406
FXUS63 KLSX 010803
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
303 AM CDT Sat Jun 1 2024

.KEY MESSAGES...

- Rain will gradually taper from west to east today, with a few
  thunderstorms possible this afternoon along and east of the
  Mississippi River. After patchy morning fog, Sunday looks
  largely dry with seasonable temperatures.

- Our return to a relatively-unsettled pattern begins Monday,
  with daily chances for showers and thunderstorms through late
  Wednesday. This is accompanied by near/slightly-above-normal
  temperatures, but exactly how warm we get will depend on the
  uncertain timing and location of that rain.

- A pattern change begins late Tuesday that will pull
  temperatures back closer to normal, if not below normal, and
  usher in drier weather by late-week.

&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 301 AM CDT Sat Jun 1 2024

Widespread light to moderate rain enveloped the bi-state area
overnight to mark the start of meteorological summer, forced by a
weak surface low, modest moist isentropic ascent, and a meager
low- level jet. Nearly all thunderstorms (and heavy
rain/associated impacts) were relegated to the Ozarks, where
better deep moist convergence near a stationary front interacted
with more appreciable MUCAPE. The surface low, and mid-level
shortwave associated with it, will gradually lift north and east
through the region today. As a result, rain will be slow to exit
and cloud cover will remain largely fixed in place. This will keep
our temperatures from warming above the upper 70s at best in
central Missouri, with cooler temperatures further east where
clouds/rain last longer. Along the "cold front" of this weak low,
scattered thunderstorms will develop near the St. Louis
metropolitan area and south, tracking east through the afternoon.
With weak shear and unimpressive upper-level forcing, any
convection will be on the weaker side.

Low-level (925mb) moisture will linger through the night across part
of the forecast area, keeping broken to overcast cloud cover in
place for at least those along and east of the Mississippi River.
West of that, gradual clearing and weak low-level subsidence will
lead to a threat for fog overnight where saturated ground will
provide a sufficient moisture source. The best chance for this fog
would be in the Ozarks and central Missouri, where cloud cover will
be less (though not completely clear). That said, the speed of the
clearing is uncertain and will dictate if and and how far east fog
develops. If clouds exit earlier than currently forecast, fog could
impinge on eastern Missouri and western Illinois as well.

A weak surface high and subtle ridging aloft embedded within large-
scale quasi-zonal flow will work to keep Sunday largely dry, and
with 850mb temperatures steadily rising through Sunday, surface
temperatures will rise back closer to normal. There is a low (15-
20%) threat for thunderstorms in central Missouri during the late
afternoon and evening as a decaying convective complex ejects from
the Plains, but MLCAPE will be somewhat limited and modest
subsidence from the ridge aloft will be in place. All of this is to
say that any activity later on Sunday in central Missouri looks
meager, if it reaches that area at all.

MRB

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 301 AM CDT Sat Jun 1 2024

The quasi-zonal flow will remain fixed overhead throughout much of
the CONUS through late Tuesday, leaving the region open to shortwave
impulses that would interact with sufficient moisture and
instability to threaten a continuous potential for showers and
thunderstorms from late Sunday through Wednesday. That said, this
forecast is more a function of the uncertainty for when each wave of
rain would arrive - it will not be actively raining this entire
time. Along with the persistent rain chances, this flow pattern will
draw 850mb temperatures to near the 75th climatological percentile
and promote near to slightly-above-normal temperatures (mid/upper
80s) at the surface.

There is growing agreement that one of these waves will track into
the Mid-Mississippi Valley during the afternoon and evening on
Monday, but the exact track and amplitude of this wave varies in the
deterministic guidance and ensemble member suite. Regardless, this
is our first more tangible threat for showers and thunderstorms. The
amplitude/track uncertainties also lead to somewhat varied
depictions of shear parameters on Monday afternoon, but even the
more bullish guidance sources keep strong to severe thunderstorms a
remote possibility at this time. In that scenario, the best threat
for any stronger convection would be across the Ozarks where
instability will maximize amidst the passing shortwave. The SPC Day
3 Convective Outlook highlights this area in a Marginal Risk, which
we are not messaging at this time given the highly-conditional
nature of the threat.

Early Tuesday morning, guidance continues to depict a strong upper-
level trough ejecting out of the Pacific Northwest and tracking east
through the northern Great Plains into the Great Lakes region. This
feature spawns a surface low that takes a similar track, which will
send a cold front south and east through our region. Exactly when
this cold front reaches the bi-state is relatively uncertain, but is
still reasonably expected during the Tuesday evening to Wednesday
morning timeframe. In addition to being another focus for showers
and thunderstorms, this upper-level wave and its surface reflection
will be the harbinger of a pattern shift that has potential to usher
in below-normal temperatures and overall drier conditions.

What looks all but certain is that the wave will help establish a
ridge-trough pattern over the CONUS, placing the forecast area in
northwest flow aloft to promote largely-dry weather. However, model
depictions of the flow pattern quickly diverge with the location of
a closed upper-level low varying from the Ohio River Valley to the
Mid-Mississippi Valley. Others diverge even further from these
solutions, but these two are the most prevalent in the guidance.
Where the ULL settles will be pivotal in how much we cool late next
week. While ensemble guidance overall trends cooler, the ensemble
interquartile range spans from near-normal to nearly 10 degrees
below normal. The current forecast, which keeps temperatures close
to normal, uses the National Blend of Models given the high
uncertainty near the end of the period. That said, if the patterns
being shown in the ensemble guidance come to fruition, we`ll likely
be cooler than currently forecast.

MRB

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1035 PM CDT Fri May 31 2024

A weather system continues to gradually move through the region,
impacting all local terminals with rain and increasingly low
ceilings. The rain is mostly prevalent over central and eastern
portions of Missouri, primarily impacting KCOU and KJEF. KUIN,
KSUS, KSTL, and KCPS are on the eastern edge of this rain, and are
expected to see intermittent rain through the night. Brief,
embedded heavier showers and possibly a thunderstorm may produce
lower visibilities at time, but confidence in these directly
impacting terminals remains low. Additionally, low stratus will
continue to creep into the region from southwest to northeast,
eventually bringing IFR ceilings to all local terminals by mid
Saturday morning. While the greater rain chances will clear out
during the morning, the ceilings will be slow to improve. VFR
flight conditions are current forecast by the end of the current
forecast period; however, given the timing and hints from some
model guidance, I can`t rule out that some local terminals remain
MVFR or IFR into tomorrow night. While the better rain chances
clear out during the morning, the chance for scattered showers and
possibly thunderstorms will carry into the afternoon for most of
the local terminals. This convection is expected to remain east of
KCOU and KJEF at the moment.

Elmore

&&

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

$$

WFO LSX