013
FXUS63 KLSX 280930
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
430 AM CDT Tue Apr 28 2026

.KEY MESSAGES...

-There is 40-80% chance for showers and thunderstorms this afternoon
and evening, a few may be strong to severe across southeast Missouri
and southern Illinois this afternoon.

-The remainder of the work week will be seasonally cool and
 largely dry.


&&

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 328 AM CDT Tue Apr 28 2026

Yesterday`s cold front is now stretched KHRO in northwest Arkansas
through southeast Missouri to near K1H2 in central Illinois. While
this feature will continue to sink southward through the morning
hours, an approaching mid-level shortwave and strong southerly low-
level flow will cause the front to stall this afternoon near the
Missouri-Arkansas border. Showers and thunderstorms will blossom
north of the front across the forecast area this afternoon from the
influence of the shortwave and warm air advection overriding the
surface front.

A few thunderstorms across southeast Missouri and southern Illinois
could become strong to severe between 2-8 PM today, though
uncertainty remains in if the ingredients will be present for these
storms to organize and maintain themselves. Instability is our
limiting factor thanks to a 800-700 mb warm nose that will be
reinforced by the ongoing warm air advection. The HREF indicates
that around 750 J/kg of MUCAPE will be available across far
southeast Missouri, though the 50-60 kts of 0-6 km shear may be too
strong to balance this low amount of instability. Storms in this
environment have the potential to shear out before producing severe
impacts. Any storms that form from this elevated instability and
become sufficiently strong will be capable of quarter sized hail.
SBCAPE is at even more of a premium due to the anticipated warm
nose, yet the HREF indicates as much as 1000 J/kg (worst case
scenario). If this surface based instability is realized and storms
are able to remain organized, damaging winds and hail will be the
main threats. Tornadoes are not anticipated to form from these
storms due to very weak low-level shear.

The mid-level shortwave will exit the area tonight, and combined
with an incoming surface high, will push showers and thunderstorms
eastward into the Ohio River Valley. At the same time, a low-level
secondary cold front will slowly slide through the forecast area
early Wednesday. The combination of these factors will result in a
dry and seasonally cool day with temperatures peaking in the 60s
areawide.

Delia

&&

.LONG TERM...  (Wednesday Night through Monday)
Issued at 328 AM CDT Tue Apr 28 2026

Heading into the second half of the work week the 500 mb flow will
start to shift from its previously stagnant pattern. By Thursday the
mid-level low will be churning over the Great Lakes region, having
moved eastward from south-central Canada, though the mid-Mississippi
Valley will remain in northwest flow. At the surface a weak frontal
boundary will be stretched across the central CONUS, bisecting a
portion of the forecast area. Lift along the front as well as aloft
will be weak, so despite ensemble guidance indicating about a 40%
chance for showers along the front Thursday, confidence is low in
widespread, significant precipitation.

Another, more robust mid-level shortwave will push through the mid-
Mississippi Valley Friday as the mid-level low shifts east out of
the Great Lakes region. At the same time a mid-level ridge will
build into the Intermountain West, leaving the mid-Mississippi
Valley in strong northwest flow in the wake of the shortwave. This
flow will prevail through the end of the weekend, and extends down
to the low-levels, where 850 mb temperatures will cool into the 10th
percentile through Saturday. Seasonally cool temperatures with highs
in the 60s will persist through Saturday as a result. By Sunday the
low-level flow will become more westerly, advecting the thermal
ridge into the area and kicking off a clear warming trend that is
evident in the interquartile high temperature spread. The 25th
percentile jumps from 60-70 between Saturday and Sunday, though the
interquartile spread is about 6-10 degrees from the weekend onward.
This uncertainty is due to uncertainty in the evolution of the mid-
level and surface pattern beyond Sunday. Ensemble guidance shows
chances for precipitation returning to the region for next week,
though specifics remain elusive at this point.

Delia

&&

.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 416 AM CDT Tue Apr 28 2026

An MVFR stratus deck has moved into the region in the wake of
yesterday`s precipitation and cold front. It`s currently impacting
KUIN, KCOU, and KJEF with MVFR flight conditions. The southeastern
edge of the stratus is right over KSTL and KSUS, causing the
terminals to scatter between 3,000-3,800 ft. Uncertainty remains
in if the stratus will progress southeast as guidance indicates,
or if it will stall in it`s current location as observations are
pointing toward. I`ve leaned towards the observations within the
forecast and put VFR ceilings in at KSTL, KSUS, and KCPS. The MVFR
stratus is expected to lift during the late morning into early
afternoon, bringing all terminals to VFR flight conditions.

Showers and thunderstorms will move through the region this
afternoon into the evening hours, impacting mainly the mid-
Missouri (KJEF, KCOU) and St. Louis metro terminals (KSTL, KSUS,
KCPS). Confidence is lower in showers making it into KUIN, so
have kept showers out of this terminal for the moment. Showers and
thunderstorms will come to an end overnight, leaving conditions
dry for the remainder of the forecast. MVFR ceilings are expected
to move into the St. Louis metro terminals during the evening with
the showers and thunderstorms, though it is expected to be rather
progressive as another surface high pressure moves into the
region and shunts the MVFR stratus to the east with the
precipitation.

Delia

&&

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

$$

WFO LSX