962
FXUS63 KLSX 011858
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
158 PM CDT Tue Oct 1 2024

.KEY MESSAGES...

- The seven day period will be dry with largely near to above
  normal temperatures.


&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 158 PM CDT Tue Oct 1 2024

Winds continue to gust into the 20s across the forecast area this
afternoon, but will diminish with sunset as we lose our deep,
daytime mixing and as a surface high quickly elbows its way into the
mid-Mississippi Valley. The post-frontal dry airmass will continue
to push southwards through the region, dropping dewpoints into the
30s to 40s, a noticeable change from the previous days with
dewpoints in the 60s. Combined with temperatures up to 10-12
degrees below yesterday`s highs, today really does feel like fall,
y`all.

The surface high will settle over the forecast area tonight and
combined with the low dewpoint values and clear skies, will support
effective cooling across the forecast area. Lows in the 40s will be
widespread, with some of our coolest sites in the hollows of
Missouri and Illinois expected to flirt with the upper 30s.

By Wednesday the 500 mb shortwave associated with today`s cold front
will exit the upper-Mississippi River Valley, and quasizonal flow
establishes across the northern half of the CONUS. 850 mb
temperatures will gradually warm through the day from their minimum
overnight along southwesterly flow. Despite the low-level warming,
the cool start to the day will limit overall high temperatures in
the mid 70s.

Delia

&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 158 PM CDT Tue Oct 1 2024

The quasizonal mid-level flow will continue through much of Thursday
ahead of another shortwave impulse rolling through the upper-
Mississippi Valley. This will be the first of two waves impacting
the region over the weekend. Ahead of the first wave warm air
advection and abundant sunshine will help temperatures warm 5-15
degrees above normal (mid 70s) on Thursday. This first impulse will
bring a weak cold front through the region sometime Friday, though
guidance differs on the exact timing. The weak front will bring
little sensible change to our weather, and Friday`s highs will be 5-
10 degrees above normal. A second shortwave and associated cold
front will follow a similar track and push through the region Sunday
into Monday. Despite several days of southwesterly flow, moisture
return will struggle to overcome the dry airmass in place ahead of
these cold fronts. The very dry atmosphere keeps chances for rain
very low, with the global ensembles depicting at most a 20%
chance of rain over the 24 hour period surrounding the second
frontal passage. The post-frontal airmass will be cooler, bringing
near to below normal temperatures and lower humidity values to
the region for the first half of the work week.

Confidence is high that cooler air will make it into the region
during the first half of next week, and all ensemble members show
this trend. But the strength and timing of the cold air mass remains
uncertain at this point, as the GEFS ensemble system is favoring a
more delayed cold front passage and warm temperatures lingering into
Monday. The remainder of the ensemble members support a cold front
passage over the weekend, with cool air reaching the forecast area
by Monday. In the wake of the cold front, northwesterly mid-level
flow will persist across the central CONUS as the shortwave
amplifies over the eastern US during the first half of next week.
Temperatures will remain cooler than the weekend through the first
half of the work week, but how cool remains uncertain as the
interquartile spread for high temperatures is around 10 degrees.


Delia

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1144 AM CDT Tue Oct 1 2024

Dry and VFR flight conditions will prevail through the TAF period
in the wake of the cold front. Gusts along the northwesterly to
northerly winds will diminish this afternoon and winds will go
light and variable to calm as the center of a surface high moves
over the region overnight. Winds will become southerly to
southwesterly tomorrow as the high moves off to the east.

The dry post-frontal airmass will keep the region largely free
from fog tonight. While steam fog may form in the rivers, it is
not expected to be widespread or able to advect into the river
valley terminals (KSUS, KCPS, KJEF).

Delia

&&

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

$$

WFO LSX