Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
757 am EDT Thursday Oct 30 2014
high pressure over the Ohio Valley this morning will move into
the Appalachians tonight. A vigorous upper level trough digging
through the Carolinas will support the development of a strong
coastal storm tracking just east of the New England benchmark this
weekend. This will lead to a brisk north northwest wind with much
colder air supporting snow showers over the western alleghenies
Saturday night. 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 /through tonight/...
patchy dense fog over portions of the lower susq valley should
lift over the next 1-2 hours as a light northwest wind advects drier air
into the area.
Cool air spreading downwind of the lower Great Lakes along with
weak upslope flow is supporting an expansive area of stratocu
clouds across the northwest alleghenies. These clouds will prevail
through the morning as the low level moisture gets trapped under the
subsidence inversion associated with surface high migrating slowly
eastward from the Ohio Valley. Model data suggests breaks in overcast this afternoon which may
be a little too optimistic. Downslope flow should lead to more sun
east of the Allegheny Front.
Shortwave ridging aloft and high pressure at the surface will combine to
bring dry weather to central PA from this afternoon into tonight. High temperatures
will range from the middle 40s in the northwest to near 60f in the lower susq.
Amplifying upper trough pivot into the upper Great Lakes by 12z
Friday with a surface low developing near Lake Huron. Model consensus
supports a schc of showers over far northwest PA by the end of the period.
Short term /Friday through Friday night/...
all medium range guidance still shows a potent shortwave diving
southeast through the Great Lakes and carving out a deep trough over the
northeast Continental U.S. By Friday night. Track of this feature now
appears relatively certain based on lack of spread in model
runs...all of which indicate a path across eastern Ohio Friday PM.
Best lg scale forcing over PA...as implied by model 300-500mb q-vec
convergence...should be across the western half of the state.
Latest gefs/sref output support categorical probability of precipitation across the
Allegheny plateau Friday afternoon/Friday night and only chance probability of precipitation across the
susq valley. 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 alleghenies Friday night appears low given
latest oper model soundings...as well as gefs/sref probs. Have
removed chance of snow and raised overnight lows to above freezing
everywhere Friday night.
Long term /Saturday through Wednesday/...
all medium range guidance cuts off upper low south of PA Saturday.
Remnants of associated clipper low should die over the central
Appalachians as energy is transferred to developing coastal low
near Cape Hatteras. As coastal storm lifts NE...a tightening
pressure gradient and northwest flow will draw 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 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 /12z Thursday through Monday/...
dense fog has finally scoured out at lns with a return to VFR
expected by 13z. Outside chance bfd/jst may dip to IFR ceiling but
decided to go with prevailing low MVFR with the 12z issuance based
on persistence and guidance.
High confidence in MVFR ceilings prevailing over the western 1/3 of the
airspace early today with ceilings remaining at or below 050 for much of
period. Scattered-broken low VFR ceilings will be across the central and eastern
sections of the airspace through the afternoon with layered middle and
high clouds increasing late tonight into Friday.
Friday...VFR/MVFR early with rain showers developing western sections by evening.
Reduced conds likely Friday night west-central 1/2 of airspace with
rain showers mixing with/changing to snow showers western 1/3.
Sat...MVFR/IFR with scattered rain/snow showers west. Varying
conds from VFR-MVFR central/east with occasional -shra. North-northwest wind gusts
25-40 miles per hour.
Sun...rain/snow showers ending west. Low ceilings west becoming
VFR. Gusty north-northwest winds gradually decreasing area-wide.