Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
530 am EDT Friday Oct 31 2014
a weak ridge of high pressure will drift east across the
Appalachians early today. A vigorous upper level trough digging
through the Carolinas will support the development of a strong
coastal storm expected to track just off the East Coast this
weekend. This will lead to much cooler air being ushered into the
region on a brisk northerly wind. High pressure will return to the
area behind the departing storm system on Monday with moderating
temperatures into the middle of next week.
Near term /until 6 PM this evening/...
look for mostly cloudy to overcast skies to continue through
the day today in the form of one or more layers of strato cumulus and
Alto cumulus. The culprit for the clouds was relatively weak
isentropic lift from the southwest right around the 285k Theta
surface. Some breaks in the clouds are expected across mainly the susq
valley and locations to the east.
Early morning temperatures across the susq valley and scent zones /where
the formal growing season has yet to end/ will be in the u30s and
lower 40s. Across the northern and western mountains...temperatures will be
in the low to middle 30s around sunrise.
Will keep continuity with previous forecast and maintain dry
conditions for the southeast half of the County Warning Area...while initially isolated
showers across the northwest mountains...spread to the southeast and expand in
coverage this afternoon and evening as the potent upper trough
/over the western glakes this morning/ dives southeast toward the smokey
mountains and upper diffluence greatly increases over central and
western Penn atop an inverted surface trough.
The primary forecast problem for today is determining just how far
east the showers will move as the upper low to our west develops
rapidly and drops south. The track of the low well to our west and
then south of the area suggests there will be some measure of
shortwave ridging/subsidence to the east of the system...over
parts of the middle Atlantic and northeast.
High temperatures today will vary from the low-middle 40s across the northwest
mountains...to the middle 50s throughout the lower susq region.
Short term /6 PM this evening through 6 PM Saturday/...
models are in good agreement with the development of a
sharp/highly amplified upper trough over the eastern U.S. During
the upcoming 24-36 hours. The upstream energy can be seen clearly
on water vapor imagery as a potent upper trough over northestern wisc moves
southeast and from least a pair of systems. The primary chunk of upper
energy is currently moving south-southeast across northestern Wisconsin.
The upper flow over the local area is expected to back quickly to
the southwest and eventually south as a potent low closes off and
dives south-southeast from the lower glakes toward the Carolina
At this point it looks like most of my eastern forecast area could
remain dry into tonight...while the best chance of rain showers
will be over about the western half of the County Warning Area. The lack of deep
moisture should result in fairly light rainfall with this
feature...perhaps up to a quarter inch over the West Mountains
Risk of snow over the higher elevations of The Laurels by later
Friday appears low given latest model soundings.
Long term /Saturday night through Thursday/...
Friday night into Saturday the large upper level low will drop
down south as all medium range guidance cuts off upper low south
of PA and moves it down into the Carolinas. Late Friday
night...early Saturday morning as remnants of an associated clipper
low should die over the central Appalachians as energy is
transferred to developing coastal low near Cape Hatteras. However
given the moisture...the advection of colder air...and the upslope
flow...there is a good chance for snow showers over the SW
alleghenies early Saturday morning. This is reinforced by westward
graphics and model soundings. Have adjusted weather grids accordingly.
As the low drops south it will become a coastal storm and lift NE.
This will cause a tightening pressure gradient and bring strong northwest
flow drawing much colder/drier air into PA. Operational runs and
nearly all ensemble members keep coastal low far enough out to sea
to not directly impact central PA. However...cold northwest flow will
likely produce scattered lake effect snow showers across the Allegheny
plateau and parts of the central mountains Saturday night. Given
dryness of air mass moving in...Don/T expect any sig accums...but
a coating of snow does appear possible over the West Mountains
Temperatures this weekend will almost certainly be the coldest yet this
season. Ensemble mean 800 mb temperatures point toward daytime highs from
the u30s-l40s over the alleghenies to 45-50f across the lower susq
valley. The coldest night of this outbreak will likely be Sunday
night...when surface high builds over the region. Those locations that
have yet to see a freeze will likely get it by Monday am.
High confidence in a period of dry weather Sunday/Monday...when
all guidance builds surface high and associated dry air mass across PA.
An approaching cold front will bring the next chance of rain showers by
midweek. Deep upper trough will lift out quickly early next
week...allowing a much milder return SW flow to overspread the
region. Temperatures likely to return to above normal readings by Tuesday.
Aviation /10z Friday through Tuesday/...
31/09z update...no significant changes to 06z tafs.
31/06z infrared Sat and surface observation show a fairly uniform...overcast low VFR
cloud deck /with bases generally at or below 050/ blanketing most of the
airspace. Although surface winds are expected to remain light/vrb to
calm...felt the guidance was too bullish on lowering visbys in
br/fog given extensive cloud cover. Therefore the 31/06z scheduled
tafs reflect an optimistic/prevailing low VFR flgt category in
the 0-6hr timeframe before dropping ceilings to MVFR at western tafs
after 12z. Not expecting precipitation to impact the western airspace until
this evening as low pressure tracks east-southeastward from lower Michigan across WV to the
Carolinas. In general...look for conds to trend lower tonight into
early Saturday with precipitation coverage increasing over the western and
*Noteworthy change in the outlook period...the period of strongest
north-northwest winds on the backside of the developing coastal storm has
shifted to Saturday night into Sunday.
Sat...MVFR/IFR with scattered rain/snow showers west. Varying
conds from VFR-MVFR central/east with occasional -shra. North-northwest winds
increasing Saturday night.
Sun...gusty northwest winds. Rain/snow showers ending west. Low ceilings
west becoming VFR.
Mon-Tue...no sig weather.