the cap breaks, allowing for isolated convection to develop this
afternoon. If this does happen, areas along and south of I-70 are
where it is most favored. SBCAPE will peak at 3,000-3,500 J/kg per
the 00z HREF and REFS among deep-layer shear of around 25 kts. This
will allow for an updraft or two to become strong to severe, and
capable of damaging wind gusts. The damaging wind threat will
increase if a storm/storms can develop a cold pool and become
sustained as an MCS, with this threat diminishing this evening with
the setting sun. There is a low chance for additional convection on
Monday, and that chance will hinge on how convection evolves this
evening and tonight in the broader region, which is uncertain at
this time.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Issued at 334 AM CDT Sun Jul 26 2026

Guidance continues to show the upper-level ridge weakening slightly
over the Midwest on Tuesday as a backdoor cold front pushes into the
area. This front has been trending a little quicker, increasing
confidence that most of the area north of I-70 will see relief from
the heat. While guidance has narrowed in on FROPA timing, there is
still enough variability in the magnitude of the post-frontal air
mass to lead to uncertainty in how many locations south of I-70 will
see relief. We will continue monitoring the potential of extending
heat headlines into Tuesday afternoon.

For Wednesday through the weekend, northwesterly flow on the eastern
flank of the ridge will persist over the Middle Mississippi Valley.
Through the low to mid-levels, flow will be north-northeasterly,
which will keep the thermal ridge positioned just west of the
region. Temperatures as a result will hover around seasonal normals
for highs (upper 80s) and lows (upper 60s). Narrow ensemble spread
supports this solution, though there is increased variability on
Friday thanks to a potent shortwave moving through the Midwest. With
both deterministic and ensemble-based guidance honing in on this
shortwave, confidence in rain has increased, with a 40% chance for
scattered showers and thunderstorms now forecast Friday into
Saturday.

&&

.AVIATION /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 615 AM CDT Sun Jul 26 2026

Fog impacting KCPS and KSUS is expected to dissipate early in
the period, with mainly VFR flight conditions expected then
through the day into this evening at all local terminals. There
is a low chance for afternoon showers and thunderstorms at
KSUS, KSTL, and KCPS. If these occur and directly impact the
terminals, reduced visibilities and gusty winds can be expected.


&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Extreme Heat Warning from 11 AM this morning to 9 PM CDT
     Monday for MOZ018-019-026-027-034>036-041-042-047>052-
     059>065-072>075-084-085-099.
     Heat Advisory until 11 AM CDT this morning for MOZ041-042-
     047>050.
IL...Extreme Heat Warning from 11 AM this morning to 9 PM CDT
     Monday for ILZ058>060-064-065-069-070-074-079-095>102.

&&

$$

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