111
FXUS63 KLSX 250802 CCA
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
258 AM CDT Sat May 25 2024

.KEY MESSAGES...

-Severe thunderstorms are expected late Saturday night into Sunday
 morning and again during the afternoon/evening, with damaging
 winds, a tornado, and large hail possible.

-Locally heavy rainfall is possible with both rounds of severe
thunderstorms on Sunday.

-The upcoming work week will be dry with near normal temperatures.


&&

.SHORT TERM... (Through Sunday)
Issued at 253 AM CDT Sat May 25 2024

A cold front, associated with a retreating surface low and mid-level
trough, will continue to push through the forecast area early this
morning. As of 3 AM the front was located from roughly West
Plains, MO through the St. Louis metro to Champaign, IL. Behind
this front, a surface high and mid- level height rises will result
in a dry day with a largely clear sky.

By Saturday evening the nose of the upper level jet will advance
into the mid-Mississippi Valley at the same time as a negatively
tilted mid-level trough. At the surface a low will form across the
Central Plains, it`s attendant warm front surging northward
through Missouri overnight. The large scale ascent, enhanced by
the strengthening north-south oriented LLJ, will spawn convection
in the warm sector across portions of Kansas, Nebraska, and
western Missouri. Convection will become organized into an MCS and
track eastward through the forecast area along the nose of the
veering LLJ and supported by a shortwave sliding through the
advancing mid-level trough. The MCS is expected to move through
the area during the early to late morning hours. Where exactly the
MCS tracks remains uncertain, some guidance sources push it along
the I-70 corridor, while others push it across southern Missouri
and Illinois.

1500-3000 J/kg of MUCAPE and strong shear throughout the profile
combined with cold pool support from the MCS will allow portions of
the MCS to become strong to severe. The main concerns with any area
of the MCS that become severe will be damaging winds and embedded
tornadoes, although large hail can`t be ruled out.

The MCS will exit during the morning hours, and the path the MCS
takes through the CWA will determine where the outflow is deposited,
and thus where the best chance for afternoon re-development will be.
Thunderstorms are expected to redevelop during the afternoon and
into the evening as the mid-level trough pushes into the forecast
area. Moisture and instability will advect into the forecast area
Sunday behind the morning convection within the warm sector, but how
much destabilization occurs in the wake of the MCS remains
uncertain. Ensemble minimum surface based CAPE from the HREF shows
1000-2000 J/kg of instability present across the entire forecast
area tomorrow afternoon, increasing confidence that at least some
destabilization will be able to occur. Confidence is highest that
the best storm environment will be along and south of the outflow
boundary left over from the morning convection. Here instability
will be high with MLCAPE values between 2000-4000 J/kg.
Additionally, favorable bulk shear (30-50 kts of 0-6 km), and 7-8
C/km mid-level lapse rates will support severe thunderstorm
development. Multicell clusters and bowing segments that become
severe will be capable of significant damaging winds, as well as
large hail and tornadoes.

The NAEFS situational awareness table indicates integrated water
vapor transport (ie moisture advection through the atmosphere) will
be in the 90th percentile Sunday. The high moisture content combined
with the potential for two rounds of thunderstorms and training
thunderstorms introduces a heavy rain risk. There is a particular
risk for heavy rain and localized flash flooding in areas that
saw heavy rain tonight. The probability-matched mean QPF from the
HREF indicate that with both the morning MCS and afternoon
convection that QPF could exceed 2 inches/3 hours.

These storms will track southeastward over the evening, chased by
the attendant cold front of the previously mentioned surface low.
Additional thunderstorm development along the front is not
anticipated to be widespread given the weak convergence along the
front. The front will exit the forecast area by Monday.

MRM

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 253 AM CDT Sat May 25 2024

Behind this system largely dry and near seasonable temperatures
will persist into the second half of the work week under a
northwesterly flow regime. Another pattern change will take place
towards the end of the period as the mid-level ridge that will be
over the Rockies during the work week shifts eastward and throws
the mid-Mississippi Valley back into southwest flow aloft. This
will bring above normal temperatures and chances for precipitation
back to the forecast.

MRM

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1144 PM CDT Fri May 24 2024

In the wake of convection this past evening, VFR conditions exist
at the TAF sites currently. There are some areas of fog where
heavy rain fell today, but there isn`t much to suggest the
terminals will be impacted by this. Dry and VFR conditions will
persist through the valid TAF period, with the exception of KSTL
which will see an increasing threat for thunderstorms by the end
of its forecast period.

MRB

&&

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

$$

WFO LSX