Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
802 am EST Wednesday Mar 4 2015
low pressure will track north of Pennsylvania this morning...as
its trailing cold front sags south through the region. Another wave
of low pressure will track south of Pennsylvania later today into
Thursday. High pressure will build southeast into the state Friday
Near term /until 11 am this morning/...
temperature outside finally up to 38 degrees. However...colder air
to the east. Still ice on the ground outside here...and with a wide
range in temperatures and conditions...did not cut back anymore
on the advisory. The Winter Weather Advisory expires at 7 am.
Earlier discussion below.
Did adjust the advisory some...Clearfield still near 32...while
other sites above 32.
Overall the ll jet and associated push of warm air aloft has changed
precipitation to plain rain over much of the Allegheny plateau as of 07z.
Meanwhile...freezing rain continues to fall across portions of central PA.
While air temperatures are just above freezing...ground temperatures
Model soundings...as well as the latest guidance...continue to
indicate the low level cold air will erode from west to east...with
the last of the freezing rain ending across the region by sunrise.
Far SW part of the County Warning Area above freezing...keeping an eye on the
rainfall...as we already have water over some roads as of late
Flood Watch in effect for the SW now.
Short term /11 am this morning through 6 PM Thursday/...
based on colder tops to the SW over Ohio...still seeing moderate
rain at times...along with some bright banding...and other
offices...decided to issue a Winter Storm Warning for tonight
and Thursday...for the southern portion of the County Warning Area. While amts could
be a little high...big concern over rapid temperature drop
expected late tonight.
Earlier discussion below.
Surface frontal boundary will sag south of PA by late Wednesday.
Concern is that rather cold air at low levels will work in...at
the same time that energy from the SW will result in more precipitation.
More details below.
Models continue to show an amplified and moist SW flow aloft and
jet entrance region dynamics supporting a broad stripe of precipitation
along a well defined baroclinic zone ahead of phasing upper level
energy over the dsw/rockies today and moving eastward through the Ohio
Valley and middle Atlantic on Thursday. Several waves of low pressure are
forecast to develop and move east-northeastward along a SW-NE oriented cold front
which should progress and settle southeastward through the middle Atlantic region
by early Thursday...as Arctic high pressure builds into the upper
plains. Ample moisture is forecast by the models to move into a
deepening cold sector along/north of the southeastward-advancing cold
frontal boundary which could result in a late season heavy snow
event for south central PA Wednesday night into Thursday. There are model
differences with the timing of the front which is likely tied to the
consolidation of shortwave energy upstream. That said...the
consensus forecast axis of maximum quantitative precipitation forecast /0.50 inch or greater/ has been
fairly consistent painted over the southern 1/3 of the County Warning Area. Similar to
yesterday...used a multi model blend with pseudo bias correction
factor to derive gridded quantitative precipitation forecast amts which generally range from 0.25
to 0.75 inches across the southern 1/2 of the County Warning Area. The column will be
in a state of transition and cooling down from northwest to southeast Wednesday-Wednesday
night as the cold front pushes southeast across the area. This
should support a transition from rain to snow. A period of sleet
may impact forecast snow accumulations with the duration a key factor
in this forecast. With increasing confidence and greatest risk in the
potential for heavy snow/6+ inches...which is supported by robust
wpc ensemble as well as the gefs mean...have issued a Winter Storm
Watch for the south-central counties from Wednesday evening
through Thursday evening. However...lingering precipitation remains
likely across southern PA in region of 8-7h fgen forcing. Model blyr
temperatures indicate precipitation will fall as light rain across the southeast
counties. However...model soundings just cold enough to support a
rain/snow mix across the Laurel Highlands...where a light accumulate of
up to an inch appears possible by Wednesday evening. Drier air across
northern PA should translate to a mainly dry day there with just a
chance of -shsn.
Long term /Thursday night through Tuesday/...
an active late-winter precipitation pattern will impact central
PA through midweek followed by what is hopefully the last shot of
Arctic air for the season Thursday into Friday. The pattern should
dry out late in the week through the upcoming weekend with high
pressure in control most of the time. The general model and ensemble
consensus is to maintain a ridge in the west and trough in the
east through the weekend with some signs of a pattern shift by the
middle of next week with troughing forecast to develop along the West
Coast. Below normal temperatures should moderate closer to normal early
next week with some hints at even warmer temperatures later next week.
What is hopefully the last shot of Arctic air this season should
arrive behind the cold front. Friday morning lows are likely to be
20-30f below normal with single digit lows +/- zero degrees. Such
readings may break daily records.
Aviation /13z Wednesday through Sunday/...
a cold front /traversing northern and western higher terrain at
12z/ will continue to sag southeast across the region today.
Westerly winds will pick up to 10-20 miles per hour in the western higher
terrain...with winds elsewhere turning to the west/northwest and increasing
to 5-10 miles per hour. Widespread ceiling restrictions to mainly IFR conditions
will persist across the west today...with central and eastern
sections settling into generally persistent MVFR conditions. Light
rain will overspread the southern portion of County Warning Area early in the
day...but as colder air works in from the north and another wave
of low pressure slides along the frontal boundary...precipitation
intensity will increase heading into tonight with precipitation type
changing back over to snow. Biggest impacts will be from kjst to
kmdt/klns as widespread IFR/LIFR conditions develop Wednesday night into
Improving conditions will set in for Friday into Monday...as cold
high pressure builds into the area.
Wednesday night...sig reductions in snow developing mainly across the
Thursday...slowly improving conditions...lingering MVFR northwest and southeast.
Sun...MVFR with chance -shsn mainly north and west. VFR southeast.
Flood Watch until 7 PM EST this evening for paz017-024-025-033-
Winter Storm Warning from 7 PM this evening to 7 PM EST
Thursday for paz024>028-033>036-056>059-063>066.