Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
239 am EST Friday Dec 6 2013
a series of low pressure waves will track northeast from the
Tennessee Valley across the Middle-Atlantic States tonight through
Friday...bringing periods of mixed precipitation to the area. High
pressure will provide a brief respite in the recently active early
season winter weather pattern on Saturday. Confidence is steadily
increasing for a period of frozen and freezing precipitation
Sunday into early Monday that should eventually change to rain. An
Arctic frontal passage early next week will drop temperatures well
below normal through next Wednesday.
Near term /until 6 am this morning/...
fine line on kccx radar showing location of cold front...just west
of kunv and kaoo at 0330z. Near term models agree in tracking
cold front southeast across the area overnight.
Although rain tapered off a bit across central PA earlier this
evening...regional radar mosaic showing another batch of rain
approaching from western PA at 0330z. Upstream water vapor loop
shows a shortwave over Illinois at 03z...which should lift NE
across the east Great Lakes overnight. Lg scale forcing ahead of this
feature should help support next batch of precipitation late this evening
through around 10z. 02z rap frontogenesis patterns match well with
band of snow falling from central Indiana to Northwest Ohio. Rap indicates
this band of enhanced fgen lifts across northwest PA between 06z-09z as model
soundings become cold enough for snow. So...believe a light snow
accumulate is possible over Warren/McKean cos late tonight.
Further south...will ramp up chance of rain overnight...as shortwave
and associated ll jet/plume of anomalous precipitable waters lifts through. Blend of
latest lamp/rap/NAM indicate temperatures behind cdfront will coast
downward to the u20s by dawn over the northwest mountains...the 30s most of
central PA...and the m40s across the lower susq valley.
Have increased wspeeds overnight based on upstream reports and
BUFKIT soundings...which support a period of gusts around 20kts in
in the northwest flow in wake of frontal passage.
Short term /6 am this morning through 6 PM Saturday/...
next shortwave lifting out of the S plains will spawn a wave of
low pressure along quasi-stationary surface front S of PA on Friday
with another slug of precipitation for Friday afternoon and evening.
Strong frontogenetic forcing will develop under the entrance
region of a very strong jet streak that will race down the St
Lawrence River valley. The biggest issue with the second wave will
be the amount of wintry weather associated. Latest ensemble and
oper guidance suggests air mass will be cold enough to support
snow across the north counties...as the heaviest precipitation arrives Friday
afternoon. Further south...latest model soundings support rain changing
to a wintry mix across the central counties Friday evening...and
mainly rain over about the southeastern 1/3 or so of my forecast area.
Blended model quantitative precipitation forecast supports a 3-5 inch snowfall across the north
tier...with just enough of a wintry mix over the central counties
to potentially create slippery roads during the evening. Across
the Laurel Highlands...warm air aloft should preclude much snow.
However...the higher terrain down there still looks susceptible to
a period of freezing rain Friday evening. Have therefore...moved back the
start time of their advisory to 22z.
Deepening cold air will advect into the state late Friday night with
lingering light snow/mixed precipitation tapering off quickly around
Long term /Saturday night through Thursday/...
1040+ mb modified Arctic surface high pressure over the upper Midwest
Saturday will migrate eastward into the northern Middle-Atlantic
States by Sunday morning...setting the stage for what appears to
be a classic overrunning mixed precipitation event for later Sunday into
05/12z deterministic model and ensemble guidance shows low pressure
initially developing along an inverted surface trough axis extending
from the lower MS/Tennessee Valley northeastward up the west-side of the Appalachians.
The main low should lift northward through the Ohio Valley/Great Lakes into southeastern
Canada on days 5-6. Meanwhile a secondary/weaker area of low pressure
should form near the Virginia/NC coast and track north-northeastward just offshore southern
Downstream from a vigorous middle/upper level trough digging through
The Four Corners/southern rockies...a surge of intense 850-700mb warm
air/Theta-E/moisture advection will spread quickly northeastward behind a
broad southwesterly flow aloft and overrun a retreating cold sector
associated with the retreating Arctic high. The synoptic pattern
looks very conducive for a period of freezing-to-frozen precipitation as
moisture and strong warm air advection aloft flows over the top of a shallow sub-fzg
airmass that should remain entrenched along/east of the alleghenies
via low level cold air damming/surface pressure channeling...enhanced by northeasterly
ageo flow and wet bulb effects. Surface-850mb temperatures are forecast to warm
enough to change freezing ptypes to plain rain in most areas
around Monday afternoon before the Arctic cold front crosses the region
Monday night. The 12z gefs mean shows a high probability of a few
inches of snow/sleet on the leading edge of the precipitation shield
followed by potential significant ice accums especially from The
Laurels/scntrl mountains into the interior north-central part of the County Warning Area.
Will continue to highlight this system in the severe weather potential statement but confidence
is increasing in at least an advisory type event...if not warning
assuming moderate-heavy icing via freezing rain.
In the wake of this system...a very cold pattern should grip the
region through the middle of next week with temperatures well below
normal. Some lake effect snow showers are possible in the cold
cyclonic flow aloft although the low level winds may have too much of a SW
compnt which would Orient bands more along the Lake Erie shore
into SW New York.
Aviation /06z Friday through Tuesday/...
Ares of showers will continue to stream through the region
overnight. This steady wave of rain moving through central PA.
LIFR will continue over the lower susq terminals where fog is
still locally dense. Otherwise...MVFR and lower flight categories
will continue into this morning as more rain...snow...and wintry
mix overspread the region.
Friday...MVFR in rain southeast. IFR in snow/wintry mix northwest.
Sat...MVFR snow showers in the mountains. VFR in central/eastern
Sun...reductions developing with approaching storm that will bring
a wintry mix spreading south to north into Sun night.
Monday...widespread MVFR/IFR associated with a passing cold front.
Winter Weather Advisory from 1 PM this afternoon to 5 am EST
Saturday for paz004>006-010>012-017-018-037-041-042.
Winter Weather Advisory from 5 PM this afternoon to 5 am EST
Saturday for paz024-033.
short term...Fitzgerald/la corte/steinbugl