Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
1106 am EDT Monday Mar 30 2015
a cold front will push across eastern Pennsylvania early this
afternoon. This will be followed by another fast moving
disturbance that will bring periods of snow across the region
north of Interstate 80 later tonight and Tuesday....with mixed
precipitation a little further south...and light rain across the
southern third of the state. Slow moderation in temperatures is
expected for the latter half of the week ahead before a new cold
front approaches for Friday.
Near term /through tonight/...
as of 1030 am...the surface is approaching the susq valley...with just
some spotty...light and brief rain and snow showers.
Gusty west-northwest winds with mostly cloudy to cloudy skies and a few bands
of brief snow showers will impact the remainder of state during
the midday and afternoon hours. Wind gusts will generally peak
between 25-30 kts...but a few gusts to around 35 kts are possible
where vertical mixing can reach 3 kft above ground level and tap the 35-40kts of
well-aligned west-northwest wind at that level.
Diurnal temperature curve will be greatly flattened today considering the
timing of the cfropa and several degree f of cold advection in its
Temperatures will climb another 8-10 degree f...into the l50s across our
far southeast zones this afternoon...while the temperature increase across the
central zones and western mountains will only be another 2-4f.
As the 850-700 mb thermal trough crossed the northwest mountains later this
afternoon and Earl this evening...a few snow showers could whiten
grassy areas across the ridge tops in vicinity of kbfd. However...with
skin temperatures expected to be in the middle to upper 30s...snow accums
should not be a problem.
Models are in better agreement tonight in the track and intensity
of the quick moving wave expected to approach for later tonight
and Tuesday. I leaned closer to the sref probability of precipitation to have precipitation moving
into my northwestern zones after midnight. The precipitation will initially be
snow as it breaks out over the northwest and a light coating is possible
Short term /Tuesday/...
the bulk of the fast moving storm system will affect the region
Tuesday with a wet snow expected across the northern higher
elevations...a rain snow mix in the transition zone between the
rain over southern parts of the state.
The shortwave aloft is forecast to be rather flat...but embedded in
fast flow allowing for impressive jet dynamics under the
approaching left exit region. Strong warm advection on the lower
branch of this jet streak surges into the region as low level
ageostrophic winds increase out of the north helping to tighten
the isotherms. What we see is the development of some deep layer
instability with middle level lapse rates exceeding 6c/km by early
afternoon over much of the state...while an area of strong
frontogenesis tracks through central and northern areas.
The system will be a quick hitter and not blessed with a
tremendous amount of moisture inflow...so quantitative precipitation forecast will not be
excessive...generally on the order of ... highest
amounts over the northern half of the state. I favored the ensemble
snow accumulate guidance which suggested most of my area will see
little or none...with the best chance of a few slushy inches being
along the New York border. Temperatures even there will be in the middle 30s for
much of the day so unless it snows fairly hard...most accums 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 Sunday/...
high pressure should bring dry weather for midweek before the
pattern turns increasingly unsettled during the last part of the
The latest naefsbc has not changed the overall thinking that a
warming trend will develop ahead of the late week cold front.
The details of the front at this range remain fuzzy as the
operational European model (ecmwf) and GFS have timing and intensity issues. The
GFS drops the front into PA Thursday night while the European model (ecmwf) still has
it through the lower gr lakes. The differences between the two
operational models mean Friday could either be mainly dry with the
front settling south...GFS...or wet as the front enters the
From there it gets a little more complicated again with a
trailing wave or waves made to surge out of the Tennessee Valley into the
region Friday night into Saturday. The ec ensemble hints at a wavy
front with perhaps a couple of lows rippling along it eventually
moving through central PA...while the GFS/gefs are more in line
with a single more coherent low taking a more southern track.
Because of the uncertainty with these scenarios...I kept probability of precipitation in
the chance range for Thursday into Friday night before beginning to
dry things out for Saturday.
It seems likely there will be somewhat of a cool down over the
weekend after the front passes and we see the upper low over
hudson's Bay sink south once again. While we will cool down...the
larger scale flow over western noam is expected to have more of a
Pacific origin so temperatures should not continue the trend of being
well below normal as we have seen for much of the last 2 months.
Aviation /15z Monday through Friday/...
surface cold front will push through susq valley taf sites kipt...kmdt
and klns between 15-17z today.
Gusty west-northwest winds average 10-15 kts...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
from bfd to points east toward kavp.
MVFR to IFR in mixed precipitation for the central third of the state
Tuesday morning...and a few periods of lighter /plain/ rain or
showers will occur across the southern third of Pennsylvania.
Some brief and lighter rain and snow showers for Tuesday afternoon
into early Wednesday...in the wake of the low pressure area.
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.
near term...Lambert/la corte
short term...la corte
long term...la corte