133
FXUS63 KLSX 121028
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
528 AM CDT Fri Jun 12 2026

.KEY MESSAGES...

-Near normal temperatures and a clear sky will prevail today.

-Showers and thunderstorms return to the region Saturday with
multiple chances for severe thunderstorms.

-Below normal temperatures and dry conditions will end the weekend
 and kick off the work week.


&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 313 AM CDT Fri Jun 12 2026

A cold front is dropping southeast through the region early this
morning, already having made it through the St. Louis metro. Cooler,
drier air is moving into the region in its wake, scouring out cloud
cover and bringing relief from the past several days of summer-like
heat. Under the influence of the cold air advection 850 mb
temperatures will drop to 14-16C, resulting in high temperatures in
the low to mid 80s area wide, normal for this time of year.

Thunderstorm chances, including the risk for severe thunderstorms,
return to the forecast Saturday. Mid-level zonal flow over the mid-
Mississippi Valley will encourage multiple mid-level disturbances to
pass over the region Saturday, increasing chances for thunderstorms.
Uncertainty exists in the development of specific rounds, and
similar to many recent events, the fate of later rounds depends on
the development of earlier ones. Some guidance, though not many of
the most recent hi-res CAMs, indicate an MCS rolling east into the
mid-Mississippi Valley during the morning. Modest instability during
the mid-morning hours, combined with the MCS structure, has the
potential to produce instances of damaging winds as it progresses
east. Confidence in the development and maintenance of the MCS
through our forecast area, and it`s influence on later convection,
is low.

There is much higher confidence in thunderstorms forming along a
cold front that will drop into the region during the late
afternoon/evening hours from the northwest. Initial thunderstorm
development along the front will be isolated, though will quickly
transition to bowing line segments as widespread initiation forces
cold pool conglomeration. 30-40 kts of 0-6 km shear will support
thunderstorm maintenance, particularly before the bowing segments
and associated cold pools develop. How much instability is present
will determine how many of these storms are able to become severe.
If the morning MCS materializes and moves through the area, there is
at least one scenario where instability struggles to rebuild across
the area in its wake and limits further severe thunderstorm
development. If the MCS does not develop, or if the warm, moist air
advection along the areawide southwest flow is not limited by the
MCS, 1500-3000 J/kg of MLCAPE and widespread severe thunderstorms
are expected across the area. With any isolated severe
thunderstorms, all hazards are on the table (large hail, damaging
winds, a tornado), but as bowing segments develop the hazards will
narrow to damaging winds and a brief, weak tornado or two. This
second round of severe thunderstorms is more likely and confidence
is higher that severe weather will occur with the afternoon/evening
storms.

Flash flooding is also a concern with thunderstorms that form ahead
of and along the cold front Saturday evening. Thunderstorms will be
capable of high rainfall rates given the 1.4-1.7" PWATs (near 90th
percentile), warm cloud depths, and high instability. And CAMs are
indicating that multiple thunderstorms could pass across the same
location, and antecedent conditions are quite moist from the past
48 hours of thunderstorms. The HREF LPMM 24 hour rainfall amounts
ending 00Z Sunday (and thus not encompassing the entire system)
suggest localized amounts of up to 5" (worst case scenario). Flash
flooding is possible even where rainfall amounts fall short of
this worst case scenario, given the moist soils across the region.

Delia

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 313 AM CDT Fri Jun 12 2026

The cold front will push through the region overnight Saturday into
Sunday, bringing more robust cold air advection to the region
compared to today. 850 mb temperatures will drop into the upper
single digits to near 10C, resulting in high temperatures Sunday
into Monday in the 70s, 10 degrees below normal for this time of
year. Into the first half of the work week the mid-level flow will
become more northwesterly as a trough over the Great Lakes region
becomes more amplified. Low-level flow will become southwesterly by
Tuesday however, restarting warm air advection and slowly warming
temperatures back to near normal by the end of the forecast period.
The northwesterly flow will persist through the end of the forecast
period, and disturbances within the flow will bring a return of
showers and thunderstorms to the region starting Tuesday.

Delia

&&

.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 524 AM CDT Fri Jun 12 2026

Dry and VFR flight conditions will continue into Saturday morning.
There is a 40% chance for showers and thunderstorms to develop
west of the region early Saturday morning and move east through
the mid-Missouri (KJEF, KCOU) and St. Louis metro (KSTL, KSUS,
KCPS) terminals. Confidence in this occurring is low, so have left
any mention of this out of the TAFs for now. The better chance for
showers and thunderstorms will be Saturday evening along a cold
front, though this is after the current TAF period.

Winds will remain less than 10 kts through the day coming from the
northwest. Overnight into Saturday as the surface high over the
region shifts east, winds will become easterly, though still
remain less 10 kts.

Delia

&&

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

$$

WFO LSX