131
FXUS63 KLSX 301050
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
550 AM CDT Mon Jun 30 2025

.KEY MESSAGES...

- Scattered showers and thunderstorms, a few capable of damaging wind
  gusts, are expected this afternoon and evening across the area.

- The week will start off relatively cooler before temperatures
  climb and heat builds toward the end of the week and weekend.


&&

.SHORT TERM... (Through Tuesday Night)
Issued at 236 AM CDT Mon Jun 30 2025

An upper-level trough is digging into the Northern Plains and Upper
Midwest per water vapor imagery this morning, its associated cold
front surging southeastward through the Dakotas and Minnesota. South
of the front across the Central Plains are several areas of
convection, all producing upper-level clouds that are streaming into
the region. Despite the cloud cover, patchy fog has developed thanks
to recent rainfall. This fog is expected to diminish as day time
heating commences.

As for the cloud cover, convective debris is anticipated to impede
upon the CWA through the day, helping keep temperatures in the mid
80s to around 90 degrees this afternoon. How much this cloud cover
keeps temperatures subdued will dictate how unstable the pre-frontal
atmosphere becomes over the CWA. On the lower end, instability may
not peak above 1,000 J/kg of SBCAPE if cloud cover is too dense, and
on the higher end, values could get as high as 2,000-3,000 J/kg. In
the former scenario, convection along and ahead of the front would
be mostly, if not completed limited. In the latter, scattered
showers and thunderstorms would form across the area during the
afternoon and evening hours once the cap erodes. In the strongest
of convection, collapsing cores and downdrafts will be amplified by
a wedge of dry air near the surface, leading to a damaging wind
threat. Deep-layer shear is expected to be around 20 kts, leading to
updrafts being disorganized and short lived. Given that, any
damaging wind threat is expected to be fairly isolated. With a
majority of guidance showing that the prefrontal environment will
become sufficiently unstable for convection, the going forecast is
for scattered showers and thunderstorms.

The front moves through the CWA tonight as the trough digs into the
Ohio Valley, deepening northwesterly flow over the region. An area
of high pressure, characterized by seasonable temperatures (highs in
the mid to upper 80s), will advect into the CWA for Tuesday, leading
to a relatively cool, lower humidity day.

Elmore

&&

.LONG TERM... (Wednesday through Sunday)
Issued at 236 AM CDT Mon Jun 30 2025

Northwesterly flow will still be aloft over the Middle Mississippi
Valley per guidance consensus. The northwesterly flow and surface
high still hanging around will keep conditions similar on Wednesday
as they are forecast for Tuesday. Though, temperatures will be a
couple degrees warmer thanks to mid-level ridging building into the
Midwest, causing 850 mb temperatures to rise by a degree or two.

This slight bump in temperatures on Wednesday will mark the
beginning of gradual warm up that will end with another stretch of
summer heat. On Thursday, guidance consensus is that an upper-level
ridge will quickly build into the Midwest and hold on through the
holiday weekend. Guidance trends over the last day or two have
increasingly amplified the ridge, building confidence in another
round of heat. Friday through Sunday, even the 25th percentile of
ensemble guidance has high temperatures around 90, with median
values in the low to mid 90s. With a majority of guidance supporting
dew points in the low to mid 70s, this airmass will be capable of
heat index values reaching and exceeding 100 degrees. Ensemble-based
probabilities top out around 40% for such conditions over the
weekend.

With guidance trending the ridge stronger over the region,
confidence is decreasing in what were already low rain chances this
weekend. There is a weak signal for a shortwave to traverse the top
of the ridge Saturday into Sunday among deterministic guidance,
though what models do have this feature differ wildly on its
phasing. If it ends up being stronger, rain chances would increase
over the weekend and temperatures may not get as hot.

Elmore

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 539 AM CDT Mon Jun 30 2025

The main focus for impacts this TAF period is the chance for
scattered showers and thunderstorms ahead of a cold front this
afternoon and evening. The exact timing of these storms is
dependent on how storms over the Plains this morning behave, and
the window for impacts could shift an hour or two earlier or
later than currently forecast. Some of the storms will be capable
of strong downdrafts, leading to erratic changes in wind speed
and direction if outflows pass over the local terminals. The
chance for showers and thunderstorms will end late this evening or
early overnight, with VFR flight conditions expected through the
remainder of the period.

Elmore

&&

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

$$

WFO LSX