743
FXUS63 KLSX 120333
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1033 PM CDT Tue Jun 11 2024

.KEY MESSAGES...

- Temperatures will warm well above normal over the course of the
  next week. The warmest conditions are expected Thursday, Sunday
  and Monday with heat index values reaching 100-105 in some locations.

- There is a slight risk for severe thunderstorms Thursday afternoon
  into Thursday evening. The primary threats will be large hail
  and damaging winds.


&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 212 PM CDT Tue Jun 11 2024

A broad view of the CONUS shows ridging in the 700-250mb layer over
parts of the southwest and south-central U.S. resulting in
northwesterly flow over the mid-Mississippi Valley. Mid to high
clouds have occasionally streamed overhead with limited impact on
sensible conditions. Surface high pressure has moved far enough
southeast for southerly flow to draw slightly warmer temperatures
into the Midwest, along with a modest increase in moisture.
Surface observations at 19z show dewpoints have climbed well into
the 50s to near 60 over central Missouri with 40s hanging on over
interior sections of Illinois. Temperatures generally ranged from
the mid-70s to near 80 degrees.

As we go through the remainder of the period, we will be watching a
an train of disturbances track west to east over the northern
section of the country. The disturbances are expected to trickle
southward with each consecutive pass. The parent upper low continues
to track across southern Canada with a cold front that trails
southwestward into the Upper Mississippi Valley. Modest upper
ridging over the northern Lower 48 will transition to zonal flow
Wednesday. Shortwaves embedded within the zonal pattern will
introduce potential for showers and thunderstorms over the Upper
Midwest, progressively reinforcing the surface cold front southward.
The first of these passes over the Wisconsin/Illinois border, which
is far enough to keep most precipitation out of the area. While CAMs
show an isolated shower or two along the front at times, soundings
depict a capped environment, limited moisture, and no upper
support to reasonably suggest any activity will survive this far
south. The dry forecast has been maintained.

Southwesterly flow will usher 850mb temperatures into the mid-
teens to near 20C Wednesday, along with a subtle increase in
moisture. Dewpoints in the low to mid-60s will accompany
temperatures well into the 80s and low-90s, which is only the
leading edge of the heat to come Thursday into early next week.

Maples

&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 212 PM CDT Tue Jun 11 2024

Yesterday`s guidance proves the NAM was the northern outlier with
most now showing the cold front in in the vicinity of the
Iowa/Missouri border Thursday afternoon. The front sinks southward
Thursday evening into Thursday night, bringing the greatest
potential for showers and thunderstorms, not without a few caveats
to consider. Deterministic guidance shows a rather impressive
west/east oriented corridor of SBCAPE reaching 3000-4000 J/kg with
the commonly excited NAM unsurprisingly over 5000 J/kg in some
places. The 75th percentile of LREF more reasonably projecting
between 2500-3000 J/kg with averaging pulling numbers down slightly.
Low/mid-level moisture pools along and ahead of the front with PWAT
values of 2 to 2.25 inches falling within in the top 5th percentile
of climatological means. Moisture and instability do not seem to be
in question, but surface to mid-level flow nearly parallels the
boundary, lessening potential magnitude of convergence underneath a
strong mid-level capping inversion. Bulk shear of 30-35 knots and
mid-level lapse rates of 7-8C should be sufficient enough to aid in
severe potential over northeast Missouri and west-central
Illinois. However, shear drops below 30 knots near I-70.
Additionally, there is no indication of a definitive low level jet
feature. While there is certainly potential, there is some
uncertainty as to how soon convective initiation occurs and just
how far south convection tracks before instability wanes through
the evening. While the cap may be an initial inhibitor, what does
break through could lead to large hail, provided updrafts are
robust and deep enough to overcome melting in the very warm
surface and mid-level layers. Parallel flow should quickly congeal
any individual cells into a linear segment that drifts south with
time. Soundings do show dry air aloft that will also exhibit wind
potential, but how long this survive and how far south will be
the question with greatest potential likely remaining north of
I-70.

Thursday night`s cold front is expected to stall over southern
sections of the area Friday. Where the front stalls could keep
conditions uncomfortable Friday into Saturday as dewpoints in the
70s pool along and south of the front. This is accompanied by
light and variable surface flow, limiting any breeze that could
otherwise provide some relief for those outdoors. LREF graphical
output shows the greatest spread in temperature quartiles (5-9
degrees) south of I-70, signaling the front likely stalls
somewhere over southern Missouri into southern Illinois. North of
the boundary could be a different story. Though still warm,
dewpoints range from upper 50s in the far north to the mid-60s
along I-70. This isn`t a dry airmass, per say, but it looks to be
relatively better than what is draped through southern sections of
the CWA.

Surface high pressure over the Upper Midwest moves into the Great
Lakes as upper height rises build from west to east Saturday. Mid-
level warm air advection kicks southwest to northeast Saturday
afternoon with surface flow turning out of the south later in the
day. This pushes 20-23C air northward with a resurgence of heat late
in the weekend into early next week. Some guidance suggests a few
showers and thunderstorms along the front as a weakening upper
shortwave trough traverses the central Plains into the Upper
Midwest. Much of the lift passes to the northwest, though, keeping
most of this potential over northwest Missouri into Iowa.

Though NBM quartiles are tightly clustered through the next several
days, I think the magnitude of warmth (upper 90s) is a bit
aggressive. Not only would this approach record highs in some cases,
the ample amount of low level moisture (dewpoints in the 70s) and
180-200 degree surface wind direction does not typically result in
near triple digit heat. Low to mid-90s is more reasonable.
Additionally, the GFS MOS is much lower with upper 80s to low-90s
and ECM MOS 90th percentile in the mid-90s. It`ll still be warm
regardless, but should limit the spread of triple heat index values
Sunday and Monday. It`s safe to say that a majority of the area will
be in the low to mid-90s with few upper 90s around urban areas.

The forecast becomes a little murky beyond Monday. Global
guidance shows a system over the Gulf of Mexico that could either
eject westward into Texas or northward along the Mississippi
Valley as a cold front drops southward out of Iowa. This will all
depend on the evolution of the southeastern U.S. ridge, which
plays out quite differently between long range deterministic
guidance.


Maples

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1022 PM CDT Tue Jun 11 2024

Dry and VFR conditions are expected through the period.  Winds
will out of the southwest at 8 knots or less.

Britt

&&

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

$$

WFO LSX