Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
413 PM EST Wednesday Dec 17 2014
brisk westerly winds will bring in colder air this afternoon
through Friday night. Scattered...mainly mountain snow showers
are expected through the end of the week as cloudy and cold
conditions persist. A weak low pressure area is expected to move
well south of the region this weekend...but most or all of the
precipitation will stay to the south of the Keystone state.
Near term /until 6 am Thursday morning/...
gusty westerly winds will advect seasonably cold air into the
region late this afternoon and tonight. Narrow...east-west bands
of snow showers will occur across mainly the higher terrain of
the north and west this afternoon...while any snow showers
throughout the Central Ridge and valley region from the relatively
shallow stratocu deck will be isolated and brief /thanks to
decreasing lapse rates in the 850-700mb layer/. Occasional breaks
in the cloud deck will occur across the scent and southeastern counties.
No snow accumulation will occur throughout the central valleys
and near the Allegheny Front where temperatures will be in the middle to
upper 30s. However...a coating to one half of an inch is possible
across the higher terrain close to the PA/New York border where more
persistent/steeper low-middle level lapse rates will be found.
Temperatures will be nearly steady or slowly falling for the rest of the
daylight hours...ranging from 30-32 across the northwest mountains and
laurels...to the middle 40s in the lower susq valley.
A compact...though moderately strong middle-level shortwave will track east-southeast
across the forecast area between about 00-07z Thursday /at the northern
edge or left exit region of a potent 130 knots upper jet/ with a
distinct pocket of relatively steep...850-700mb lapse rates in
excess of 5c/km.
This feature should support a few hour period of scattered-numerous
snow showers /this evening until shortly after midnight/ with the
potential for a quick ground whitening - even across the central
mountains and susq valley. Mins tonight will fall into the 20s and
even some upper teens in the highest elevs of The Laurels. Wind
chills will brush the single digits there.
West to northwest winds will average 10-15 kts with gusts in the 20s
late today through tonight.
Low temperatures at daybreak Thursday will be near 20f across the higher
terrain of the north and west...and in the middle to upper 20s
throughout the susq valley.
Short term /6 am Thursday morning through Thursday night/...
cold air will stay in place across the forecast area Thursday into
Thursday...winds at the surface and aloft will veer just slightly to
the northwest. Speeds of 10-15 miles per hour and gusts into the l20s are
possible over the mountains. Little of no change in the blanket of
stratus and stratocu clouds is expected Thursday and Thursday
night...and the relatively shallow cold air blowing over the lower
glakes will create more shsn across the northern and western mountains
Accumulations will be minor...and confined to the far northwest and very
highest elevations. As stated by previous shifts no more than 2
inches is expected throughout the snowbelt of northwest Warren County on
the high end...so no les advisories planned at this time. Glimpses of
sun may develop again across the lower susq...nudging temperatures into
the lower 40s /near normal for middle Dec/. Maximum temperatures across the northern
and western mountains will be in the u20s to l30s.
Similar temperatures and sky cover expected again for Thursday night with
isolated to scattered mountain snow showers. Little in the way of
additional snow accumulate is expected though as moisture will
continue to be quite shallow with the subsidence inversion base
at or below 5 kft above ground level. A few highly localized amounts in the 2-3 inch
range are possible across the favored perennial snowbelt of Warren
County during the 24 hour period ending 12z Friday.
Long term /Friday through Wednesday/...
the pattern will remain seasonably cold and brisk through the end
of the week. Large scale subsidence associated with surface high
migrating eastward from the upper Midwest across Ontario/Quebec
should bring an end to nuisance/minor snow shower activity and
lingering flurries over the northwest alleghenies by Friday night. Clear skies
and calm/light winds should allow min temperatures to dip into the teens
in the northern mountains Saturday morning.
Concerning the weekend system...
Hopes for a significant storm this weekend continue to fade with
the latest models continuing to show a much weaker/suppressed and
disjointed system as the consensus solution. In addition..the
risk for accumulating light snow has also notably decreased...but
not totally evaporated. That said there is still a non-trivial
chance that most precipitation remains south of PA and the weekend turns
out dry for most locations. Early week model simulations for a
major storm have all but trended to a virtual non event for our
The progressive split stream energy aloft moving east from the
middle MS valley never really organizes as it attempts to overrun a
retreating area of modestly strong hp/ridging over the northeast
and eastern Canada. As such downplayed precipitation potential and trimmed
probability of precipitation back by 10-20 percent. The new wrinkle in this forecast cycle is
for a second piece of energy /what appears to be the main surface low/
lifting north from near the Florida/Georgia coast and tracking up/offshore
the East Coast sun through Tuesday.
The split flow pattern over the Continental U.S. Is expected to become highly
amplified with a very deep negatively tilted trough evolving east
of The Rockies...likely spawning an intense surface low over the
Great Lakes/middle Atlantic/northeast regions around the Christmas
evening time frame. For what its Worth...both the 17/12z GFS and ec
are forecasting a record low mslp sub 970mb cyclone in vicinity of Lake
Huron at 00z on 25 Dec. This system will likely gather more and
more attention as we move closer to the Holiday. The good news for
travelers/bad news for snow crows is that current projections
would place central PA initially in the "warm" side of the deep
storm favoring rain vs. Snow. It also looks quite windy assuming
the deep low verifies.
Aviation /20z Wednesday through Monday/...
widespread strato cumulus and persistent MVFR to IFR ceilings with
isolated-scattered and generally brief MVFR to IFR visibilities in snow
showers will occur across the northwest mountains and Laurel Highlands /invof
kbfd and kjst/ through Friday morning. Conditions across the
central mountains and susq valley will vary between low end VFR...and
Gusty west to west-northwest winds /up around 25 kts at times/ will add some
mechanical turbulence during takeoff and fap to regional airfields
this afternoon through Thursday.
A secondary cold front and compact upper air disturbance dropping
southeast across the region late today and early tonight could bring
some briefly heavier snow showers...with a coating to 1/2 inch
possible even across the Central Mountain and susq valley airfields.
Otherwise...not a lot going on till maybe just before Christmas.
There could be widespread rain shower activity around as a potent
surface and upper low tracks NE toward the Ohio River valley.
Thu-Fri...MVFR ceilings poss northwest. Otherwise no sig weather expected.
Sat-sun...light snow/reduced visibility possible ... along the Maryland
Monday...mainly VFR. However...areas of MVFR strato cumulus are
possible in developing southeasterly low level flow.