286
FXUS63 KLSX 070335
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1035 PM CDT Sat Jun 6 2026

.KEY MESSAGES...

- Scattered showers and thunderstorms will continue into this
  evening, with the main hazards being lightning, brief heavy
  rainfall, and gusty winds.

- Additional, more widespread showers and thunderstorms are
  forecast Sunday morning through Monday with the greatest hazard
  being heavy rainfall potentially leading to localized flash
  flooding.

- Summer-like heat and humidity moves into the region starting
  Tuesday, with a 30% chance of heat index values reaching and
  exceeding 100 degrees each afternoon the rest of the week.


&&

.SHORT TERM... (Through Late Monday Night)
Issued at 311 PM CDT Sat Jun 6 2026

Analysis of recent mid-level water vapor imagery reveals a closed
low slowly propagating northeastward across northern Texas.
Meanwhile, the northern stream features ridging across the Upper
Midwest with longwave troughing on either side. Low-level southerly
flow ahead of the closed low continues to filter in a warm/moist
airmass from the Gulf, helping to boost instability. The latest
ACARS sounding out of KSTL reveals around 2,000 J/kg of SBCAPE
mostly due to surface dewpoints near 70F and near dry-adiabatic
surface-850mb lapse rates. A west-east oriented swath of agitated
cumulus exists near the Missouri River this afternoon, with showers
and brief pockets of heavier rain percolating northeastward. This
activity has struggled to grow vertically due to weak mid-level
lapse rates less than 6 C/km. Additionally, most of this development
has remained below the 14kft freezing level, which has limited
glaciation and thus lightning production.

However, as southerly warm/moist advection persists, surface based
instability should increase enough to overcome the weak mid-level
lapse rates, allowing for greater vertical growth and more lightning
this evening. As a result, additional scattered shower/thunderstorm
development is still forecast (20-30%) this afternoon/evening as
broad forcing for ascent aloft increases thanks to the approaching
closed low. Given the pulse-like nature of thunderstorm activity
today, the main hazards will be abundant lightning, brief downpours,
with a low potential for an isolated microburst that could produce
damaging wind gusts. Scattered instances of hail up to 1" associated
within the stronger thunderstorms remains possible, but is not
likely, as instability in the hail growth zone is unimpressive and
14kft freezing levels will allow for abundant melting of falling
hailstones. Coverage of showers and thunderstorms is forecast to
decrease later this evening around sunset as instability wanes, with
a 20% chance for lingering showers mainly across Missouri tonight.

On Sunday, the aforementioned northeastward moving closed mid-level
low will begin phasing with the northern stream near the Upper-
Mississippi River Valley. As this system nears the area, height
falls aloft will increase, leading to broad forcing for ascent.
This, coupled with abundant instability, thanks to the warm/moist
airmass in place, will lead to increasing chances for more
widespread rain from the southwest tomorrow. As a result, the
greatest shower and thunderstorm chances (70%) will be across
central/east-central Missouri Sunday afternoon, with those chances
reaching Illinois by Monday morning. With a closer proximity to the
closed low aloft, bulk 0-6km wind shear will be slightly higher
during this timeframe (20-30kts), which may aid in better
organization of thunderstorms. Additionally, high-resolution
guidance reveals that PWATs will be around 1.9-2" (99th percentile),
indicating that plenty of column moisture is available for
convection to tap into. As a result, the main hazard expected with
thunderstorms on Sunday and Monday, aside from abundant lightning,
is the threat for pockets of heavy rainfall leading to localized
flooding if the same locations experience multiple rounds of
thunderstorms.

Peine

&&

.LONG TERM... (Tuesday through Next Saturday)
Issued at 311 PM CDT Sat Jun 6 2026

By Tuesday, the mid-level closed low will be fully phased with the
northern stream and will be departing to the east. Long-range
deterministic guidance reveals a mid-level shortwave ridge
influencing the region beginning on Tuesday, which would induce
height rises aloft resulting in the start of drier conditions. The
exception to this would be across Illinois early on Tuesday, where
there is a 30-50% chance for lingering showers/thunderstorms with
closer proximity to the exiting low. Confidence in prevailing dry
conditions increases Tuesday evening lasting into Wednesday night as
the mid-level ridge axis moves across the area, with chances for
measurable precipitation remaining below 20% indicated by the LREF.

After the shortwave ridge axis shifts eastward on Wednesday, deep
southerly flow returns, which will aid in ample warm/moist advection
from the Gulf. A steady increase in temperatures/moisture is
expected Tuesday into the end of the week, with at least 50% chances
for highs >90F progged by the LREF Tuesday through Friday. The NBM
is still on the hotter end of guidance and is generally 5F warmer
than the LREF for this period. Regardless, with temperatures in the
lower to mid 90s possible, coupled with high humidity values, next
week will feature the warmest conditions we have experienced since
last summer. Lastly, the LREF indicates around a 30% chance for heat
indices >100F Wednesday through Friday, further increasing
confidence in warm/humid conditions.

Peine

&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1026 PM CDT Sat Jun 6 2026

Showers and thunderstorms have dissipated across the region this
evening and a largely dry forecast will prevail into the morning
hours. During this time, VFR flight conditions will prevail and winds
will be variable and largely less than 5 knots. While guidance
previously was quite bullish with MVFR ceilings moving into the
region overnight and into tomorrow morning, low level dry air and
diminishing precipitation will keep ceilings VFR for much of the
TAF period.

The exception will be with any showers and thunderstorms that
develop tomorrow afternoon and evening across the area. Where
these develop MVFR ceilings and a brief reduction in visibility to
MVFR conditions are expected. Confidence is highest in
precipitation falling at the mid-Missouri terminals and KUIN,
where a PROB30 group has been introduced during the afternoon
hours, though precipitation could fall outside this window as well.
Confidence in the timing of precipitation within the St. Louis
metro terminals is much lower. So while showers and thunderstorms
are possible (30-40% chance) throughout much of the day across the
metro area, there isn`t a strong enough signal to outline in the
TAF at this point.

Delia

&&

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

$$

WFO LSX