080
FXUS63 KLSX 071855
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
155 PM CDT Fri Jun 7 2024

.KEY MESSAGES...

- Multiple rounds of thunderstorms will impact the region between
  tonight and Sunday morning, with the best chance for rain
  overnight tonight. Locally heavy rainfall is possible across
  portions of central Missouri overnight.

- The best chances for severe thunderstorms will be Saturday
  afternoon, mainly across southeast Missouri. Confidence in
  thunderstorm development is low due to uncertainty in
  instability and forcing, but if thunderstorms develop they will
  be able to become severe and capable of damaging winds, large
  hail, and locally heavy rainfall.


&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 151 PM CDT Fri Jun 7 2024

The surface high over the mid-Mississippi Valley has kept the sky
clear today, allowing us to warm to just a few degrees above normal.
The high will continue to shift eastward over the evening as low-
level flow ramps up across the central Plains. Aloft, a disturbance
is still expected to pass through the northwesterly flow overnight,
and combined with the strengthening LLJ, will blossom thunderstorms
across Nebraska this evening. These storms will grow upscale into an
MCS, and slide into the forecast area overnight, driven by the
southeastward mid-level flow. A lack of instability (<500 J/kg
MUCAPE) from central Missouri eastward will cause the MCS to begin
decaying as it enters the forecast area and will keep severe weather
at bay. The mid-level disturbance and decaying MCS will exit the CWA
during the mid morning hours, leaving a developing warm front and
mesoscale convective boundaries littered across the area.

Through the day tomorrow, southwesterly low-level flow and some
sunshine is expected to build instability along and south of the
warm front. Guidance differs on how much instability forms and how
far north it extends, but confidence is increasing that at least
2000 J/kg of SBCAPE will form across the southern CWA with up to at
least 4000 J/kg of SBCAPE possible. Despite the increasing
instability, thunderstorm development during the afternoon and
evening remains uncertain. There is little support for thunderstorm
development with neutral mid-level heights and weak-surface
convergence around the warm front. While warm air advection
induced thunderstorms could still form off the existing
boundaries, confidence in more than isolated convection is low.
The best location for any thunderstorm development will be across
southeast Missouri where confidence is highest that instability
will be present.

Any storms that do form will be able to organize in the 30-
50 kts of 0-6 km shear, initially isolated thunderstorms will be
capable of large hail and damaging winds. With the deep layer shear
vector largely parallel to the anticipated boundaries, storms are
expected to grow into broken line segments during the evening, which
would result in a shift to mainly damaging winds as the main threat.
Localized heavy rainfall and nuisance flooding are the other
concern with storms that form into broken line segments or train
due to the orientation of the shear vector to the boundary. The
HREF PMM guidance indicates up to 1-2 inches of rain over 3 hours
is possible across portions of southeast Missouri Saturday evening
and overnight.

Convection will exit the forecast area overnight while another MCS
forms across eastern Kansas and tracks east-southeastward into
Missouri along a small mid-level disturbance. The track of the MCS
will depend on the location of the warm front, but confidence is
increasing that this system will track south of the I-70 corridor,
and mainly across southern Missouri. and exit the forecast area
during the early morning Sunday. Instability will wane overnight,
and convective elements within the MCS are not expected to be strong
to severe.

MRM

&&

.LONG TERM...  (Sunday through Next Friday)
Issued at 151 PM CDT Fri Jun 7 2024

The MCS will push the warm front to the south of the forecast area
Sunday, and in it`s wake a surface high will nose into the forecast
area from the northwest. Another mid-level shortwave will slide
through the area during the day Sunday, and spawn showers and
thunderstorms along the warm front. But with it expected to be south
of the forecast area, the CWA is expected to be largely dry Sunday.

Over the weekend a mid-level trough will form across the Great Lakes
region and dive into New England, amplifying the northwest flow
across the mid-Mississippi Valley and westward. By the start of the
work week, 70% ensemble members indicate that a mid-level cutoff low
will form across south-central Canada, pushing south along the
northwesterly flow into the mid-Mississippi Valley by mid-week. This
solution would result in an increased chance for showers and
thunderstorms during the first half of the work week. About 30% of
ensemble members do not show this cutoff low, and instead build a
shortwave ridge in over the mid-Mississippi Valley for the first
half of the work week. This second, drier solution would also be
slightly warmer than the other solution.

Ensemble guidance does agree on a shortwave trough pushing through
the central CONUS mid-week. It`s associated surface low will move
across the Great Lakes region and drag a surface front through
the mid-Mississippi Valley, resulting in increased precipitation
chances mid-week. In the wake of this shortwave trough a
progressive mid- level ridge will build in across Texas and place
the forecast area on the northern edge of the ridge. Warm air
advection will push 850 mb temperatures into the upper teens to
low 20s across the region starting Thursday, near the 90th
percentile for this time of year. Temperatures will jump above
normal as this ridge builds into the area, and this weekend could
be the first time KCOU and KUIN hit 90 degrees this year.

MRM

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1228 PM CDT Fri Jun 7 2024

VFR flight conditions will continue across the area through this
evening ahead of a large complex of showers and thunderstorms that
will move into central Missouri near midnight and progress
eastward over the remainder of the night. This complex will weaken
as it moves east but is still expected to impact all the terminals
with thunderstorms and MVFR visibilities. Visibilites will
improve as rain exits the terminals. This system will exit the
region Saturday morning. Winds will be light and variable
overnight, becoming southerly to southwesterly for most of the day
tomorrow before veering to the northwest Saturday afternoon.


MRM

&&

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

$$

WFO LSX