Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
736 am EST Tuesday Mar 3 2015
high pressure centered over the the Pocono Mountains early today
will drift east to the southern New England coast by this
afternoon. An area of low pressure over the Central Plains states
will move quickly northeast to the upper Great Lakes by tonight. A
surge of deep moisture ahead of this low will ride up an over the
cold air in place across Pennsylvania and lead to another round of
significant wintry/mixed precipitation this afternoon into early
tonight. The precipitation will likely turn to rain across
southern sections early Wednesday. Colder temperatures will follow
for the latter part of the week...and steady snow will return
Wednesday evening into Thursday as a wave of low pressure ripples
along a nearly stationary front that will be draped just to our
south. High pressure and cold air will return for Friday into the
first part of the weekend.
Near term /until noon today/...
surface high pressure /1030 mb/ was centered over northeastern
Pennsylvania and will be drifting east across the lower Hudson
River valley during the late morning hours. Cirrus shield has
overspread all of the County Warning Area and thickened-up substantially over the
past few hours.
Regional 88d mosaic loop shows plenty of 10-20 dbz returns moving
quickly east into the western portions of our County Warning Area...
observation confirm nothing more than a few flurries or light sleet
pellets reaching the ground all the way back through Ohio and wva.
Expect conditions to change rather quickly late this morning and
early this afternoon /per latest rap and hrrr/ as an anomalously
strong 50-60 knots 850 jet /sref/ transports a plume of deep
moisture up and over this dome of low level cold air.
Only some minor changes /upward adjustments/ were made to the
probability of precipitation for today with little or no change in the onset of the
Early morning lows with winds light and variable were in the
single digits above and below zero across the northern mountains...in the
lower to middle teens throughout the Central Ridge and valley region
of the state...and upper teens to around 20f in the southeast.
Still expect precipitation onset from the southwest to hold off until
Short term /noon today through 6 PM Wednesday/...
conditions will go downhill fairly quickly today with an
anticyclonic arc of deep moisture from Baja California monsoonal flow
advecting through the Ohio Valley and arriving into/blossoming
over the alleghenies by late this morning in the form of snow
initially...spreading eastward across my County Warning Area through the midday
and early afternoon hours.
Very mild upper boundary layer flow will bring a quick mix and
eventual changeover to freezing rain for all as the day
progresses. There may be a complete changeover to rain across the
far south and southeast. Consensus blend of 00z-06z operational
models and ens guidance is still consistent with previous
solutions for an initial few to several hour period of snow/sleet
this afternoon...followed by a significant bout of mixed
precipitation/freezing rain later this afternoon through early tonight.
Overall it appears that prolonged ice accretion is the main
threat throughout the central and northern mountains of the state /and to a
lesser degree across the middle and lower susq valley/...preceded by
the coating to 1 inch of snow far south and 1 to 2 inches of snow
in most areas (with up to 3 inches north).
The north may not completely mix out the shallow and initially
bitterly cold air...so expect areas of freezing rain to persist
there through the pre dawn hours on Wednesday. Ice accums should
be on the order of 0.10-0.20 of an inch across the ncent mountains of
PA...with lesser amounts in most places to the south of Interstate
Winter weather advisories continue for a period of snow/sleet
accumulate...followed by freezing rain...generally from midday today
through the early morning hours on Wednesday.
Long term /Wednesday night through Monday/...
an active late-winter pattern will affect 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 consensus is to
maintain a ridge in the west and trough in the east...limiting
temperature moderations to no better than climate normals.
Models show an active precipitation zone extending from the Southern Plains
east-northeastward into the northern Middle Atlantic States...in between a positively
tilted trough pushing through the plains/Midwest/Great Lakes and
above normal ridging from the Gomex into the western Atlantic Ocean.
Several waves of low pressure are forecast to move along a SW-NE
oriented cold front which should slowly progress eastward toward the
coast Wednesday-Thursday as northern stream energy crosses the Great Lakes. There
are some differences with model quantitative precipitation forecast but the consensus points
toward the southern PA for the highest precipitation amts. Used a multi model
blend with pseudo bias correction factor to derive gridded quantitative precipitation forecast
amts which conservatively 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 from northwest to southeast Wednesday-Wednesday night as the cold front
pushes east. This should support a general transition from rain to
snow across the County Warning Area. With a decent amount of moisture available/stripe
of above normal precipitable water located just south of PA...the potential exists
for moderate to heavy snow over the southern portions of the area.
However given the uncertainty at day 3-4 range in the frontal
waves and thermal profiles in a state of flux going from warm to
cold...confidence in big amounts is questionable. For now the day
3+4 numbers range from 0-2" north of I-80...2-4" from i80 to the
PA Turnpike/I-81 and amounts in excess of 4" possible along/S of
the Turnpike/I-81. This is subject to change as any slight shift
in the quantitative precipitation forecast axis is plausible at this point. The takeaway here is
that winter weather will likely continue to impact the area through
midweek with the greatest risk for heavy snow over the southern
portions of the County Warning Area.
What is hopefully the last shot of Arctic air should arrive behind
the cold front Thursday with single digit lows +/- of Friday
morning. May be the coldest morning until next winter. Some snow
showers are possible over the weekend with a reinforcing cold
front. Temperatures will remain at or below average through the period.
Aviation /12z Tuesday through Saturday/...
ongoing VFR conditions at 12z will persist into the middle/late
morning hours as high pressure quickly cedes to an approaching
winter storm system. Winds through middle morning will be less than
5 miles per hour.
A fast moving storm system will lift across the Great Lakes...and
quickly spread snow/sleet into western sections by noon and across
all of central PA by middle afternoon. Warmer air surging in on
increasing SW flow will bring a transition to a wintry
mix/freezing rain event by late afternoon into the early
evening...before changing to rain /fzra east/ late tonight into
early Wednesday as heavier precipitation gradually tapers off. Widespread
significant restrictions to ceilings/visibilities will accompany this storm
at all terminals for an extended period starting late this
morning/early this afternoon. Low level wind shear will also develop as 850mb
winds increase to around 50 kts.
Late Wednesday/early Thursday as colder air works back to the south...low
pressure moving NE along the front will likely result in a nasty
mixture of precipitation. Potential is there for heavy wet snow
and significant freezing rain as well.
Wednesday...some improvement to conditions but some reductions will
continue /esp south/ in rain transitioning back to snow later in
Wednesday night...sig reductions in snow developing mainly across the
Thursday...slowly improving conditions...lingering MVFR northwest and southeast.
Winter Weather Advisory from 3 PM this afternoon to 7 am EST
Wednesday for paz006-011-012-018-037-041-042.
Winter Weather Advisory from 1 PM this afternoon to 4 am EST
Wednesday for paz004-005-010-017-024>026-033>035.
Winter Weather Advisory from 3 PM this afternoon to 4 am EST
Wednesday for paz019-027-028-036-045-046-049>053-056>059-