489
FXUS63 KLSX 061120
AFDLSX

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

.KEY MESSAGES...

- Isolated to scattered showers and thunderstorms are forecast today
  through Monday, mainly during the afternoon and evening.

- The primary thunderstorm threat is locally heavy rainfall,
  especially on Sunday and Monday.

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


&&

.SHORT TERM... (Through Sunday Night)
Issued at 211 AM CDT Sat Jun 6 2026

Quasi-zonal flow extends across the Central Plains eastward through
the Middle Mississippi Valley as a cutoff spins over the Southern
Plains per water vapor imagery. A bifurcated low-level jet is
fueling convection over the Middle Missouri Valley and Great Lakes.
Of immediate concern, upwind propagation vectors favor the
convection over the Middle Missouri Valley moving south-
southeastward toward the CWA through this morning. However, with the
western prong of the low-level jet weakening through the morning and
the air mass south of the convection being increasingly dry and
stable, confidence remains high that this convection will struggle
to reach the CWA. A reasonable worst-case scenario is that outflow
helps force isolated to scattered weak showers and thunderstorms
across Knox, Lewis, Adams, and Brown counties around sunrise.

Our next round of convective potential comes this afternoon as the
atmosphere ahead of the aforementioned cutoff drifting northeastward
becomes increasingly unstable. The 00z HREF and REFS means have
approximately 2,500-3,000 J/kg of SBCAPE across the CWA during peak
heating today, with this instability and weak forcing from the
cutoff inducing CI at roughly 3pm. Deep-layer shear of 10-20 kts
favors disorganized single cells to multi-cells primarily west of
the Mississippi River where lift will be greatest; though, there is
at least a low threat (15-20% chance) for showers and thunderstorms
area-wide. There is a low chance that residual outflow boundaries
and/or a weak front just north of the CWA may provide a focus for CI
in proximity to greater instability this afternoon and evening. The
greater instability favors relatively stronger updrafts compared to
elsewhere in the CWA, leading to a localized threat of hail up to 1"
in diameter and damaging wind gusts. However, the weak shear
suggests that updrafts will tend to be too short-lived to provide a
severe threat; therefore, the SPC Day 1 Marginal Risk will not be
publicly messaged.

Diminishing instability with the loss of day-time heating will lead
to decreasing convective coverage this evening. The exception will
be areas west of the Mississippi River where forcing from the
approaching cutoff will keep a low chance (30%) for showers and
thunderstorms going through the overnight hours. As this shortwave
continues to pass overhead during the day Sunday and instability
builds once again, convective coverage will increase. Thanks to more
widespread lift via the shortwave, the entire CWA has a 80-90%
chance of seeing scattered showers and thunderstorms at times Sunday
into Sunday evening. Deep layer shear is expected to be weak once
again, with the greater storm coverage and PWAT just under 2"
supporting a threat of locally heavy rainfall.

Elmore

&&

.LONG TERM... (Monday through Friday)
Issued at 211 AM CDT Sat Jun 6 2026

With the aforementioned cutoff only slowly lifting northeastward
through the Midwest, guidance consensus is that rain chances will
linger across the Middle Mississippi Valley on Monday. Uninhibited
Gulf flow will keep dew points in the low 70s, yielding SBCAPE
around 2,000 J/kg to as much as 3,000 J/kg per the 90th percentile
of ensemble guidance. While deep-layer shear of around 20 kts leads
to low confidence in organized severe convection, when coupled with
PWAT around the 99th percentile of climatology, there is concern
for localized heavy rainfall.

Confidence in the forecast decreases as we get into the middle of
next week through the end of the week, with both deterministic
guidance and ensemble clusters differing on the phasing of an upper-
level trough over the northwestern CONUS and expansive ridging over
the southern CONUS. A deeper trough and one that encroaches more upon
the Midwest will keep the ridge further south relative to the CWA,
favoring temperatures topping out around 90 degrees Tuesday through
Thursday (50-80% chance per global ensembles). If the ridge is able
to build further northward into the CONUS, then portions of the CWA
could see high temperatures warm into the mid 90s (30% chance). The
current forecast (NBM) favors the warmer solutions and represents a
realistic "worst-case" scenario. If this scenario comes to fruition,
then the first heat headlines of the season will be needed for
portions of the CWA.

Elmore

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 617 AM CDT Sat Jun 6 2026

The main concern for impacts this TAF period will be scattered
showers and thunderstorms. Currently, confidence is greatest in
these impacting KCOU and KJEF this afternoon; however, there is
still a low chance for impacts at KSUS, KSTL, KCPS and KUIN this
afternoon and evening. Chances will decrease at these 4 terminals
after 00z, but may linger into the overnight hours for KCOU and
KJEF. Lastly, KCOU and KJEF will be on the eastern edge of MVFR
stratus late in the period Sunday morning. Given the lead time and
low confidence in these conditions making it to these two
terminals, impacts have been left out of the TAFs for now.

Elmore

&&

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

$$

WFO LSX