Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
743 PM EDT Monday Mar 30 2015
a potent but relatively fast moving area of low pressure will move
southeast from the western Great Lakes tonight and ride along the
Mason-Dixon line Tuesday...bringing highly elevation dependent
snow accumulation of a few to several inches across the northern
half to two thirds of Pennsylvania. Mixed precipitation or rain
will fall across the southern tier of Pennsylvania. 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 much of central and northern
Near term /until 6 am Tuesday morning/...
upper trough sliding eastward this evening over southern New York
state along with steep low level lapse rates is maintaining
scattered snow and rain shower activity across the north central
mountains into middle evening. Expect these to decrease by late
evening (03-04z) as upper trough axis slides east and llvls
Not as optomistic for brief clearing across the north early
tonight as a result of the lingering shower activity...but should
still see some breaks across the south central mountains...and the
lower susq will remain mostly clear to partly cloudy. Winds
will tail off to light and variable in many locations after
midnight...allowing min temperatures to settle into the middle 20s to lower
The stage will be set for yet another early Spring /wintry/
weather event for much of central and northern Penn beginning
around or shortly before daybreak Tuesday across the western
mountains...then spreading across the remainder of the region during
the middle to late morning hours. More to follow in the short term
Short term /6 am Tuesday morning through 6 PM Tuesday/...
models are in good agreement on the track of a potent clipper
along the Mason-Dixon line during the day Tuesday.
Three or four factors greatly complicate the forecast for snowfall
amount across the region Tuesday. The first being the exact track
and quantitative precipitation forecast...second is the timing of the heaviest snowfall rates during
the morning /a few hours faster mean more snow/...and third being a
large degree elevation dependency.
In any case...a several hour period of moderate to strong uvvel
associated with the left exit region of a 100 knots upper jet will
bring a moderate to briefly heavy snow Tuesday morning...gradually
tapering to areas of lighter snow/snow showers for the middle to late
afternoon. Likely snowfall will vary from 1-3 inches in the
valleys...and up to 4 or 5 inches of heavy wet snow across the
higher terrain near and to the north of the rt 322 corridor.
There may be little or no snow south of the Turnpike and west of
Interstate 81...along and just south of the low track.
After coordination with surrounding offices...we wanted to wait
for at least one more run to confirm the timing track and quantitative precipitation forecast
amounts before coordinating an advisory for a borderline and
highly elevation influenced event.
Multi model temperatures for Tuesday/S maxes are in the middle 30s across
the northern mountains...and upper 30s to middle 40s in central Penn...with
highs around 50f in the south.
Unless it snows fairly hard...most accums after 15z will likely
be on grassy surfaces. Over central areas where a rain-snow mix is
likely for a time...it should be too warm to stick.
Long term /Tuesday night through Monday/...
surface low will exit the New Jersey/Delaware-Maryland-Virginia coast Tuesday night 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.
The 12z model and ensemble data seems to be coming around to the
idea 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
Aviation /00z Tuesday through Saturday/...
gusty west-northwest winds average 10-15 kts for the next several hours...with
gusts of 25 to 30 kts at times will be the rule in the wake of the
cold front through the rest of the daylight hours.
VFR will continue across all central PA and susq valley taf
sites...while MVFR ceilings will be found for the rest of today across
the higher elevation airfields of kjst...kfig and kbfd.
A fast moving clipper type low will spread a shield of steady
light to MDT snow across the northern third to half of Penn very
late tonight and Tuesday morning with conditions dropping to
IFR/LIFR from bfd to points east toward kipt and kavp.
IFR in mixed precipitation or snow appears likely for the central third
of the state Tuesday morning/early afternoon...while a few
periods of lighter rain/snow showers will occur across the
southern tier counties of Pennsylvania.
Some brief and lighter rain and snow showers for late Tuesday
afternoon into early Wednesday...in the wake of the low pressure
Looking cold but dry on Wednesday.
A sharp...but brief warming for Thursday...with a gusty SW wind
Tuesday...chance of light snow/rain...mainly across the northwest half
with MVFR reductions.
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.