829
FXUS63 KLSX 182025 CCA
AFDLSX

Area Forecast Discussion...CORRECTED
National Weather Service Saint Louis MO
325 PM CDT Tue Jun 18 2024

.KEY MESSAGES...


- A few showers and possibly weak thunderstorms are possible
  across the Ozarks and southern Illinois late this afternoon, and
  again tomorrow afternoon across northeast Missouri. Storms
  tomorrow afternoon have a slightly higher chance of being
  strong, but chances are low overall.

- Chances for showers and thunderstorms are higher (30-50%)
  between late Saturday and Sunday, but confidence in timing and
  coverage remain low.

- Otherwise, temperatures will gradually rise through the week
  and peak over the weekend when heat indices will likely reach
  between 90 and 100 degrees, with the potential to climb above
  100 in the St. Louis metro area.

&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 323 PM CDT Tue Jun 18 2024

The primary forecast item of interest in the short term continues to
be the potential for isolated showers and thunderstorms, both later
this afternoon across the Ozarks and southern Illinois and again
across northeast Missouri tomorrow afternoon.

As of 2 PM, deep south to southwesterly flow is in place across the
mid-Mississippi Valley region, with a large upper ridge across the
eastern CONUS, and a corresponding trough across the west. Embedded
within this southwesterly flow is a subtle trough just to our south,
which is slowly advancing northward this afternoon and spreading
ample mid and upper cloud cover into southeast Missouri and southern
Illinois, along with some very light stratiform rain. Elsewhere,
fairweather afternoon cumulus has developed, but is being suppressed
by warmer temperatures aloft. As the afternoon progresses, the
advancing impulse may support isolated convective development within
the broader cloud/precipitation shield across the Ozarks and
southern Illinois, although meager instability and weak wind shear
will prevent this activity from becoming particularly threatening
aside from a strike of lightning or two. Any showers than manage to
diminish through the evening. Otherwise, temperatures will peak in
the mid 80s to near 90 degrees today, which is several degrees
cooler than yesterday.

Early in the day tomorrow, a weakening cold front will slowly sink
into northeast Missouri, perhaps augmented by outflow from overnight
showers and thunderstorms. While this boundary is expected to stall
as it approaches, it may bring a round of scattered showers and
thunderstorms into northeast Missouri in the morning, with possible
redevelopment during the afternoon as the atmosphere destabilizes.
In spite of strong surface heating (temperatures in the low to mid
90s), instability is not expected to be particularly robust thanks
to poor mid-level lapse rates, with SBCAPE potentially reaching 1000-
1500 J/kg. Meanwhile, 0-6km shear values will also be marginal (20-
30kt), as most of the stronger upper level winds will remain near or
above the equilibrium level per model soundings. Still, just enough
instability and shear will be present to support a few stronger
clusters capable of producing isolated damaging wind gusts and small
hail. Most of this stronger activity is likely to remain to our
north, but a strong storm or two can`t be completely ruled out
across northeast Missouri. Elsewhere, warm and dry conditions will
prevail tomorrow.

BRC

&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 323 PM CDT Tue Jun 18 2024

Over the course of Thursday and Friday, the previously mentioned
upper ridge of high pressure across the eastern CONUS will gradually
expand westward, causing temperatures to slowly increase and largely
suppressing convection as it does so. While the upper ridge is
expected to reach its peak heights on Friday, temperatures will
continue to increase heading into Saturday as a cold front
approaches from the northwest. While confidence is very high that
temperatures will climb to well above average, likely at least into
the mid-90s, ensemble mean 850mb temperature percentiles are not
particularly anomalous in either the NAEFS or GEFS ensembles, which
my limit the upper ceiling and mitigate potential heat impacts.

Further complicating this picture is the arrival of a cold front
sometime late Saturday, which remains likely to bring a round of
showers and thunderstorms into the area. The timing of this feature
may impact temperatures to some degree, but thus far this has not
been well reflected model forecasts. In fact, ensemble temperature
forecast spreads remain narrow Sunday and similar to Saturday,
suggesting that this front may not bring a meaningful cooling trend.
This would be especially true if convection arrives overnight
Saturday and diminishes before heating commences during the day.

Meanwhile, there have been some minor changes in the potential for
showers and thunderstorms between late Saturday and Sunday
afternoon. This continues to be the most likely timeframe for
showers and storms over the next week (30-50%), and this time range
has narrowed slightly from the previous forecast. There will also be
at least some potential for strong or severe thunderstorms, although
much will depend on the timing of storms and available instability.
Still, the passage of a shortwave trough will likely spread at least
modest mid-level flow into the area, perhaps enough to produce
sufficient shear for updraft organization. Early deterministic model
forecasts suggest upwards of 30 to 40 kt of deep layer shear and
1000-2000 J/kg of SBCAPE, although these numbers should be taken
with a grain of salt.

BRC

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1135 AM CDT Tue Jun 18 2024

VFR conditions are expected to prevail throughout the 18Z TAF
period. VFR cumulus will impact local terminals this afternoon,
and may briefly dip to MVFR levels. A few showers and a weak
thunderstorm or two may develop in southeast Missouri this
afternoon, and there is a low chance (20%) that a storm could
develop near STL. Chances remain too low to include in the TAF,
but an amendment may be needed later this afternoon if storms
develop nearby. Gusty southerly winds will persist this afternoon,
and diminish by sunset. A low chance of valley fog exists at a
few terminals overnight, but confidence remains too low to include
in the TAF at this time.

BRC

&&

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

$$

WFO LSX