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FXUS63 KLSX 182331
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
631 PM CDT Thu Jul 18 2024

.KEY MESSAGES...

- Cool, dry mid-summer weather continues into this weekend.

- Daily thunderstorm chances return next week.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 210 PM CDT Thu Jul 18 2024

High pressure is centered just to our north over Iowa this
afternoon with the core of this cool, dry air mass overhead.
Lots of afternoon cumulus developed as the low level moisture that
remains near ground level convects up into this cool, dry air.
Today is likely the coolest and driest day of this air mass, but
conditions aren`t changing quickly. We`ll have several more nice
days ahead.

High pressure nudges slightly to the east tonight, with our area
generally on the southwest side of the high overnight. This is a
favorable position for morning fog as dry advection ends, winds
are light, and the sky remains clear. Efficient radiational
cooling will send temperatures down into the 50s for most of us
tonight, although urban areas stay in the 60s. Fog is most likely
in valley locations across the Ozarks but also in the larger river
valleys where cool air and low level moisture pool overnight.
Confidence in widespread dense fog is low at this time, so we have
opted against a Dense Fog Advisory unless confidence changes.

Expect a very similar story on Friday. The center of the high
settles in across northern Illinois with a very light flow, mostly
out of the east across our area. The air mass gradually moderates
in place each day as the July sun works to warm it. Highs should
be 1 to 3 degrees warmer than today. Humidity gradually increases
as well through evapotranspiration, but fresh moisture from the
Gulf of Mexico remains inaccessible well to the south helping to
limit cloud cover to those diurnal fair weather cumulus.

Kimble
&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 210 PM CDT Thu Jul 18 2024

Upper flow pattern remains fairly stable across our area this
weekend and into next week with a ridge over the western US and a
broad trough over the central to eastern portion of the country
(with a deeper core over Hudson Bay). At the surface, high
pressure broadens across the southern Great Lakes to New England
keeping the cool, dry air mass over our area and moderating in
place each day. Greater moisture remains over the Central Plains
where daily thunderstorms triggered in the narrow corridor of
remaining instability are aided by weak shortwave troughs aloft.
This should stay well to the west of our area, though, with Gulf
of Mexico moisture still locked away to the south. Temperatures
remain mild, but moderate further into the mid 80s at times.

Troughing gradually develops over the central Plains this weekend
into next week, eventually expanding the area of daily showers and
thunderstorms eastward and into our area. Gulf of Mexico moisture
remains limited until Sunday night when we get our first fresh
feed of moisture back into our area. So by next week temperatures
remain near or below normal but with dewpoints creeping back up
toward 70 degrees. In addition to making it feel more humid, it
brings back the instability that leads to scattered showers and
thunderstorms. The weak flow pattern is unfavorable for organized
severe thunderstorms, but even pulse type thunderstorms could
produce local microbursts or pockets of hail. Where storms form
each day will largely depend on outflow from storms on prior days
which of course has very low predictability. It is also possible
that a more compact upper low could develop and provide more
widespread showers and storms near its center, wherever it is,
leading to cooler temperatures as well. In general the
thunderstorm threat is highest in the afternoon and early evening,
but cannot be ruled out any time from Sunday evening through at
least the middle of next week.

Kimble
&&

.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 630 PM CDT Thu Jul 18 2024

VFR flight conditions through the forecast period with light winds.
Otherwise, could see some patchy fog in the river valleys late
tonight, so added mention of MVFR visibilities at KJEF and KSUS
between 10z and 14z Friday.

Byrd
&&

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

$$
WFO LSX