Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 181814
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1214 PM CST Thu Dec 18 2014

.UPDATE:
Issued at 1210 PM CST Thu Dec 18 2014

Snowfall lingering over the region late this morning should
continue to wind down heading into the afternoon. I`ve continued
some mention of drizzle...primarily in the deeper low level
moisture on the Missouri side of the Mississippi River.

Truett

&&

.SHORT TERM:  (Through Late This Afternoon)
Issued at 323 AM CST Thu Dec 18 2014

Vigorous little shortwave over northern Kansas is producing a broad
area of snow over parts of north central Missouri back toward west
central Missouri.  The going forecast looks pretty good in general;
however current radar and latest short range model guidance suggests
that there will be more precipitation across parts of northeast
Missouri and west central Illinois than previously forecast.  Low to
mid level frontogenesis and isentropic lift will increase through
15Z as the wave passes north of the area.  The shortwave should be
dampening out as it moves into an area of longwave ridging, so
precipitation should be on the decline through the morning, and
there is still a wedge of low level dry air to overcome, but
measurable precip does look likely across most of the area.
Accumulations still look pretty light with most areas receiving an
inch or less except for parts of central and southeast Missouri
where an inch or two of wet snow is likely to accumulate.  With
temperatures hovering right around freezing, expect some slick spots
on any untreated roads this morning, but temperatures should be
rising above freezing in most locations by mid to late morning so
that should alleviate any road issues.  Additionally, expect precip
to be winding down pretty rapidly after 15Z as the shortwave dampens
out and the primary band of frontogenesis and isentropic lift
weakens.  The one place where there may be lingering problems is
northeast Missouri and west central Illinois where temperatures may
struggle to break freezing this afternoon.

Carney

.LONG TERM:  (Tonight through Wednesday)
Issued at 350 AM CST Thu Dec 18 2014

Relatively quiet weather will prevail over the area for the rest of
the week with a large surface ridge extending from southern Canada
through the Upper Midwest and Great Lakes region down into the
Mid-Mississippi Valley.  Another wave will ripple through in the
quasi-zonal flow aloft Friday night.  The lower atmosphere looks too
dry to allow any precip to reach the ground from this wave...all
except for southern portions of the area.  Have kept chance/slight
chance PoPs across southeast Missouri and southwest Illinois with
little or no qpf associated with them.

Saturday and Sunday look warmer with southwest flow deepening ahead
of the next trof with temperatures rising to around normal on
Saturday and above normal on Sunday.  Medium range guidance looks to
be in good agreement that the trof will deepen across the Central
U.S. Monday into Wednesday.  This should bring us another period of
cloudy and occasionally rainy weather with the best probabilities of
precipitation Monday night into Tuesday.  The atmosphere will likely
be too warm for much frozen precipitation until possibly Tuesday or
Tuesday night.  QPF looks pretty light on the GFS and ECMWF by the
time the atmosphere is cold enough to snow, but there may be a few
periods of flurries or light snow Tuesday and Tuesday night.

Carney

&&

.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.

Truett

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX





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