Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
643 am EST Friday Feb 27 2015
a large area of high pressure will build across the area through
the first half of the weekend. A low pressure system will lift out of
the Midwest late in the weekend...passing over the region Sunday
night. A stronger area of low pressure will likely track west of
Pennsylvania late Tuesday or early Wednesday.
Near term /until 6 PM this evening/...
at 11z radar continues to show patchy light snow falling from
around kunv sewrd into the lower susq valley along axis of inverted
surface trough. Returns should continue to diminish...as drier air
works in. Blend of latest hrrr and 4km NAM indicate any accums
after 12z will be negligible. Satellite imagery shows clearing skies
across northwest PA early this am...as dry...Arctic air mass
overspreads the area. Expect this partial clearing to progress
into the eastern half of the state later this morning.
Upstream observation suggest skies may become mosunny for a while this
morning. However...diurnal heating...combined with cold pool
aloft...should yield a fair amount of strato cumulus this afternoon...especially over
the Allegheny plateau. Gefs mean 925mb temperatures around -15c should
translate to highs no better than around 10f across the higher
terrain of the alleghenies...to the l/m20s across the susq valley.
Short term /6 PM this evening through 6 PM Saturday/...
clearing skies...a calm wind and snow cover will likely result in
the lowest temperatures until next winter tonight as the center of high
pressure moves over Penn.
The ideal radiational cooling conds should allow readings to
bottom out from 10-15f below zero across the western mountains...to
0-5f above zero across the lower susq valley. The coldest rural
valleys in northwest and ncent Penn could see the Mercury settle to
around -20f at 11-12z Saturday. Some locations across the northwest mountains
are likely reach wchill advisory criteria...despite a light/calm
wind. However...confidence in widespread apparent temperatures blw -15f
not there yet for an advisory.
Fair and still cold weather expected Saturday...as surface high
drifts over the area. A fast-moving middle level shortwave is prognosticated by
all models to track across the area late in the day. Low precipitable water air
mass will preclude any precipitation concerns. However...cirrus may dim
the sunshine at times. Ens mean 850/925mb temperatures remain well blw
normal for the date...likely supporting highs only in the 20-25f
Long term /Saturday night through Thursday/...
a relatively mild/active southwest flow aloft should prevail
through the period as the persistent upper ridge over the western
U.S. Temporarily breaks down. An Arctic high pressure system over
central PA at the start of the period will retreat into Atlantic
Canada by Sunday night. The combination of shortwave energy in the
southern stream ejecting from a dsw trough and a digging northern stream
trough moving southeastward from south central Canada toward the upper
Midwest seems to drive an expanding area of warm air advection type snowfall
spreading eastward sun-Monday behind the southerly return flow into
a retreating cold sector associated with the departing Arctic high.
The precipitation should be enhanced along an expanding baroclinic zone
across the central and eastern US...as upper ridging builds over
the southeast U.S.
While there appears to be little indication of a strong surface
low...models still generate 0.25-0.50 inch quantitative precipitation forecast amts along the
baroclinic zone. Thermal profiles seem to favor snow across the northwest
1/2 of central PA with mixed ptypes in the south/southeast.
Preliminary 24hr snow/ice amounts were derived using a multi
model quantitative precipitation forecast blend with pseudo internal bias correction factor and
wpc westward. This results in advisory level snowfall over the northwest 1/2 and
0.10 inch ice/freezing rain over the lower susq valley. Confidence in the amts is
average at best given uncertainty in quantitative precipitation forecast and low-level temperatures.
The lack of a defined surface cyclone also raises some reg flags.
Another cold hp system builds into the region Monday-Tuesday with another
SW flow warm air advection moisture surge along baroclinic frontal zone likely
impacting the area into the middle of next week. Timing
differences are evident with the GFS more bullish on precipitation
returning early Tuesday vs. Slower European model (ecmwf) (tue ngt). Mixed ptype issues
are expected again with this next system.
Aviation /12z Friday through Tuesday/...
large-scale forcing ahead of a shortwave over the Great Lakes
bringing some light snow to portions of central mountains early this
morning. This will reduce visibilities/ceilings to MVFR /occasional IFR/
primarily over the western higher terrain through 12z. Brief
period of MVFR ceilings poss further east through 10z.
Another push of Arctic air arrives immediately behind the
shortwave for Friday. Orographic flow will keep MVFR conditions in
western higher terrain through midday...with improving conditions
in the afternoon as anomalously strong 1040-1045 mb surface high
pressure builds into the commonwealth from the Ohio Valley. VFR
will be the rule in central and east. Winds will be west/northwest at around
10 miles per hour.
Friday night and Sat...no sig weather expected.
Sun...snow likely northwest mountains mixed precipitation possible elsewhere.
Monday...mixed precipitation with ceiling/visible reductions possible...mainly am.
Tuesday...VFR am. Mixed precipitation with ceiling/visible reductions possible late.
the coldest February on record is still looking likely. Monthly
mean temperatures through the 26th...ipt 18... 20.7.
The current record coldest February..ipt 19... 21.4 both set
So far...this month is the third coldest ever at ipt...and fourth
coldest ever at MDT.