Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
1057 am EST Tuesday Mar 3 2015
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 6 PM this evening/...
after several hours of virga...first bands of precipitation (snow) have
arrived across northwest/west central/central mountains and Laurel
Highlands late this morning. Some quick moderate rates have been
noted around Erie and band is intensifying as it pushes eastward.
A quick 1 to locally 2 inches will accumulate in spots before the
initial intense snow band moves east of the Laurel Highlands and
sout central mountains. The southern third of central PA will
likely then see a several hour lull in precipitation later this
afternoon...while the north transitions to sleet and then
Near term details appear to be in good shape...but concerned that
we'll exceed advisory criteria for ice later tonight north of
intersate 80. Sref threats Page indicate 70-80% confidence in
20-.30" or more of ice and successive hrrr runs continue to delay
warming at the end of its 15-hour window...meaning that much of
north central PA will likely never rise above freezing. Critical
timeframe will be between 21z and 04z...and may be a whites of
their eyes decision based on radar evolution and adjustments of
hrw-arw and hrrr consensus late this afternoon.
Short term /6 PM this evening through 6 PM Wednesday/...
prolonged ice accretion is the main threat tonight 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 /16z Tuesday through Saturday/...
conditions will quickly deteriorate from west to east as moderate
snow bands cross through the central and north central mountains
through midday and early afternoon hours. Warmer air surging in
on increasing SW flow will bring a transition to a wintry
mix/freezing rain event by middle to late afternoon into the early
evening...before changing to rain /fzra north/ 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 until 4 am EST Wednesday for paz004-
Winter Weather Advisory from 3 PM this afternoon to 4 am EST
Wednesday for paz019-027-028-036-045-046-049>053-056>059-