Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 081149

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
549 AM CST Sun Dec 8 2019

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 336 AM CST Sun Dec 8 2019

Zonal upper level flow persists over the central CONUS, with a
deep cut-off low over the Hudson Bay and a broad trough moving
onshore over northern California. At the surface, the mid-
Mississippi Valley is situated between high pressure departing to
our east and weak low pressure over the western Plains. The resultant
southerly winds across the region continue to advect an airmass with
ample low level moisture into the region. With patchy fog already
across portions of the area, and what looks to be dense fog extending
into Reynolds and Iron counties, I have hoisted a Dense Fog Advisory
through mid morning in our far south. Fortunately the northward
progression of the dense fog appears to have halted, so I
anticipate the dense fog to remain in SEMO and far southern IL.

Looking beyond this morning, a series of shortwaves will continue
to eject southward from the northern upper low while the western
trough continues to progress eastward. This will help deepen the
surface low over western KS/OK/TX today before the cyclone zips
through the mid-Mississippi Valley tomorrow. Ahead of the low,
warm and moist advection will continue, pulling temperatures
higher than what we saw yesterday. However, this warm up will
likely be tempered by increasing cloudiness through the day.

WAA will intensify this evening as the low approaches the region,
persisting into Monday before a cold front sweeps through the
area. The increasing lift associated with the enhanced WAA may
result in light rain or drizzle beginning late this afternoon,
becoming more likely in the overnight hours. The best chances for
rain will be across our south, where the most focused WAA will
reside. As dry air behind the cold front moves into the area
tomorrow afternoon, precip chances will come to an end.


.LONG TERM...  (Monday Night through Saturday)
Issued at 336 AM CST Sun Dec 8 2019

Cold high pressure will build into the region behind Monday`s cold
front. This will bring noticeably cooler temperatures to the region,
though much of the latest suite of ensemble guidance has come in a
bit warmer than previous runs. Nonetheless, we`ll still see temps
around 10 degrees below normal, which compared to today`s highs in
the 50s, will feel quite chilly.

By midweek, another upper level trough is forecast to move onshore
over the western CONUS before pushing into the central CONUS toward
the end of the week. Confidence is fairly high that this will shunt
the cool anticyclone to the east of the region, which in turn will
help temps build back to near normal on Thursday and Friday. Confidence
is also high that this will bring at least one more round of precip
to the central CONUS by the weekend, though when and where exactly
this occurs remains uncertain.



.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 534 AM CST Sun Dec 8 2019

Fog will continue to affect parts of southeast and east central
Missouri and southwest Illinois through mid morning including KSTL
and KCPS through 14Z. Then mainly dry and VFR condition are
expected through mid evening before rain moves into the area along
with MVFR ceilings. The greatest chance for rain will occur after
06Z at the St. Louis terminals with lesser chances at KCOU and
KUIN.  Winds will remain out of the south through the period.


Fog is expected to dissipate by 14Z. Then mainly dry and VFR
condition are expected through mid evening before rain moves into
the area along with MVFR ceilings developing after 06Z. The
greatest chance for rain will occur after 08Z and continuing
through the end of the period with visibilities be reducing MVFR
after 14Z. Winds will remain out of the south through the period.



MO...Dense Fog Advisory until 9 AM CST this morning for Iron MO-
     Madison MO-Reynolds MO.



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