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FXUS63 KLSX 190357
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1057 PM CDT Wed Sep 18 2024

.KEY MESSAGES...

- While rain chances are increasing at times this weekend into
  early next week (up to around 50%), confidence remains low (less
  than 10%) in rainfall capable of relieving widespread drought
  conditions.

- Confidence remains high (90%) that above normal temperatures
  will continues through end of the week, and while cooler weather
  is expected to end the weekend and start next week, exactly how
  cool remains uncertain.


&&

.SHORT TERM...  (Through Late Thursday Night)
Issued at 218 PM CDT Wed Sep 18 2024

A tightly-wound shortwave can be seen in current water vapor imagery
moving into the Northern Plains as a Rex Block slowly drifts
eastward over the eastern CONUS. Between these two features, the CWA
is beneath weak northwesterly flow aloft, with high clouds from the
shortwave moving into the region. An expansive area of high pressure
at the surface coving much of the eastern half of the country
continues to keep our weather calm. With ample sunshine and weak
southerly surface flow, temperatures are so far tracking similarly
today as yesterday, with highs in the upper 80s to around 90 degrees
expected this afternoon.

Tonight, temperatures will fall to around 60 degrees for lows, and
while the dry air in place over the area should help temperatures
cool quickly, upper-level clouds will at least initially inhibit
radiational cooling to some degree. If clouds are able to clear soon
enough and temperatures can cool more than forecast, another round
of fog, mainly in valleys, is possibly across southeastern Missouri
and southwestern Illinois.

The upper-level pattern mostly holds in place tomorrow, with the
shortwave beginning to make its way into Canada as a trough slowly
drifts eastward over the Intermountain West. Given the rather
stagnant upper-level pattern over the CWA, mostly a repeat of today
can be expected, with dry and warm conditions present. This stretch
of relatively calm weather comes to an end late Thursday night/early
Friday morning, as a weak shortwave passes north of the CWA and a
weak low-level jet veers into the region, combining to spawn
scattered showers and a couple of thunderstorms. Coverage,
placement, and timing of this convection is uncertain at this point
given the spread among CAMs, though the general timing is it could
enter the CWA as early as midnight and exit as late as 8am Friday.

Elmore

&&

.LONG TERM...  (Friday through Next Wednesday)
Issued at 218 PM CDT Wed Sep 18 2024

At the start of the weekend on Friday, guidance still differs on the
phasing of an upper-level trough and possible cutoff over the
western CONUS. Solutions vary from the trough becoming cutoff to
remaining intact with the main flow, and handle differently the
speed and amplitude of the trough/cutoff when it begins to progress
eastward. In general, the crest of an upper-level ridge ahead of the
western trough will be poking into the Middle Mississippi Valley
Friday into Saturday, keeping the thermal ridge in place over the
region characterized by 850 mb temperatures around 20 degrees C per
ensemble means. This climatologically favors surface temperatures
around 90 degrees. Thanks to mostly clear skies on Friday,
temperatures will reach the low 90s, but increasing cloud cover on
Saturday from the approaching trough/cutoff leads to uncertainty in
how warm temperatures will be able to get then. On the warmer end,
temperatures may be close to if not similar to those on Friday, and
on the cooler end, values could be roughly 5 degrees cooler compared
to Friday.

As for rain chances, deterministic guidance shows subtle
disturbances rounding the crest of the ridge through the Middle
Mississippi Valley on Saturday ahead of the main trough/cutoff.
Moisture return over the CWA is still expected to be meager ahead of
this feature, leading to low chances (20-40%) for rain mainly along
and west of the Mississippi River.

Guidance begins to diverge notably Sunday into early next week, with
varying solutions in the evolution of the trough/cutoff as it enters
the Midwest. The general consensus is that this trough/cutoff will
be the first in a series of troughs or shortwaves to move through
the Midwest, leading to an unsettled pattern as we end the weekend
and start next week. The spread in the handling of the upper-level
pattern has lead to a broad smattering of low rain chances (roughly
30%) Sunday through Wednesday. In reality, rain chances will likely
be confined to much smaller time frames during this stretch.
Moisture return is expected to remain meager, with any rainfall that
occurs not amounting to much - ensemble medians are around 0.5" for
total accumulation Saturday-Wednesday.

The spread in the evolution of the upper-level pattern also leads to
a wide spread in temperatures among guidance through the first part
of next week. At times, the difference between the 25th and 75th
percentile reaches 10 degrees during this stretch. While this degree
of spread makes it difficult to pin down scenarios at this lead
time, the general pattern and ensemble means support cooling
temperatures to somewhere around climatological normals, with values
varying from about 5 degrees above normal to 5 degrees below
normal.

Elmore

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1043 PM CDT Wed Sep 18 2024

Mid to high clouds are moving east of the Mississippi River late
this evening with clearing from west to east. Surface observations
around most sites are showing rather large dewpoint depressions,
which denote a dry surface layer. Among the few exceptions are
sites near river valleys, including KSUS. Already light/calm
winds and narrow dewpoint depressions provide enough argument for
a brief period of MVFR fog. This has been added at KSUS from
10z-13z Thursday morning. Support for fog is otherwise limited to
southeast MO and portions of southwest IL, which will well outside
the terminals. Aside from KSUS, VFR is expected through the
remainder of the period with typical diurnal wind behavior.

Maples

&&

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

$$

WFO LSX