Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 162355
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
555 PM CST Fri Nov 16 2018

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 332 PM CST Fri Nov 16 2018

The main issue for tonight is the potential for low stratus and/or
fog. The combination of weak warm advection and abundant sunshine
and warming has resulted in significant melting of the snow cover
across eastern MO and into southwest IL. This has lead to a good
infusion of moisture into the boundary layer. High pressure
currently centered in the western Ohio Valley will retreat to the
east tonight, resulting in a light southeast component to the
surface winds while not far above the surface the winds will
increase to 20+ knots out of the south-southwest as low level WAA
becomes better established. High clouds will also be on the
increase, especially overnight as they spread into the area from
the Plains in advance of the next system. These later factors
typically don`t bode well for widespread dense fog. That doesn`t
mean we won`t see some patchy fog and I have a bit more confidence
going this route for now. Lows will be in the 30s.

Present indications are our next threat of precipitation/wintry
weather will occur Saturday afternoon into Sunday, although this
will not be a widespread event and precipitation amounts will be
light. A cold front will be located near the MO/IA border at
daybreak on Saturday and this cold front will advance rather
quickly through the CWA as the upper trof broadens and heights
aloft begin to fall. The models are still showing that the
combination of low-mid level frontogenetic forcing and very weak
large scale forcing associated with passing low amplitude
migratory disturbances will produce a west-east band of post-
frontal precipitation across northern MO into central IL.
Deepening of the cold air into the afternoon in the wake of the
cold front will allow the ptype to change from rain to snow across
northeast MO and west central IL during the late afternoon. There
will be a decent warm-up ahead of the front on Saturday from I-70
southward, albeit temperatures will remain below average.
Temperatures will fall upon fropa due to the combination of post-
frontal stratus and CAA.

Glass

.LONG TERM...  (Saturday Night through Next Friday)
Issued at 332 PM CST Fri Nov 16 2018

The region across northern MO into west central IL will remain
the most favored area for measurable precipitation on Saturday
night. The band may wane at times as frontogenetic forcing changes
in intensity. Current thinking is there is potential for up to an
inch snow accumulation in this area tomorrow night as CAA
continues to bring falling temperatures. Further south the forcing
is weaker and more nebulous and thus any precipitation is
expected to be light and less organized. The cold air will be
deepening southward on Saturday night, however the availability of
ice nuclei is less certain. I can`t rule out some brief periods
of freezing rain/drizzle as the precipitation threat spreads
southward into central and eastern MO however it looks short-
lived. The more liquid precipitation threat late Saturday night
into early Sunday morning across southern MO may occur where
surface temps remain just above freezing. Frontogenetic forcing
diminishes significantly on Sunday and this should lead to more
spotty precipitation as compared to Saturday night. An advancing
short wave trof may bring another round of light precipitation to
southeast MO and southern IL on Sunday night. Once again with
this wave of precipitation the thermal structure and surface
temperatures are a bit more questionable, and it appears to be a
light rain or freezing rain situation.

Through Thursday of next week, conditions look dry and tranquil.
We begin the week in northwest flow and temperatures will remain
below average. A pattern change is underway by midweek bringing
rising heights and moderating temperatures, and highs by
Thanksgiving should be near and maybe above normal. The forecast
for Friday is a bit more uncertain as a migrating upper trof and
associated cold front could bring rain.

Glass

&&

.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 537 PM CST Fri Nov 16 2018

Main focus will be over potential fog development over snowpack
overnight. With snow melt expected to continue overnight, along
with light winds and a clear sky, believe at least some light fog
will form. Have added a tempo for IFR visbys at KSUS, but this is
potentially still not low enuf. Otherwise, an approaching system
is expected to bring RA turning to SN Sat afternoon to KUIN.
While some light RA will be possible before the onset/change over
to SN, have only focused on the SN in the forecast for now. This
same system will also bring gradually lowering cigs to the region
on Sat.

SPECIFICS FOR KSTL: Have added some VFR fog overnight. While some
uncertainty still remains, believe that if fog develops around the
area, there will be lower impact to KSTL. Otherwise, cigs will
gradually lower on Sat as a system approaches the region. There is
a chance of RA or SN reaching the terminal, but chances are too
low at this time to add to the TAF.

Tilly

&&

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

$$

WFO LSX


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