Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
1116 am EST Wednesday Dec 11 2013
a cold cyclonic flow over the Great Lakes will maintain below
normal temperatures through the end of the week...with lake
effect snows focused over far northwest Pennsylvania in the
perennial snowbelt region. A pair of weak clipper systems passing
to the north will reinforce the cold air. For most of the area...the
next few days will be cold and dry. The active early season winter
pattern looks to resume with another storm system likely impacting
the area Saturday into Saturday night...followed by more cold and
brisk conditions into early next week.
Near term /through tonight/...
surface observation/Sat imagery shows mostly sunny skies across central
PA. Even with increasing southwesterly surface winds afternoon temperatures today
will wind up being a couple degrees colder than yesterday after
mins bottomed out this morning in the teens and single digits.
Only precipitation at this point is intermittently snow showers across the
far northwest...bfd. Water vapor Sat imagery shows weak shortwave
crossing the Great Lakes and into Ohio Valley. This will help
redirect low level flow into somewhat more favorable flow for le
snow into northwest PA...at least expanding coverage of light snow
showers into the afternoon hours. Les advisory remains in effect
for Warren Colorado...with the bulk of accums expected between 18-06z based
on the latest run of the mean hires consensus short-range model.
For most areas...it will just be brisk and cold with daytime maximum
temperature departures averaging 15 degrees below early December climatology.
Will continue to highlight near to below zero wind chills in the
severe weather potential statement for the early morning and nighttime hours.
Short term /Thursday through Friday night/...
a series of northern stream impulses are forecast to race eastward across the Great
Lakes around the base of large polar vortex over Hudson Bay. The
vortex should gradually shift to the north and east by the end of
the period and allow for at least modest 500mb height rises and
slightly moderating temperatures as the 1000-500dm thicknesses gradually
increase. There is some uncertainty focused on the upstream side
of the trough with placement diffs over Ontario...which appears to
impact the low-level temperature gradient and magnitude of high pressure cell
moving over Montreal Canada by 12z Sat. This feature looks to play
a role in another winter mixed precipitation event into the weekend.
In general...expect the lake effect snow mechanism to fade into
day 3 with the focus shifting to developing low pressure/inverted
trough over the Tennessee Valley and associated warm air advection/overrunning precipitation possibly
reaching the area by early Saturday morning. Day 2/Thursday looks like
the coldest day based on consensus guidance with highs 20 degrees
below normal. Wind chills will remain a concern and will continue
to highlight in the severe weather potential statement. Sub-zero apparent temperatures seem likely during
the nighttime and early morning hours across the west-central mountains
Long term /Saturday through Tuesday/...
the active early/cold-season pattern across the eastern Continental U.S. Will
resume this weekend...likely bring more winter weather to the area
from Saturday into early Sunday. A consensus of the 11/00z
GFS/CMC/ECMWF deterministic model data and their respective
ensemble means /including the naefs/ show the most likely outcome
being a Transfer of surface low pressure from the Ohio Valley to the
middle Atlantic coast by Saturday night...with subsequent deepening
of the coastal low as an Arctic vortex plunges into the Great
Lakes on Sunday.
The most critical region in terms of model spread initially lies
upstream over the Midwest where several shortwaves are expected to
converge...and then translates eastward to the coast as the Miller-b
cyclogenesis process runs its course. The disjointed/complex
nature of the split flow pattern expected to evolve and merge/phase
over the eastern states makes forecasting specific hazards somewhat
of a low confidence venture. The integrity of the retreating? Cold
air banked along/east of the Appalachians will undoubtedly be crucial
for p-types. The Transfer of energy to the coast/dumbelling of
middle-level shortwaves will ultimately dictate where moisture will
converge and where dry slots will be forced. Lots of variables to
While there remains better than average convergence of solutions
on a storm system which supports going at or above medium range consensus
MOS probability of precipitation...as usual with systems several days out...the details
will need sorting-out at shorter ranges to determine sensible weather
impacts. At this point...model thermal profiles indicate that
precipitation arriving early Saturday should start as snow.
However from this point on ptypes become increasingly problematic
with the oprn European model (ecmwf) trending slower vs. The other global guidance
with its surface low track...and very aggressive in locking in very
cold air especially near the surface. Meanwhile the 00/06z GFS and 00z
gefs mean are more bullish on allowing warmer air to reach at
least the southern half of the County Warning Area Sat night...perhaps suggesting ptype
transition to pl/fzra. For now will continue to broadbrush
multiple ptypes in an attempt to indicate a wintry mix...however
if this cycles European model (ecmwf) verifies it would be an all snow event. To
add a final layer of uncertainty to the forecast...experience and
local research suggests wide range of outcomes is possible in
Miller-b situations. I personally can recall a large variation
from one system to the next. The bottom line is that this system
will have the potential to bring winter weather impacts to the area
this weekend...and anyone with travel plans should stay tuned to
the latest forecasts.
A cold and blustery west-northwest flow should follow behind the departing
coastal low with lake effect/upslope snows over the northwest mountains and
western high terrain later Sunday and possibly into Sunday night.
High pressure should arrive by early next week. Temperatures should moderate
into Tuesday/Wednesday as flow veers to the west/SW ahead of a clipper wave
dropping across the northern Great Lakes. Large scale height falls associated
with another polar shortwave spreading across the Great Lakes/mid-Atlc
may spin-up another low off the coast...but at this point its
latitude would be too far north to impact the local area.
Aviation /16z Wednesday through Sunday/...
VFR will dominate most of the region today. Bfd will see reduced
conditions continue in lake effect clouds and snow showers. Jst
could also see brief reductions as upslope flow brings clouds and
some snow showers...mainly after 00z/12.
A weak cold front will move across the region this afternoon bringing
thickening clouds from west to east and scattered snow
showers...mainly in the mountains due to strong winds aloft combined
with instability from the front...winds aloft should mix
down...bringing gusty west winds as high as 25mph this afternoon
On Thursday...high pressure will build over the Tennessee Valley and southern
Appalachians bringing widespread VFR conditions to the region once
again. Though bfd could linger in the MVFR due to lake enhanced
clouds/snow shower into Thursday afternoon.
Thu-Fri...no widespread sig weather.
Sat-sun...snow or mixed precipitation with low ceilings/visibilities possible.
lake effect Snow Advisory until 7 am EST Thursday for paz004.