Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
1047 am EST Friday Dec 19 2014
high pressure over the Great Lakes will slide east into
Pennsylvania this weekend. A deep upper trough and associated surface
low will lift northeast through the Great Lakes during the middle
of next week.
Near term /until 6 PM this evening/...
shallow cold...moist air trapped below a moderately strong
subsidence inversion /based at around 2-3kft above ground level/ will continue to
produce widespread stratus and stratocu through the rest of today.
There/S a chance for some breaks of sunshine across the southeast during the
middle to late afternoon hours as shallow vertical mixing under the
low late Dec sun angle tries to poke of a few holes in the
Orographic forcing of the shallow layer of moisture via a well-
aligned nwrly flow should continue to produce a bit of very light
snow or -fzdz across Cambria/Somerset counties through at least 18z.
However...given the very light nature of the precipitation and that some
is falling as snow...have opted to continue covering the minor event
with an Special Weather Statement rather than a west-southwest. Surface observation have been indicating both
-shsn and -fzdz and model soundings indicate borderline cloud top
temperatures of around -10c...so will continue to forecast scattered -shsn and patchy
-fzdz over the Laurel Highlands this am.
Maximum temperatures should be similar to Thursday...with u20s over the
highest terrain of the alleghenies...to the u30s across the lower
Short term /6 PM this evening through 6 PM Saturday/...
high pressure and associated drier air mass will build in from the Great
Lakes tonight...providing the area with fair weather. For much of central
PA...tonight should feature mclear skies. However...low level
moisture trapped beneath inversion may result in persistent
stratocu across the Laurel Highlands this evening.
Warm air advection aloft in advance of shortwave lifting out of the miss valley
will likely spread some thin cirrus into the region on Saturday.
However...all model data supports a dry day with surface ridge and low
precipitable water airmass over the state. Maximum temperatures should be fairly close to
seasonal norms in the 30s.
Long term /Saturday night through Thursday/...
high pressure will shift slowly eastward from the Great Lakes
into northern New England by 12z Monday. A moisture-starved northern
stream shortwave should produce a period of cloudiness Sat
night...but likely not produce any precipitation. Latest model soundings
point toward a relatively bright Sunday in the wake of this
feature with near seasonal temperatures. Split flow regime across the
lower 48 will transition to a highly amplified pattern by early
next week across the lower 48...with a deep negatively tilted
trough evolving over the eastern 1/3rd of the Continental U.S..
The models seem to be forming a consensus on the pesky low
that refuses to be well forecast...lifting it slowly from Florida
Panhandle north-northeastward just off the East Coast sun-Monday along a low level
baroclinic zone that may not budge past the Gulf Stream as the
flow aloft turns southwesterly/parallel to the coastal front.
Increased probability of precipitation for Monday over the southeast based on multi-model/wpc
blend. Complex surface pattern with some hints at low level cad along
with warm air advection aloft via southerly flow could make for a messy mixed ptype
An intense triple point system developing on Dec 23-24 will
likely steal the headlines given its timing and large portion of
real estate that will be affected. While the details remain
unclear at this point...temperatures should be warm enough to
support rain on the front side of the system before brisk west-northwest
winds drive strong cold air advection east of the Appalachians. The timing of the
cold/occluded front will be key for precipitation transition from
rain to snow showers. As the system spins up into eastern
Canada...the strong cold air advection circulating through the Great Lakes and Ohio
Valley should lead to lake effect/upslope/wrap around snow showers
over the western alleghenies by Christmas day. A period of strong
gusty winds are highly probable as the dynamic system intensifies
shifting nwwd from New England into Ontario/Quebec.
The take home message is that a volatile period of weather will
impact the area through Christmas day with potential wind...rain
and snow impacts all on The Table. Stay tuned.
Aviation /16z Friday through Tuesday/...
surface trough dropping southward this morning will gradually wash
out. Ribbon of higher low level moisture along and in advance of
this feature will continue to support spotty light snow/fzdz across
the Allegheny Mountains through around 18z...bringing icing concerns
for aviation interests. IFR conditions ongoing mainly at kjst
with widespread MVFR elsewhere across the central and northwest mountains low
end MVFR strato cumulus cloud deck will persist across the susq valley
through much of the day.
Orographic forcing of shallow layer of moisture should continue to
produce a bit of light snow or fzdz around kjst through late morning.
Otherwise...arrival of drier air in wake of trough should produce
gradually lifting ceilings with partial clearing later today...especially
where west wind downslopes the mountains across the susq valley and southeast
half of County Warning Area. Northwest winds will be around 10 kts with higher
High pressure builds in tonight...with mainly VFR conditions. Some
MVFR ceilings may develop again over the northwest mountains at kbfd. And light
winds may produce some local fog restrictions in spots.
Sat-sun...no sig weather expected.
Monday...low ceilings with light rain/snow possible. Becoming breezy.
Tuesday...low ceilings with rain likely. Breezy.