283
FXUS63 KLSX 071049
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
549 AM CDT Fri Jun 7 2024

.KEY MESSAGES...

- A decaying complex of thunderstorms is expected to move into the region
  late tonight through early Saturday. Thunderstorms are not
  expected to be severe, but will be capable of locally heavy
  rainfall up to around 2 inches.

- Another round of thunderstorms is expected late Saturday
  afternoon and evening; however, where these form will depend on
  the exact timing and tracking of tonight`s thunderstorms, which
  is uncertain at this time. Where this second round of
  thunderstorms forms, thunderstorms will be capable of isolated
  occurrences of large hail, damaging wind gusts, and locally
  heavy rainfall.


&&

.SHORT TERM... (Through Saturday Night)
Issued at 313 AM CDT Fri Jun 7 2024

Current water vapor imagery shows northwesterly flow aloft over the
Middle Mississippi Valley. At the surface, an area of high pressure
is centered over the Central Plains, with surface winds in the CWA
out of the west-northwest. Through today, this northwesterly flow
along the southern-southwestern periphery of an upper-level trough
will remain in place over the region, helping the surface high to
shift eastward. With the surface high moving overhead, our weather
will remain calm and clear, with temperatures topping out just above
climatological normals thanks to clear skies and deep mixing of the
boundary layer. Temperatures at 850mb are forecast to reach 16-17
degrees C this afternoon across the CWA, climatologically
correlating with surface temperatures in the mid to upper 80s.

Tonight, a subtle shortwave will pass through the Central Plains and
into the Middle Mississippi Valley. Ahead of this shortwave, a low-
level jet will ramp up and pair with southerly surface flow to
advect warm moist air into the Central Plains, aiding in
thunderstorm development this afternoon. These storms are expected
to grow upscale into a complex that moves east-southeast toward the
CWA tonight as the low-level jet veers. The current thinking is that
these storms will outrun the better instability and weaken as they
near and enter the CWA. However, I can`t rule out some isolated
occurrences of wind gusts up to 50 mph as storms move into central
Missouri after midnight. Of greater concern is that storms may train
over the same locations given the positioning of a warm front
southwest of the CWA and the nose of low-level jet moving into the
CWA overnight. Where storms are able to train, a quick two inches of
rainfall may occur, leading to a low risk of flash flooding.
However, the progressive nature of the overall storm complex is
expected to keep the threat of flash flooding low and isolated.

The exact evolution of this storm complex will have implications on
the development of additional thunderstorms Saturday afternoon and
evening as another subtle shortwave moves out of the Plains into the
Midwest, tracking a surface low along the previously mentioned warm
front. South of the front and/or any outflow boundary from the first
round of convection, an unstable airmass characterized by around
3,000 J/kg of SBCAPE and about 40 kts of 0-6 km bulk shear will
exist. As the shortwave approaches, isolated to scattered showers
and thunderstorms will develop along the front/outflow boundary,
which may be draped across southeastern Missouri and southwestern
Illinois. Given the instability and shear profile, initial storms
will be discrete and likely supercellular, carrying a threat of
damaging wind gusts and large hail, the latter in particular for
elevated storms north of the boundary. Given that shear vectors are
nearly parallel to any initiating boundary, upscale growth into a
complex or lines of storms is expected, which would translate to
primarily a damaging wind threat. Southwesterly surface winds and
weak low-level shear leads to very low confidence in tornado threat
with these storms. Any severe threat is expected to be isolated at
this time. Given the orientation of the boundary and the low-level
jet ramping up along it Saturday evening and night, another round of
training convection is possible, leading to localized heavy rainfall
where they train. This may lead to a flash-flood threat, but varying
solutions on where exactly storms will track makes this threat
uncertain at this lead time.

Elmore

&&

.LONG TERM... (Sunday through Thursday)
Issued at 313 AM CDT Fri Jun 7 2024

At the start of the period on Sunday, deterministic guidance shows
the previously discussed front moving gradually south of the CWA
beneath northwesterly flow and a shortwave aloft. How far south this
boundary is able to progress Saturday night relative to the CWA will
determine how far north rain chances reach into the CWA through at
least Sunday morning. Per ensemble clusters, if the front is able to
positioned further north, rain chances may reach as far north as
roughly the I-70 corridor with chances lasting longer into the day
across the southern half of the CWA. If the front progresses further
south, then only portions of far southeastern Missouri and far
southwestern Illinois may see the chance for rain, with chances
winding down earlier in the day. The upper-level trough centered
over the Northeast will broaden and dig slightly southward during
the day Sunday, helping to push the front southward and well away
from the area Sunday into Monday as high pressure moves into the
region.

This will be an unseasonably cool airmass for the area to end the
weekend and start next workweek. Even the 75th percentile of
ensemble guidance has daily highs Sunday-Tuesday roughly 5-10
degrees below climatological normals, increasing confidence in this
cool down. How cool exactly remains uncertain, as both deterministic
guidance and ensemble clusters show widely varying solutions of the
phasing of a shortwave located over south-central Canada on Sunday.
Two more likely solutions seem possible: one where the shortwave
either becomes cut off or slows drastically over southern Canada or
the Northern Plains, and one where the shortwave digs equatorward
along the backside of the trough through the Plains and Midwest. If
the former solution is realized, temperatures would be cooler, but
closer to normal and dry conditions more prevalent. If the latter
solution is realized, then temperatures would run even cooler as a
second cold front moves through the area along with a chance for
rain sometime late Monday into Wednesday. The chance for rain would
not extend through that entire stretch - just occur at some point
within that window.

The evolution of this shortwave early in the week will drive how
soon the eastern trough and northwesterly flow aloft will shift
eastward away from the region. If the shortwave does not pass
through the Plains and Midwest, the through will be quicker to
depart the region and allow for upper-level ridging to edge in. This
will result in temperatures during the middle of next week rising
back to around climatological normals. However, if the shortwave
does dig through the Plains and Midwest, some deterministic guidance
and ensemble clusters show that the warm up will be delayed and
slower relative to the first solution.

Elmore

&&

.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 538 AM CDT Fri Jun 7 2024

VFR flight conditions are expected through the day today ahead of
a complex of thunderstorms tonight. This complex will begin
entering the area from the west later this evening and exit by
around the end of the TAF period tomorrow morning. There is still
some uncertainly on where exactly this complex will track
relative to the local terminals, and therefore, where the heaviest
rain will fall. Any of the local terminals could see visibility
lower than currently forecast depending on the location of the
heaviest rain. Low ceilings will accompany the northern portion
of this complex, impacting KUIN with MVFR flight conditions.
Confidence is currently high that these lower ceilings will remain
north of other local terminals.

Elmore

&&

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

$$

WFO LSX