Area Forecast Discussion
Issued by NWS State College, PA-- Remove Highlighting --
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FXUS61 KCTP 091609
Area Forecast Discussion
National Weather Service State College PA
1109 AM EST Fri Dec 9 2016
A broad and very cold upper level trough over the Great Lakes
and southeastern Canada will remain in place for the next few
days. This weather feature and a well-aligned westerly low level
flow off the lower Great Lakes will bring a prolonged period of
lake effect snowfall and significant snow accumulations to
northwest PA today through Saturday. High pressure will briefly
build across the commonwealth late Saturday and Saturday night
causing the Lake Effect snow bands to lift north of the region
with a brief period of clearing elsewhere. Then, a weak low
pressure system is likely to track east from the Ohio Valley and
impact Pennsylvania with another bout of wintry weather Sunday
into Monday. Unsettled weather will persist next week with
additional storm chances during the mid...and late week periods.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
-- Changed Discussion --An anomalously cold cross-lake flow will maintain a long-duartion,
multi-band lake effect snow event across the northwest PA. Model
soundings continue to display a deep, moist boundary layer with an
inversion height around 700MB (and 850mb temps of around -11 to
-14C), sufficient to maintain moderate to heavy banding from
relatively warm lake waters of around 50F.
Multiband event continues late this morning...and passage of a
shortwave currently over northwest PA will support lake effect
snow streamers extending well downwind of the lakes into the
central mountains...with perhaps a quick coating possible in the
ridge/valley region of Central Pa and possibly even into parts of
the Susq Valley...although this is not imminent per regional radar
mosaic or from high resolution HRRR.
Multiple bands of moderate to heavy Lake Effect Snow will continue
to stream southeast and well inland from Lake Erie through tonight as
the sfc-850 mb flow stays well aligned from about 285-290 deg and
the wind in the cloud bearing layer averages around 30 kts.
Snow/water ratios will average at a minimum...17-20:1 and the
Laurels and North-Central mountains will experience periods of
moderate-briefly heavy snow, and a quick 1-3 inches of snow accum
in some of these narrow LES bands (and favored areas for
orographic snow to the west of rt 219 in the Laurels). Will have
to keep a close eye on persistence/snowfall intensity of the
snowbands across the Laurels with respect to the possible issuance
of a Winter Weather Advisory.
Will maintain the Lake Effect Snow Warning (Warren County) and
Lake Effect Snow Advisory (Mckean County) through 17Z
Saturday, before the llvl mean wind gradually backs to about 250
Deg carrying the LES bands north of the PA/NY border Saturday
Little change to forecast storm total snow amounts as 12-18" with
locally 20"+ is expected over the NW third-half of Warren County
by Saturday. A light coating to one inch will occur in select
areas of the central Ridge and Valley Region of PA today...with
some spots receiving additional light accums tonight and Saturday.
Skies in this region of the state will stay mostly cloudy to
Across the Mid and Lower Susquehanna Valley Region, it will be
partly to mostly cloudy with generally just some scattered
flurries. A brief heavier snow showers is possible near and to the
NW of the I-81 corridor especially across the higher terrain of
Schuylkill County...where a coating to one inch is possible.
Highs this afternoon will be in the mid to upper 20s across the
Mtns, lower to mid 30s in the Central Valleys, and mid to upper
30s in the Susquehanna Region.
-- End Changed Discussion --
.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
Slow backing of the mean Low-level wind will occur after midnight
tonight with an expected quasi-single heavy LES snow band
continuing to impact parts of nrn Warren and Mckean counties.
Additional significant accumulation will occur across these 2 NW
PA counties...while some areas of light additional accumulation is
possible across the west-central mtns and Laurel Highlands.
Partly to mostly cloudy skies will mainly flurries will persist
SE of the I-99/RT 220 corridor. though one or two long/narrow LES
bands could snake their way well into central PA with localized
1-3 inch amounts. 09/07Z RAP indicates the likelihood of one
single and rather intense band with a good upstream connection to
moisture off the full length of Lake Huron and central Lake Erie,
aimed near or just to the north of a KFIG/KPSB and KUNV line late
today/early this evening.
Gradually improving conditions heading into Sat across the NW
Mtns as lake effect snow bands become more scattered and slowly
wind down. Still potential for a renegade snow band (with Lake
Huron connection) to stretch into the central mtns Fri night with
localized impacts though as trough lifts out. However, WSW fetch
off the lakes should continue to produce snow showers until Sat
evening across the N Mtns.
Breezy winds abate Fri night, with clearing skies across Southern
Pa late Friday night and Saturday as surface high pressure builds
in from the Ohio Valley.
.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
High pressure will bring a brief period of tranquility Sat
evening. However, a developing overrunning scenario is expected
Sunday into Monday, as weak low pressure lifts into the Eastern
Grt Lks and high pressure remains lodged over northern New Eng.
Enough cold air is in place to support a light snowfall across
much of area by Monday AM, with GFS bringing light precip into the
area on Sunday earlier than the ECMWF. Model consensus low track
west of PA should bring in enough warm air to cause snow to change
to rain (or a period of more problematic freezing rain) across
Southern PA, and esp the Lower Susq Valley, a before precip tapers
off later Monday.
A bit cooler than normal temps indicated early next week, but
longer range models indicate a significant cold front passage
Wed with noticeably below normal temps filtering in for the rest
of next week.
.AVIATION /16Z FRIDAY THROUGH TUESDAY/...
-- Changed Discussion --Cold W-NW flow pattern downwind of the Great Lakes will continue
to support a prolonged period of MVFR/IFR conditions over the
western 1/3 of the airspace into the weekend, with frequent lake
effect/upslope snow showers at KBFD/JST. MVFR cigs and ocnl vsby
reductions in -shsn are possible over the central terminals with
VFR prevailing across the eastern sites. Expect sfc wind gusts
20-25+kt from 270-300 degrees for most of today before winds
subside after 10/00z. Lake effect snow showers should decrease in
coverage and intensity by later Saturday as area of high pressure
briefly slides over the airspace. Low pressure tracking from the
lee of the Rockies to the Ohio Valley will bring a broad area of
snow/rain to the area Sunday into early Monday.
Sat...MVFR/IFR western 1/3 with snow showers decreasing.
Sun-Mon...Widespread MVFR/IFR in snow/rain. Becoming breezy Monday
night with snow showers over the wrn 1/3.
Tue...VFR to MVFR cigs. Snow showers ending.
-- End Changed Discussion --
Lake Effect Snow Advisory until noon EST Saturday for PAZ005.
Lake Effect Snow Warning until noon EST Saturday for PAZ004.