276
FXUS63 KLSX 130835
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
335 AM CDT Thu Jun 13 2024

.KEY MESSAGES...

- There is a threat of severe thunderstorms this evening, mainly over
  northeast Missouri and west-central Illinois. The primary
  threats are large hail and damaging winds. There there is a low
  threat for tornadoes.

- Temperatures will run 5-15 degrees above normal through at least the
  middle of next week. The hottest days (today, Sunday, and
  Monday) will have highs at least in the mid 90s and nighttime
  lows no cooler than 70 degrees.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 322 AM CDT Thu Jun 13 2024

A few storms have developed over north central/northeast Missouri
as of 0730z and will continue to move southeast early this
morning. It should diminish by daybreak.

Otherwise, the main issue in the near term will be the temperatures
for today.  As the upper level ridge builds in and the surface ridge
continues to slide off the southeast, surface flow will pickup from
the south to southwest. This will usher in plenty of moisture with
dewpoints rising into the upper 60s to low 70s by this afternoon. In
the meantime 850mb temperatures will rise to between 22 and 25C with
surface temperatures topping out in the low to mid 90s, though could
see mid to upper 90s in the urban heat islands. With such hot
temperatures and high dewpoints, will see afternoon heat index
values 100-104.

As for storm chances later today, a majority of the latest CAMs are
in good agreement that storms will fire up along the cold front that
will stretch east to west across far southern Iowa between 20z and
22z as the cap eventually breaks. Then as the front slowly sinks
south through the forecast area, this activity will move into
northeast Missouri and west central Illinois. As for what type of
convective mode initially, still feel confident that it will be
supercellular as the latest forecast soundings have MU CAPEs in
excess of 3000 J/kg, 0-6km bulk shear 30-40kts, and ML LR 7.5-8C/km.
So the main threats will be large hail and damaging winds initially.
Then with mean mid-level flow running parallel to the front, this
would suggest that the discrete development will become linear with
the primary threat transitioning to mainly damaging winds. Even
though low-level shear (0 to 1km shear around 10kts) will not be
particularly strong, a couple of tornadoes will also be possible,
given the potential for a few intense supercells within a strongly
unstable environment.

As the cold front and associated activity pushes south through the
evening, the instability and shear lessen considerably. Thus there
is still some uncertainty on exactly how far south strong to severe
storms will make it before waning. For now, best chances will be
along and north of I-70 between 23z-04z.

Still some uncertainty on exactly how far south the cold front will
move before stalling out. The majority of the latest deterministic
models have the front stall out just south of the forecast area
between 12z and 18z Friday. With moisture pooling along the boundary
and moderate instability with MU CAPEs between 2000 and 3000 J/kg,
could see some isolated activity flare up during the afternoon
hours, so added slight chance POPs in portions of southeast Missouri
between 18z Friday and 00z Saturday.

As for temperatures on Friday it will be a bit cooler, but still
above normal in the upper 80s to low 90s.

Byrd
&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 322 AM CDT Thu Jun 13 2024

As the upper level ridge slowly slides to the east Friday night
into Saturday, the next system over the Central Plains will begin
to lift northeast towards the Great Lakes region. As it does so,
the stalled front to our south will begin to lift northward as a
warm front on Saturday. A majority of the latest deterministic and
ensembles keep the best chances of showers and storms just off to
the west and northwest of the forecast area at this time where
the main upper support is. However, there is a chance for isolated
showers/thunderstorms Saturday as the front moves northward, so
have added slight chance POPs over portions of central Missouri.

Otherwise, the warmup begins in earnest as hot and humid weather
returns for the rest of the weekend and into next week. So with
dewpoints in the 60s to low 70s and temperatures in the 90s, could
see heat index values between 100 and 105, especially Sunday and
Monday. However, some of the latest deterministic models and
ensemble runs are indicating that the upper ridge may weaken and
move a bit further to the east by Sunday with shortwave energy
sliding along the western and northern periphery of the ridge.
This would increase chances of showers and storms over the region
during this period, which could affect the warmup. But confidence
is low at this time for this solution.

Byrd

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1040 PM CDT Wed Jun 12 2024

VFR conditions will prevail through approximately 00Z Thursday
night before a cold front threatens convection at the terminals
Thursday evening into the night. The best potential for
thunderstorms exists at KUIN during the early evening, with the
threat gradually decreasing as the convection moves further south.
VCTS was added at all TAF sites (except for KSUS and KCPS, since
the convection will arrive after the TAF period) during the time
convection would most likely be near the terminals. However,
there is little confidence that any terminal will be directly
impacted. Following the convection, a cold front will pass south
of the I-70 corridor and bring a drier, northerly wind to the
region.

MRB

&&

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

$$

WFO LSX