Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
254 PM EST sun Jan 25 2015
a vigorous upper level storm will spread light snow over
Pennsylvania tonight...before an intense secondary coastal low is
spawned off of the Carolina coast on Monday and blasts much of the
northeast with near blizzard conditions Monday night and Tuesday.
Periods of light snow and snow showers will persist over central
PA through Tuesday before before high pressure builds over the
region middle week...followed by another clipper late Thursday.
Near term /through tonight/...
surface observation indicate cold front at middle afternoon to be between klbe
and kidi...extending eastward through the central mountains and
eastward to the middle susq River Valley. Dry low level air bleeding
southward as surface high noses into north central PA...evidence by surface
dew Point of 15f at kipt...while kseg dew point is 27f.
Meanwhile...moisture flux and warm air advection are increasing over Ohio and
into southwest PA ahead of vigorous clipper advancing from the
middle Mississippi River valley. The snow there will spread
eastward and overspread southwest and central PA late this
afternoon through the evening hours. Snowfall overspreads most of
central PA by late evening...with timing over The Laurels between
5pm and 7 PM...central mountains between 8 PM and 10 PM...and
areas north of i80 between 10 PM and midnight...although spots
along the New York border may wait until well after midnight.
No major changes in quantitative precipitation forecast or snow amounts...but with 24 hour event
criteria in effect...we expect widespread 3 to 6 inch amounts over
all of central PA. 6"+ expected over the Laurel Highlands...but
remaining below the 24 hour warning criteria of 8"/24 hours...which
is why advisories are posted for those areas as well.
Snow/water ratios between 11/1 and 13/1 were used with wpc quantitative precipitation forecast as a
basis for snow amounts...and then edited for consistency. Banding
and fluff factor may influence totals upward slightly from what we
currently have. It appears most of the accumulation occurs late
tonight through middle morning Monday...before upper level trough and
Transfer to coastal low takes place.
Short term /Monday/...
weakening primary clipper low over the central Appalachians tracks
south of PA Monday morning while impressive secondary development
takes place off of the Virginia/NC coast and races NE later Monday.
Still concerned for western edge of heavy snow fall to brush my
far southeast counties...we maintain a Winter Storm Watch for York
Lancaster and Lebanon counties as a result for an additional
4-6"+ later Monday into Tuesday. But during Monday...periods of
light snow and snow showers will persist through most of the day.
Deep east fetch and upslope into The Laurels keeps the snowfall
going through the afternoon hours...with deformation banding
likely setting up over the southeast late in the day.
Long term /Monday night through Saturday/...
main focus in the extended forecast will be evolution of rapidly
developing coastal low east of PA Monday night and Tuesday. Model data
showing a fairly tight range of solutions...nearly all of which
place our extreme eastern counties from Lancaster north through
Schuylkill Colorado on the western periphery of the significant deformation
band snow. Model 7h fgen fields seem to imply the heavy snow will
remain just east of the County Warning Area. However...given complexity of
criteria differences across our borders...and 40% confidence of an
additional 6 inches or more over Lebanon/Lancaster/E York counties
with the secondary low...opted to issue Winter Storm Watch for
these three counties for Monday afternoon through Tuesday morning.
The more likely scenario at this time appears to be oncl light snow
continuing across the rest of central PA Monday night and Tuesday
associated with next shortwave diving into deepening upper trough axis
over PA. Tight pressure gradient west of coastal low should result in a
blustery day Tuesday...blowing/drifting snow a possibility. BUFKIT
soundings indicate gusts to around 30kts likely.
Fair and chilly Wednesday expected Wednesday...as high pressure builds over the
area. Medium range guidance still indicates next chance of snow
Thursday/Thursday night with next in series of clippers. Ecens shows a
deepening upper level trough over the eastern Continental U.S. By next
weekend...offering no respite from the cold weather.
Aviation /19z Sunday through Friday/...
cold front sagging southward across PA. Northern PA seeing some
improvement north of the front...while lows ceilings and light precipitation
persist along and south of the fnt across southern PA.
Kjst socked in with very low ceiling/visibility and light precipitation.
Downsloping farther to the east should be enough to keep southeastern
airfields /kmdt and klns/ in VFR/high MVFR conds.
An Alberta clipper will track just south of Pennsylvania tonight
into Monday...bringing another round of snow with IFR conds. This
will impact most terminals after 00z Monday...but some affects
possible earlier at kjst.
Snow/snsh continues Monday into Tuesday. Northwest winds pick up Monday night
into Tuesday as low pressure spins up off New England coast. Quiet weather
Wednesday between systems...before another clipper drops across
the area late Thursday into Friday.
Sun night...snow rapidly overspreading from SW to NE...bringing
IFR conds to the entire region overnight.
Monday-Monday night...widespread IFR/MVFR in snow/snsh.
Tuesday...areas MVFR in shsn west...and poss lingering snow in the
Wednesday...VFR with no sig weather expected.
Thursday...VFR to start...with late day restrictions spreading in from
Thursday night- Friday...widespread IFR/MVFR in snow.
Winter Weather Advisory from 6 PM this evening to 7 PM EST
Monday for paz017>019-024>028-033>036-050-056-057-063-064.
Winter Weather Advisory from 9 PM this evening to 7 PM EST
Monday for paz010>012-041-045-046-049-051>053-058.
Winter Weather Advisory from 6 PM this evening to 1 PM EST
Monday for paz059-065-066.
Winter Storm Watch from Monday afternoon through Tuesday
morning for paz059-065-066.