Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
1129 am EST Friday Feb 12 2016
an Arctic cold front will move through northwestern Penn this
evening...and push across the susq valley shortly after midnight.
This front will be followed by the coldest air of the season just
in time for Valentines weekend. High pressure will settle across
the state late in the weekend...then a strengthening storm will
lift up the East Coast early next week.
Near term /until 6 PM this evening/...
clouds will continue to lower/thicken-up through this afternoon
as warm air advection overspreads the area in advance of upper low /polar
vortex/ plunging south from James Bay. Near term models continue to
indicate isentropic lift will result in some patchy light snow
developing during the afternoon and early evening...producing some
meager accums of less than an inch. SW flow ahead of approaching
Arctic cold front will draw slightly milder air into the
region...and middle afternoon maximum temperatures will likely range from the
uteens over the Allegheny plateau...to the m20s across the susq
Short term /6 PM this evening through 6 PM Saturday/...
Arctic front will plow southeast across central PA this evening...likely
accompanied by a round of snow squalls. Steep lower tropospheric
lapse rts and strong forcing ahead of middle level shortwave should
support a band of squalls entering the northwest mountains around 00z and
exiting the eastern counties late at night. Brief whiteout conds
will likely create traffic/travel problems this evening across the
northwest counties and possibly further southeast late tonight. Can/T rule out
a rumble of thunder across the northwest mountains between 00z-03z Sat...where
models indicate a bit of cape. Elsewhere...have removed mention of
thunder base on latest Storm Prediction Center guidance and lack of any cape in model
data. 00z ncar ensemble suggests the heaviest squalls will fall
over the north mountains...closest path of shortwave.
Have opted to forego any advisories for snow at this point...as
latest Storm Prediction Center sseo and ncar ensemble output indicate snow amts from
front and following lake effect will fall short of advisory
criteria. Plan to cover the travel hazard from evening squalls in
the severe weather potential statement and follow up with Special Weather Statement as the event unfolds this evening.
Arctic air mass will pour into the region early Saturday...accompanied
by gusty northwest winds...which will produce dangerous wind chills
across at least the northern and western mountains current wind/temperature
grids resulting in marginal wchill warning readings of around -25f
across the north mountains Sat am through sun am. Thus...with such borderline
numbers...will keep watch intact and defer warning/advisory
decision to the day shift. Further south...have issued wchill
advisory along the Allegheny plateau...where confidence is high
for wchills between 15f-25f. Gefs mean 800 mb temperatures between -22f and -25f
should support high temperatures Saturday no greater than around 0f over
the northwest mountains to the mteens across the lower susq valley.
Dry Arctic air mass should preclude sig lake effect snow across
central PA Saturday. However...Lake Huron connection should should
focus the most persistent snow showers across the northwest Warren Colorado and
the Laurel Highlands...where an additional inch or so seems likely
based on ncar mesoscale ensemble.
Long term /Saturday night through Friday/...
*impact weather highlights: frigid temperatures and dangerous wind
chills for Valentines weekend. Potential winter storm early next
Polar vortex diving into northern New England will deliver the
coldest air of the season with Arctic blast into central PA
late Sat-sun. The global models and ensembles keep a mean trough
in place over the eastern US into the middle of next week. The
most notable feature during the period will be a low pressure
system tracking up the East Coast on Tuesday. The latest 12/00z
model guidance has trended west with the low track...but there is
still a decent spread. At this time precipitation with this system
appears likely Monday-Tuesday but the track will ultimately determine
precipitation type. Next week should end on a milder note.
The coldest temperatures/wchills expected Sat night over the area...as
800 mb temperature anomalies of near 3sd blw climatology graze the area. Minimum
temperatures Saturday night into early Sunday morning expected to
range from around zero in the lower susq valley to -10f in the
northwest mountains the frigid temperatures combined with dangerous
wind chills /-10 to -25 below zero/ will result in an increased
risk for frostbite and hypothermia. May need to add counties to
the current wind chill advisoryf for Sat night.
A moderation (warming trend) is expected into early next week.
The model spread has narrowed a bit concerning a potential winter
storm system impacting the middle Atlantic/northeast next Mon-Tue. Latest
gefs and ecens indicating a surface low track across eastern PA is
most likely...which would favor rain across all but perhaps the West
Mountains confidence remains low on ptypes across western PA...but
now definitely favors rain across the east. All the models are
signaling a significant quantitative precipitation forecast event...and therefore have increased
probability of precipitation in the likely range for Monday ngt-Tuesday.
Aviation /18z Friday through Wednesday/...
most of central PA will continue to be VFR into the middle
afternoon. West/SW winds will be 10 miles per hour or less much of the day.
Trough moves through the region late this afternoon and evening
followed by a blast of Arctic air. Ahead of that...some warm air advection light
snow will develop during the middle-late afternoon...bringing
generally MVFR reductions to cigs/vsby. When Arctic air arrives
/reaching northwest mountains by 00z/... more sig snow showers/squalls will
bring periods of LIFR/vlifr in this evening into Saturday morning
- especially over the northwest half of County Warning Area. These will be accompanied by gusty
northwesterly winds...and these brisk winds will continue into
Downslope flow kicks in late tonight...bringing improvement to VFR
in most places. Western higher terrain will continue to see
restrictions in snow showers/blowing snow/upslope flow into the
Sat...windy. Shsn/reduced visible poss West Mountains...mainly am.
Sun...no sig weather expected.
Monday...PM snow possible.
Monday night and Tuesday...snow likely northwest half of Penn...with rain or
snow in the southeast.
wind chill watch from late tonight through Sunday morning for
Wind Chill Advisory from 4 am Saturday to 11 am EST Sunday for