Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Pittsburgh PA
355 am EST Friday Dec 13 2013
a complicated storm system will bring snow and a wintry mix late
Friday night into Saturday night.
Near term /through today/...
a rather glorious middle-December day looks to be in store across the
region as infrared satellite imagery reveals little but cirrus and
cold air across the region this morning. Upstream conditions for
the rest of the day look relatively similar...so a forecast of
mostly/partly sunny looks good. A cold start to the day with
temperatures generally in the teens and 20s will mean a struggle
toward 30f today...as a frozen ground...snow cover...and a very
weak sun angle all work against strong diurnal temperature
Short term /tonight through Sunday night/...
..another accumulating snow expected tonight into Saturday...
Clouds will quickly advance into the area tonight as the next
system approaches the area. Warm advection increases toward the
area overnight aloft...while a period of clearing in the evening
allows temperatures to fall off pretty quickly early. This seems
likely to set the stage for the development of light snow from
west to east across the area overnight as all areas look likely to
have sufficiently cold profiles to allow for this.
Several common characteristics and a few problematic differences
still exist amongst the model guidance in terms of the evolution
of the occluding cyclone that tracks through the area on Saturday.
First...the commonalities: strong isentropic ascent seems likely
to encompass the area rapidly overnight Friday night into Saturday
morning. Isentropic Omega values of 4-8 microbars/second in the
295-305k layer along with strong moisture transport with some
Atlantic component likely to enhance moisture transport into the
southeastern zones should allow for rapid intensification of the
snow into Saturday morning. Additionally...the period of most
intense isentropic ascent seems to be Colorado-located with the lower
portion of the dendritic growth zone on both the NAM and
GFS...with the average temperature in the 295-305k layer running
roughly -12c. Because of this...snow growth should be efficient at
the same time forcing for ascent is the strongest. This would
favor higher ratios on the order of 13-15:1 across the area
initially. Furthermore...at least some modest jet coupling can be
noted on the NAM/GFS during the afternoon hours of Saturday. This
will accentuate ascent during that period.
While all these factors are working toward strong snow
accumulations...particularly Saturday morning...there are other
factors playing strongly against runaway snow accumulations across
portions of the area. First of all...thermal profiles quickly
become problematic over the southern reaches of the forecast area
by Saturday midday. Given a surface low track favored from
Zanesville to DuBois on the consensus while the low weakens and
emphasis is shifted to the coastal low...warm advection seems
likely to be more effective at the surface over Ohio than in
western Pennsylvania. Because of this and due to the expectation
that the surface low should start to move into the cold air and
decay as it GOES into Pennsylvania...a mix with/change to rain can
be expected for a period early Friday afternoon over much of the
forecast area in Ohio...and while it would seem this should
extrapolate into Pennsylvania...that may well not be the case.
With a weakening surface low and weakening surface warm
advection...the model consensus favors the 925 mb 0c line hardly
reaching Allegheny County...and failing to move north of
Westmoreland County. Additionally...the a ridge of surface high
pressure east of the ridges seems likely to lock cold air in to
the east of the Chestnut Ridge line. As a result...while a
mix/change ot rain may sneak up into southern Allegheny County and
western Westmoreland County Saturday afternoon...the expectation
is that things should remain virtually all snow east of the
Chestnut Ridge through the afternoon as well as north of Indiana.
As such...snow amounts generally north of a line from Zanesville
to Beaver to Indiana should be relatively unaffected by a mix
with/change to rain...and the same will likely be true east of the
Chestnut Ridge. In these areas in Ohio and
Pennsylvania...generally 3-6 inches of snow will be
probable...with the highest amounts likely to fall in
Further south of the aforementioned line...a mix with/change to
rain is quite likely. This will cut accumulation off Saturday
morning...with amounts likely to be much lower...particularly
south of Pittsburgh. However...in the far southeastern ridge
counties in West Virginia and Maryland...easterly flow from the
areas to the east of the mountains seems likely to lock cold air
in there concurrent with strong deformation...isentropic
ascent...and moisture transport during the afternoon Saturday. The
model consensus as well as the sref mean quantitative precipitation forecast favor more than 0.50
inches of quantitative precipitation forecast during the afternoon alone on Saturday...and model
time height sections confirm a favorable thermal and microphysical
environment for dendritic growth as well as surface accumulations.
While some riming will be likely given how cold some of the
profile comes to freezing...rapid accumulations will be possible
Saturday afternoon...particularly in Garrett County and eastern
Tucker County. Even modest snow-to-liquid ratios suggest a rapid
6-8 inches will be possible there before drier air aloft moves
into the area and cuts off efficient snow growth. As such...winter
storm watches have been issued for this.
While Friday night into Saturday will be rather messy...things
look to quickly wind down Saturday evening. However...this may not
be the end of The Mess. Cold advection at the surface looks to be
rather sluggish as the inland surface low decays in upstate New
York and a marginal thermal profile remains over the area Saturday
night. Additionally...several model runs have suggested the
dendritic growth layer should dry out progressively from southwest
to northeast on Saturday night. With deep moisture remaining below
the still fairly elevated dendritic layer along with time height
sections suggesting negative Omega values as well as an increase
in upsloping as low level flow turns westerly...drizzle and
freezing drizzle will become increasingly possible. These dry
slots are notoriously difficult to forecast...however should the
scenario unfold with surface temperatures dropping below
freezing...additional winter weather/freezing rain advisories may
be necessary for a portion of the area.
Upslope seems likely to continue into Sunday with the moist layer
gradually waning in the boundary layer. While some additional
drizzle/freezing drizzle may be possible...the dendritic growth
layer should gradually drop toward the saturated layer allowing a
change back to light snow shower activity by Sunday/Sunday night.
Cold air should definitely arrive back in the County Warning Area by Sunday/Sunday
night as well...with all areas dropping back well below freezing
as precipitation chances start to wane.
Good Luck out there on Saturday with the wintry conditions across
the region. Instantaneous carpal Tunnel may ensue if I write
more...however all watch/advisory products have been issued at
this time...so no more typing tonight. Fries
Long term /Monday through Thursday/...
the long term period features several fast moving upper level
disturbances crossing the Great Lakes. While confidence in the
southern extent of each of these systems remains low...low chance
probability of precipitation were maintained over the north to take into account any lake
enhanced precipitation. Both models also have a more potent wave cross
midweek bringing yet another Arctic push. Temperatures were
generally kept 10 degrees below normal through the week.
Aviation /08z Friday through Tuesday/...
VFR conditions expected at all ports through the morning. MVFR
stratocu will drift down from Lake Erie this afternoon reach fkl and
Outlook.../Friday night through Tuesday/...
widespread restrictions late Friday night through Sunday in both a
mix of snow/rain as low pressure ejects from the plains towards
the coast. High pressure builds overhead for Monday...with another
chance for snow possible on Tuesday.
Maryland...Winter Storm Watch from late tonight through late Saturday
night for mdz001.
Ohio...Winter Weather Advisory from 1 am Saturday to 1 am EST Sunday
Winter Weather Advisory from 1 am to 7 PM EST Saturday for
PA...Winter Weather Advisory from 1 am Saturday to 1 am EST Sunday
WV...Winter Storm Watch from late tonight through late Saturday
night for wvz023-041.