Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
131 am EST sun Dec 8 2013
high pressure over the middle Mississippi Valley will build east into
the commonwealth overnight...before becoming centered over the New
England states by Sunday night. Another round of snow and sleet
followed by freezing rain will become focused across central
Pennsylvania Sunday afternoon into Monday. An Arctic frontal passage
early next week will drop temperatures well below normal through the
middle and latter part of next week.
Near term /until 6 am this morning/...
surface high centered across northern Illinois this evening will build east
into central Penn early Sunday. Light northwest to north winds early
tonight will veer around to the east early Sunday.
Areas of broken-overcast strato cumulus will prevail across the western mountains of PA
overnight as the favorable orographic...light northwest flow squeezes out
the limited low level moisture from the upstream glakes. A few light
flurries are possible on the ridges top to the west of Route 219.
Elsewhere...a thickening shield of cirrus will advance NE across the
Low temperatures will range from the lower teens across the northern tier
counties....to the lower 20s across the Lowe susq valley/Harrisburg
Short term /6 am this morning through 6 PM Monday/...
a Winter Storm Watch remains in effect for Sunday afternoon through
Monday morning for the scent mountains and Laurel Highlands...while a
Winter Weather Advisory is in effect for portions of the lower susq
and east central mountains for the same period.
It appears likely that later shifts will expand the advisory a bit
further north and east. However...precipitation amounts appear lower across
the northern mountains...and the low-middle levels appear cold enough to
produce a longer period of snow/sleet with a much shorter duration
of icing potential.
Cold air damming associated with departing/sprawling East Coast surface
ridge axis will be overrun by a persistent southwest upper flow with
increasing moisture and pulses of enhanced isentropic lift from a
few wswrly low level jet maxes.
These subtle forcing features and varying north/south depth of the
low level cold pool of air will create another challenging forecast of a
winter weather event here in central Penn.
The lack of deep/strong warm air advection aloft will minimize the chance for heavy
quantitative precipitation forecast and rapid changeover to rain or freezing rain...but more
importantly lead to the potential for a prolonged sleet and freezing
rain event Sunday night into Monday morning /after several...to 8
hour period of light snow.
Models continue to indicate that light precipitation will break
out by late morning across the south in the form of snow and
overspread most areas north of I-80 by Sunday evening. Precipitation
is expected to remain on the light side for the daytime hours...with
only light accums of snow expected for most...though a few inches of
snow accumulate are poss along the Mason/Dixon line. Highs will be in the
upper 20s to lower 30s.
As high retreats into New England...cold/sub-freezing air will
continue to be reinforced in the low levels from the NE. But warm
air advection continues above the boundary layer...with a changeover
to sleet and freezing rain beginning late in the day near the
Maryland border adjoining the Laurel Highlands and south central
mountains. This will begin a prolonged period of freezing
precipitation with potential for significant ice accums in the
southwest...and lighter amounts to remainder of central PA. The
precipitation will last through the overnight...but question remains just
how much quantitative precipitation forecast we/ll see...as heaviest precipitation remains south of the
Maryland border. Models generally agree on a tenth or two across the
southern tier by 00z...with another tenth or two from 00-06z -though
this may be overdone as region will be between two shortwaves and
this time frame may be a precipitation minimum. Amounts increase again
after 06z during prime time for freezing rain...though models still showing
notable differences in quantitative precipitation forecast. GFS and sref on the high end with large
areas greater than 0.25 while NAM and especially European model (ecmwf) giving much
lower totals. Either way...anything that falls will be a wintry mix
or likely freezing rain. So earlier issued Winter Storm Watch remains in
effect for portions of the southern and central mountains looking
ahead...at least portions of current west-southwest will likely transition to a
warning later this
evening...surrounded by a Winter Weather Advisory.
Warmer air eventually wins out in the lower levels as well. Temperatures
gradually rise Sunday night...with a transition to plain rain
from S to north early Monday. By 12z...precipitation should already be all
rain in the southeast.
Long term /Monday night through Saturday/...
transition from wintry precipitation to plain rain should occur early
Monday...with light showers persisting on Monday as strong jet
brings some large scale lift but no shortwaves to focus the
Strong upper jet will shift off the coast into the middle
of the week. A trailing weak wave may brush the area with some
light snow on Tuesday afternoon and evening before Arctic air
surges into the region for the rest of the work week. Northwest
flow aloft and around building high pressure at the surface will
bring lows in the single digits and teens for Thursday and Friday
morning...and highs mainly in the 20s.
This weekend...flow turns westerly bringing gradual moderation to
temperatures. Another wave of low pressure prognosticated for later Sat
Aviation /06z Sunday through Thursday/...
VFR conds expected for the eastern taf sites overnight as high
pressure over the Midwest builds east into Pennsylvania. A broken-overcast
shield of cirrus/cirrostratus will gradually spread NE and thicken
up across the state overnight. MVFR deck of strato cumulus associated with
nwrly flow in cloud layer has banked itself up into the
alleghenies. These lower clouds will linger most of the night at
The wind will lighten up through the night and veer all the way
around to a NE/east surface flow in the morning. The high pressure
floats to the east a bit more and cold air damming results. This
cold air will be trapped at the surface/low levels through much of the
daylight on Sunday. Strong upper forcing rides overhead late
Sunday. But warm advection snow will start in the morning along
the southern border and lift quickly north through the afternoon. After a couple of
hours of light snow...the precipitation turns to pl/freezing rain in the southern
terminals. Gradually the air aloft warms and lns/MDT go to plain
rain before nightfall. Jst/aoo/unv will likely see a sig impact to
flight due to freezing rain/pl after initial snow burst. Hey might not see
a turn to plain rain until late Sun night. Ceilings/visbys lower
quickly during the late morning and afternoon from north-S as snow/pl work
down from above. Winds may show enough of a veer by late afternoon to
warrant some low level wind shear mention.
IFR/LIFR conditions should linger at most sites through the night
and into Monday morning.
Monday...widespread MVFR/IFR as cold front passes through NW-se.
Tue-Thu...northwest flow. MVFR/IFR scattered -shsn northwest.
Winter Storm Watch from this afternoon through Monday morning
Winter Weather Advisory from 1 PM this afternoon to 10 am EST
Monday for paz057>059-065-066.