874
FXUS63 KLSX 021741
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1241 PM CDT Sun Jun 2 2024

.KEY MESSAGES...

- Dense fog will continue to threaten parts of northeast and
  central Missouri, as well as west-central Illinois, through
  10am. A Dense Fog Advisory remains in effect until then.

- After a low (15-30%) chance for showers and thunderstorms
  across northern Missouri today, a more widespread threat for
  scattered thunderstorms materializes Monday. A few of these
  thunderstorms could produce large hail and damaging wind, but
  most will be sub- severe.

- A pattern change still looks all but certain starting late
  Tuesday that will usher in somewhat less-active weather by late
  in the week.


&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 302 AM CDT Sun Jun 2 2024

Weak high surface pressure is settled across the Upper Mississippi
Valley in the wake of a slow-moving surface low and attendant weak
mid-level wave. In between ongoing low stratus across most of
Illinois and incoming cirrus from remnant convection across the
Plains, clear skies and light winds allowed for fog to develop in
parts of central/northeast Missouri and far west-central Illinois
overnight. This threat for fog, which will be dense in spots,
will persist until diurnal heating can allow for humidity to
decrease sufficiently. A Dense Fog Advisory will be in effect
until 10am, but given we are approaching the summer solstice and
the sun angle is high, fog will likely dissipate before that. As
the day progresses, high-resolution guidance hints at an MCV or
remnant outflow boundary from the convection currently in the
Plains drifting through northern Missouri. Provided instability
can recover this afternoon with warmer temperatures returning, and
the low-level lift is sufficient, isolated to widely-scattered
showers and thunderstorms are possible in that area. Other than
heavy downpours and a few lightning strikes, no impacts are
expected from the convection.

Nocturnal convection in the central Plains will enter western
Missouri overnight Sunday into Monday morning, but all indications
suggest that this activity will weaken as it approaches us and
interacts with more stable air. Anything more impressive will more
likely follow along the nose of the low-level jet into Iowa. That
said, warm and humid southwest low-level flow will allow for
instability to build through the morning on Monday. With the
possibility of an outflow boundary existing in the region from the
earlier thunderstorms and most deterministic guidance developing a
weak mid-level shortwave in the quasi-zonal flow aloft, the
potential for at least scattered thunderstorms exists for much of
Monday. During peak heating, when instability will maximize, a few
thunderstorms may strengthen and become marginally-severe. While
deep-layer shear is uniformly unimpressive (15-25kts) among
deterministic guidance, weakly-capped MLCAPE values of 1500-2000
J/kg in this shear parameter space would threaten a few strong
thunderstorms with large hail and damaging wind across the region.
If there is a remnant boundary in the region to focus strong
convection, the severe weather threat may be locally-higher around
that feature. That said, I have no confidence in that feature
existing, much less where it would be. The SPC Day 2 Convective
Outlook was expanded north across the forecast area, and we`ve begun
messaging the potential for isolated severe thunderstorms on
Monday.

MRB

&&

.LONG TERM...  (Monday Night through Saturday)
Issued at 302 AM CDT Sun Jun 2 2024

By late Monday night, a shortwave will dig across the northern
Rockies and track east across the northern CONUS. This wave, and the
surface low/attending cold front that it develops, will put the
wheels in motion for a change in the upper-level flow pattern and
sensible weather. Before that, however, another warm and humid day
will be in store for the Mid-Mississippi Valley on Tuesday ahead of
the cold front. While capping appears stronger on Tuesday and deep-
layer shear is a bit weaker, we may see a similar setup as we do on
Monday. There are some deterministic models that even develop a
shortwave ahead of the more impressive mid-level wave Tuesday
afternoon, which would provide synoptic-scale ascent for stronger
thunderstorms. That said, there is no concrete signal that all
parameters overlap for severe thunderstorms just yet. The SPC Day 3
Convective Outlook keeps any threat for severe weather to the
northwest of the forecast area, but that may change as time goes on.

The first hint of the pattern change arrives late Tuesday night into
Wednesday, when ensemble and deterministic guidance generally agree
the main cold front traverses the region. This will be our best
potential for widespread rain in the forecast period, but the time
at which it`s currently forecast to pass will likely preclude it
from a strong to severe thunderstorm risk. That said, the front
won`t have much cold air behind it with most guidance suggesting
850mb warm-air advection persisting behind it. However, it will
scour out low-level moisture and lead to more comfortable
conditions across the region. The ridge-trough pattern that
establishes aloft would be supportive of another shortwave and
weak cold front dropping south amidst the cyclonic flow, bringing
a low threat for rain Thursday. While some deterministic guidance
suggests this, the NBM and most ensembles do not. Rain chances
within similar cyclonic flow patterns are often tricky to catch
this far in advance amidst global-scale guidance, so I wouldn`t
count on a completely dry Thursday just yet. That caveat goes on
through Friday as well: amidst cyclonic flow aloft, we may see
diurnal showers along and east of the Mississippi River that are
not currently reflected in the forecast. However, this hinges
greatly on the uncertain location and amplitude of the eastern
trough, which led to leaving the NBM PoPs alone through the
extended period.

High temperatures will gradually cool day-to-day with northwest flow
remaining established aloft into the weekend, but exactly how cool
we get is also tied to how the upper-level flow pattern evolves. The
current forecast does not cool daily highs notably until Saturday,
but some ensemble solutions suggest we may see cooler weather as
early as Thursday. Low (15-35%) rain chances return to the region by
Saturday, owing to a potential weakening in the western upper-level
ridge and longwave troughing taking hold of the CONUS. If this
occurs, the near-zonal flow aloft would threaten a low risk for
precipitation. The preponderance of guidance show a more
amplified ridge-trough pattern that keeps the region dry. Again,
the NBM PoPs were used in this late stage of the forecast, which
adequately hints at the first scenario described.

MRB

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1235 PM CDT Sun Jun 2 2024

VFR flight conditions are expected this afternoon and tonight.
There is a chance for showers and thunderstorms impacting parts of
central/northeast MO and west-central IL (KJEF/KCOU/KUIN) on
Monday morning, however uncertainty on timing and both presence
and coverage of showers and storms is above normal thus included
only VCSH or VCTS at this time.

Glass

&&

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

$$

WFO LSX