Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
223 am EST Thursday Mar 5 2015
a wave of low pressure will track south of Pennsylvania this
morning. High pressure will build southeast into the state Friday
and Saturday. A strong upper-level ridge will build into the
region early next week. This should provide generally fair weather
and several days of near normal temperatures and relatively dry
Near term /until 8 am this morning/...
water vapor/satellite derived precipitable water imagery showing an atmospheric river
of anomalous precipitable waters extending from the S Pacific Ocean newrd into
the miss valley and eventually PA and the New England states.
This deep moisture plume is overrunning an expansive Arctic high
pressure to the north. Earlier mixed precipitation across far southern Penn and
the lower susq valley has changed over to all snow between 06-07z
as the mean 925-850mb temperature dipped to just below zero c. Bright
banding near and just south of kthv and klns suggests that
sleet/rain will be mixed in with the snowflakes for the next hour.
Upper flow will be backing slightly early today ahead of the
approaching...positive tilt upper trough extending from the upper
glakes to middle miss River Valley. In addition...short
range...higher resolution models indicate a band of weak 850-700
fgen developing/moving NE from ecent Ohio to northestern Penn.
This will cause the precipitation /snow/ shield to temporarily bulge
north for several hours during the middle-late morning /09-15z/...and
likely reach slightly north of the I-80 corridor. This scenario
agrees quite well with our current snowfall forecast with respect
to amounts and areal distribution.
Colder air will continue to filter into the region on northwest flow.
Latest 21-00z sref and oper model data continue to point toward a
general 2-4 inch snowfall across the southern counties
overnight...with perhaps a few amts near 6 along the Maryland border.
Temperatures at 07z running from the l20s across the northwest mountains...to
the l-m30s in the southern valleys/lower susq region.
However...arrival of precipitation and low level cold advection will drop
readings to the teens and 20s by dawn.
Pertinent Point of previous disc and model data...
21z sref and 18z gefs threat pages still paint my far southeastern zones
as having the highest risk for heavy snow through middle day
Thursday...with a rather sharp cut off on the back side up into
the central mountains. However...leaning toward the more recent
00z and hrrr guidance...which indicates slightly lower amts than
Short term /8 am this morning through 6 PM Friday/...
blend of latest model data indicate snow will taper off by late am
across the central counties...and may last until late afternoon across
York/Lancaster counties. Arctic high and associated dry air mass will
arrive early over the northwest counties...resulting in dry/cold weather the
entire day up there.
Some partial clearing appears likely across the northwest counties late
in the day. Elsewhere...model soundings indicate a middle level cloud
deck will linger even after the snow ends. Overcast skies and cold
advection will likely Blunt any diurnal warming and result in
steady temperatures through early afternoon...then falling temperatures late.
Consall output supports daytime temperatures from only around 10f across
the northwest mountains...to the l20s across the southeast counties...another
exceptionally cold day for the time of year.
Long term /Friday night through Wednesday/...
high pressure and below normal low-level temperatures will
dominate the region Thursday night into late Friday. Unseasonably
cold air...perhaps the last of the significant Arctic surges of
our endless winter.
Based on the low temperatures near and below 0f and the
winds...the potential exists for some areas to experience wind
chills overnight Thursday into Friday which might require an
advisory across the northern tier counties.
Saturday the temperatures at 850 mb return closer to normal and
high pressure drifts to our south. Pennsylvania should remain
north of this weak high....cutting off any moisture to the
south...keeping the region in weak westerly flow into Monday. The
period should be relatively dry with any frontal precipitation
suppressed to our south and only weak northern stream waves to our
north to generate any precipitation.
So...kept the probability of precipitation low through the period and any light
precipitation would likely be snow as 850 hpa temperatures stay below 0c.
Best chance of any precipitation would be in northern areas should
one of the northern stream waves prove to be stronger than
A ridge is forecast to build over the region Tuesday into
Wednesday and our lower troposphere temperatures should get at
least slightly....above normal....both Tuesday into Wednesday.
This should provide at least 2 days of near seasonal temperatures
for a change. Relatively warm days and cold nights...a good slow
way to melt snow without flooding.
The naefsbc and gefsbc suggest a frontal passage Thursday with
slightly colder temperatures as another high pressure system is
forecast to move over the northeastern United States Thursday into
Saturday next week.
Aviation /07z Thursday through Monday/...
moisture overrunning a wavy cold front to the south of PA will
bring another round of rain/snow mainly to areas south of I-80
overnight into Thursday morning.
Widespread MVFR/IFR will continue across southern airfields overnight
through at least middle day Thursday before drier air begins moving
into the region from the west and north bringing improving
conditions. Northern sites...especially kbfd may stay MVFR or
Transition from rain to snow occurring across kmdt at this time. This
should be complete over the next 2 to 3 hours with snow then
continuing through the night.
Thursday...slowly improving conditions...lingering MVFR northwest and southeast.
Sun-Mon...MVFR with chance -shsn mainly north and west. VFR southeast.
Winter Weather Advisory until 10 am EST this morning for
Winter Weather Advisory until 1 PM EST this afternoon for
Winter Storm Warning until 4 PM EST this afternoon for paz033-