Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 182031

231 PM CST Thu Dec 18 2014

.SHORT TERM:  (Through Late Tonight)
Issued at 206 PM CST Thu Dec 18 2014

Any lingering flurries or drizzle will taper off late this
afternoon. Increased low-level moisture from today`s snowfall
along with light winds overnight will create favorable conditions
for fog formation. Later shifts will need to monitor the potential
for dense fog.


.LONG TERM:  (Friday through Next Thursday)
Issued at 206 PM CST Thu Dec 18 2014

An upper trough and its associated surface low will move across
the central and southern CONUS tomorrow and tomorrow night. Light
rain/snow is possible across southern/southeastern MO into
southern IL, but the bulk of the precipitation will occur south of
the LSX CWA. A separate vort max drops down from Canada into the
back side of the upper trough before it swings through southern IA
on Fri night/Sat. This feature could provide upper support for
sprinkles/flurries across northern MO and west central IL if
sufficient moisture becomes available.

A clipper-like system is forecast to develop over the northern
plains/western Great Lakes on Sunday night into Monday, then close
off over the northeastern quadrant of the CONUS during the middle
of the week. The GFS and ECMWF exhibit differences regarding
timing (ECMWF is slower), vort intensity (GFS is more intense),
and the result of interactions with other disturbances. If this
low pressure system develops as currently advertised, then it
would be expected to produce widespread rain/snow between Monday
and Wednesday with clouds and flurries lingering into the middle
of the week within a cyclonic flow regime aloft.

Temperatures will be seasonably cool on Friday and Saturday, then
warm to around 5-7 degrees above average on Sunday and Monday
ahead of the clipper-like system. Expect cooler temperatures for
the middle of next week within the colder air behind the clipper



.AVIATION:  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1210 PM AM CST Thu Dec 18 2014

Snowfall should continue to diminish across the area heading into
the afternoon, with a corresponding loss of visibility
restrictions as the snowfall ebbs. Bigger question is likely to
be ceiling trends. Low level moisture has IFR stratus locked
across the southern half of MO; however, surface ridge extending
from upper MSVLY to lower Ohio Valley appears to be keeping some
dry low level air locked over eastern sections of our CWA. This
appears to be keeping the stratus locked in place, and any
lowering of ceilings in this drier air has been associated with
pockets of heavier snow...fairly brief and rather spotty in nature.
Have attempted to reflect this by keeping ceilings from STL to UIN
in the high end MVFR range this afternon, while COU should remain
locked in IFR stratus. With time heading into the evening,
anticipate that IFR stratus and fog will attempt to form aong this
STL-UIN line, while the existing stratus deck over mid MO will
lower and head into LIFR range.

Specifics for KSTL: As suggested by primary AFD, STL area appears
to be on the e fringes of much more significant low level
moisture, and cloud/ceiling trends will likely be very tricky as a
result. For now have gone with high end MVFR cigs for the
afternoon as the snowfall gradually comes to a end, with ceilings
and visibilities lowering tonight as boundary layer cools and
heads into the IFR cat after 06z.





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