Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
915 PM EST Thursday Dec 12 2013
a cold cyclonic flow of air over the Great Lakes will maintain
below normal temperatures through the end of the week...with lake
effect snows focused over far northwest Pennsylvania in the
perennial snowbelt region. A series of northern stream waves
tracking eastward from the upper Great Lakes along the Canadian
border will reinforce the cold air. Another winter storm will
impact the area Saturday into Saturday night...bringing moderate
accumulations of snow to much of central Pennsylvania. Cold and brisk
conditions will prevail into early next week.
Near term /until 6 am Friday morning/...
shortwave tracking across the fingerlakes region this evening has
increased stratus across much of central and north central PA.
Upstream breaks in the stratus are evident but a good portion of
the overnight will have scattered-broken clouds north of roughly a kjst to
Backing blyr flow will bring lake effect snow showers southward
again into the northwest mountains after 06z. Quantitative precipitation forecast from higher res model data
supports a light accumulate of up to an inch by dawn across northwest Warren Colorado.
Elsewhere...tonight looks uneventful as high pressure passes south
of the state. Despite a mclear sky across the southern half of the
state...an active west-southwest breeze should keep temperatures from falling to
quickly. Model blend supports min temperatures within a few degrees of 10f.
Short term /6 am Friday morning through 6 PM Friday/...
a series of northern stream waves will move along an Arctic front
dropping southward near the PA/New York border on Friday. This should help to
reinvigorate lake effect snow showers over the northwest/north central mountains
with minor accums /0.5-1.0"/ expected through 00z Sat.
Long term /Friday night through Thursday/...
*thu evening update* Winter Storm Watch issued for central and north
central PA for 5-8 inches of accumulation Saturday and Saturday
Surface high pressure will slide across northern glakes
Friday night in wake of Arctic front...reinforcing a cold air surge
mainly north of Pennsylvania. At the same time...southern stream
wave exits the southern rockies and progresses steadily toward the
northeast coast ahead of well-defined northern stream trough
digging into the upper Great Lakes region. Isentropic lift/warm
advection ahead of the weak low level wave lifting northeastward along
inverted trough will support a broad area of snow spreading into
the area by early Saturday morning...mainly across
northern/western counties. Shortwave energy and height falls
crossing the central applchns will begin transferring to a surface
low that is forecast to develop along the middle-Atlantic coast and
deepen as it tracks northeastward from the Delaware-Maryland-Virginia/New Jersey coast to Long
Island/the southern New England coast Sat night into Sunday.
With colder air entrenched over central PA...models continue to
forecast moderate snow event for much of central PA...with a
changeover to sleet/freezing rain possible over the southern 1/3 of the
County Warning Area later Sat into Sat night as some warmer air noses in from the
southwest. 24-hour snow accumulations of 6"+ seem likely
especially along and north of I-80 with 850mb temperatures remaining -3c or
colder. Sref also supports this. Though quantitative precipitation forecast totals of about
0.50-0.60 inches forecast...totals may be hampered by lack of
phasing between the lead southern stream wave ejecting offshore
and the northern stream trough moving across the Great Lakes about 12
hours later. The lack of a closed 700/500 mb circulation is also
expected to limit the amount of deep moisture advection/lift
across the region. But drier Arctic air to the north may produce a
fluffier higher snow to water ratio which could bring amounts a
bit higher. But with longer duration of event and somewhat gradual
accumulation rates...will hold off on issuing a watch with this
package... though advisories will likely be required as event
approaches. Further south...snow possibly mixing with/changing to
sleet/freezing rain over the south-central zones as warm air tries to surge
in from the south. Sref/gefs and wpc probs show light ice amts
around 0.10 mainly over The Laurels and lower susq valley.
System moves out quickly and the winter weather should be mainly over
by Sunday morning as a dry slot moves in behind the departing
coastal low. The northern stream trough will then cross the lower Great
Lakes Sun night and begin a period of enhanced lake effect
snows/extensive clouds downwind of Lake Erie.
Weak ridging should provide a cold but dry start to next week. An
Arctic shortwave crossing the Great Lakes/northeast/mid-Atlc Tuesday-
Wednesday will likely produce some light snows with lake enhancement.
After that...models begin to diverge significantly /after being in
quite good agreement until then/. GFS keeps broad trough over the
eastern U.S. With trough digging over west rockies...while European model (ecmwf) digs
trough over central U.S. Bringing warmer SW flow to PA. Best
forecast is for a gradual moderation of temperatures with poss a more
pronounced warm up heading into next weekend.
Aviation /02z Friday through Tuesday/...
low level flow is shifting to the SW for the time being...which
will help keep the lake effect concentrated north of my forecast area.
Terminals will remain VFR overnight...with the exception of bfd
where passing snow showers could still bring reduced conditions.
Another weak shortwave will move north of the region overnight.
The main effect will be to turn the flow more off the lake once
again...leading to an increase lake snow shower coverage over my
northwestern zones. Expect persistent MVFR conditions at bfd with frequent
drops into IFR for much of the day Friday. Remaining taf sites
should be mainly VFR with isolated snow showers causing brief
Increasing southwesterly winds aloft could lead to a period of low level
wind shear overnight into Friday morning. Have included at the
sites where I except surface winds to diminish to light overnight.
Friday...shsn/reduced visibilities possible at kbfd.
Sat...snow or mixed precipitation with low ceilings/visibilities likely.
Sun...shsn/reduced visibilities poss West Mountains
Monday...no sig weather expected.
Winter Storm Watch from late Friday night through late
Saturday night for paz004>006-010>012-017>019-024>028-037-041-