Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
643 am EDT Tuesday Mar 31 2015
a potent but relatively fast moving area of low pressure will move
southeast from the western Great Lakes and ride along the Mason-
Dixon line today. This storm system will bring a highly elevation
dependent wet snowfall of a few to several inches across the
northern half of Pennsylvania. Mixed precipitation or plain rain
will fall across the southern tier of Pennsylvania...where a few
midday and afternoon thunderstorms are also possible. High
pressure will build over the region Wednesday with fair and cool
conditions. A sharp moderation in temperatures will occur
Thursday before a new cold front approaches for Friday with rain
showers...possibly changing to a period of snow at night across
the northwest half of Penn.
Near term /until 6 PM this evening/...
narrow ridge of high pressure extended from The Finger lakes of
New York to central Penn at 08z.
Potent Alberta clipper just east of Chicago will track east-southeast and
move along the Mason-Dixon line this afternoon.
Some slight variations were noted in the low pressure track via the
latest oper model and ensemble guidance...but no significant
changes in the forecast of up to several inches of snowfall across
the higher terrain of central and northern Penn /with mainly rain
falling over the southern third of the state near the Center Line
of the storm track /and just south of the mean 0c 850 mb
Coordinated with surrounding weather forecast offices to issue a rather borderline
Winter Weather Advisory for wet snow during the daylight hours
today /which is staggered by about 2 hours from west to east
across our County Warning Area...with respect to onset and ending times of the
main impact period/.
Confidence remains quite high for a swath of moderately heavy quantitative precipitation forecast
of 0.40 to 0.60 of an inch across the northern half of Penn...with
lesser amounts in the south.
Near term hi res /07z hrrr/ and short range ensemble system
depict near to slightly above advisory criteria snowfall for
portions of the west central and north central mountains today
with accums of up to 4 or 5 inches at elevations above 1800 feet
mean sea level.
Given the marginally cold low level air /and the bulk of the
precipitation/snow falling during the warmest part of the day/...we still
think that most deeper valley locations...at or below 1200 feet mean sea level will
remain near or slightly below advisory amts with around 2 inches.
The initial above freezing blyr should mean that some rain will
be mixed it at the onset...then a quick changeover to wet snow is
expected near and to the north of I-80 by late this morning.
A several hour period of moderate to strong uvvel associated with
the left exit region of a 100 knots upper jet will bring a period of
moderate to briefly heavy snow later this morning into early this
afternoon...gradually tapering to areas of lighter snow/snow
showers for the middle to late afternoon. There may be little or no
snow south of the Turnpike and west of Interstate 81...along and
just south of the low track.
Considering numerous main travel routes across central and northern
Penn...cresting terrain above 1600 feet mean sea level...we played it on the
safe side and went with a low-end advisory for this late season
Late day high temperatures will be lower to middle 30s across the
north...near 40f just to the south of I-80...and likely cresting
the 5o degree mark over the scent region and lower susq valley.
Lastly...Storm Prediction Center has 1-2 layers of counties in scent Penn within their
general thunderstorms and rain category for this afternoon as surface based convective available potential energy /along
and just to the south of the projected surface low track/ climb to
several hundred j/kg. Included the chance for thunderstorms and rain in our weather
grids across the southern two layers of counties in scent Penn this
Short term /6 PM this evening through 6 PM Wednesday/...
surface low will exit the New Jersey/Delaware-Maryland-Virginia coast tonight with high
pressure arriving from the Great Lakes. Dry weather will persist
through midweek before the pattern turns increasingly unsettled
from Thursday into Easter weekend. Low temperatures tonight will range from
around 20 in the north to 30f in the south.
Highs Wednesday will be in the upper 30s across the north...and
around 50f in the far south...accompanied by partial clearing and
a 7-12 knots nwrly wind
Long term /Wednesday night through Monday/...
yesterdays 12z.../and the latest 00z/ model run and ensemble
data...continue to indicate that one or two frontal waves will
bring precipitation to the area from Thursday into Saturday. There is still
some spread in both the track and strength of the trailing frontal
waves with the last two runs of the ec coming in much stronger
than the GFS. The Canadian is in the middle in terms of position
but leans toward the stronger ec. At any rate the details are
still unclear but a mean blend/consensus solution incorporating
wpc guidance supports increasing probability of precipitation into the likely range for at
least two 12-hour periods. A shortwave trough rotating through the
middle Atlantic and northeast states will bring a reinforcing shot of
cool air on Saturday...preceded by above normal temperatures ahead of the
cold front. The cool down should be brief with moderating temperatures
into early next week.
Aviation /12z Tuesday through Saturday/...
area of snow just to the west. Expect the snow to come down
hard at times. Current temperatures not real cold as of 6 am.
However...airmass quite dry...thus expect temperatures to fall
once the snow starts.
Expect some clearing tonight.
Cool...but sunny with VFR conditions on Wednesday.
Some showers on Thursday.
Wednesday-Thursday morning...mainly VFR...as high pressure builds across
Thursday PM-Fri...lowering conditions to MVFR with scattered rain
showers...as a cold front moves eastward. Gusty SW wind developing.
Sat...low probability for rain showers /mixed with snow showers north/.
Reductions poss mainly northern mountains
Winter Weather Advisory until 6 PM EDT this evening for
Winter Weather Advisory from 10 am this morning to 8 PM EDT
this evening for paz037-041-042-046-049-051>053-058.