Area Forecast Discussion
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FXUS61 KPBZ 121023

Area Forecast Discussion
National Weather Service Pittsburgh PA
523 AM EST Tue Dec 12 2017

Much colder weather arrives today along with snow. Heavy lake
effect snow showers will be possible through mid-week. Another
system early Thursday will bring additional accumulation. A brief
warm up follows this weekend.


Surface low pressure is currently departing the northeastern
reaches of the area. Cold advection has already started quickly
with its passage. Thus, most all areas have seen their highs
for the day with a slowly falling temperature trend this
morning, folloed by a remarkably quicker decline this afternoon
as a secondary frontal boundary surges through the area.

Large scale deformation around the surface low tracking through
is spreading snow across areas north of its track this morning
largely in line with previous expectations, even if the snow got
started a bit later than previously projected. Temperatures in
the deformation zone quickly fall toward and below freezing with
precipitation onset, and given weak snow-to-liquid ratios
through the start of the snowfall that have always been
expected, snow accumulations from the system itself this morning
should largely be relegated to areas north of Pittsburgh.

Things rapidly change as the Arctic boundary pushes into the
area as the last vestiges of deformational ascent depart to the
east. Rapid cold advection will work to quickly destabilize the
column and saturate the dendritic growth zone. This is
evidenced by the linear band of snow activity pushing southeast
of the Michigan/Ohio border, and it will continue southeastward
through our area today. As it passes, a burst of snow in a more
unstable dendritic growth regime will be likely and rapidly
cooling temperatures along the much stronger wind gusts will
follow in its wake. This boundary would seem likely to drop up
to an inch in just about every location in the lower elevations
it passes, but the calculus looks to change dramatically as it
intercepts the terrain.

As boundary layer moisture gets progressively sandwiched into
the terrain with the incoming boundary, rapid cooling aloft and
upslope WNW flow will mean a drastically increasing snowfall
intensity late this morning. A period of heavy upslope snow
showers is likely as the -18C isotherm falls toward the
ridgetops and wind gusts increase toward 40 mph. It is possible
that local blizzard conditions will result, particularly in
Garrett/Preston/Tucker Counties, however the duration of heavy
snowfall should only continue through the afternoon as once the
immediate post-frontal instability effects are exhausted, flow
looks to be too westerly for a decent moisture feed toward the
terrain. Thus, the winter weather advisory has been maintained,
even with high snow rates expected through the daytime today, as
they should fall off tonight. Fries


Right as the moisture into the ridges starts to become
exhausted, yet another mid-level short wave tracks southward
across Lake Erie, which will turn our eyes farther north for the
overnight hours. An increase in inversion heights and
reorientation of the flow over basically all of the Great Lakes
starts to occur tonight. A multi-lake fetch from Lake Huron over
eastern Lake Erie is probably the moist poignant feature to
follow. In this area, with inversion heights running up over 11
kft and lake-induced troughing downstream from the lake
increasing boundary layer friction effects, a single dominant
band is suggested by both large scale and meso-models that
tracks into the northern CWA overnight into Wednesday morning.
The models are showing good consistency with this feature, and
previous discussions and headlines have captured the threat
quite realistically with the meteorology being rehashed ad
nauseum. Timing on tongiht`s runs is similar, and thus tonight
still looks like the highest accumulation period for
Venango/Forest/northern Clarion.

Elsewhere tonight, again two other bands seem to be showing up
in large scale model guidances with remarkable persistence such
that incorporating them into the forecast became a bit more
necessary. The first and probably most impactful of these is a
Superior-northern Michigan-western Erie band with a trajectory
over northern Ohio toward Columbiana, Beaver, Allegheny, and
Westmoreland Counties. Given the long overland trajectory from
the lake, it would seem substantial accumulations would be
unlikely, however the persistence in this band in guidance
coupled with an impressive instability profile have resulted in
PoPs being increased along this corridor with general snow
expectations kicked up about an inch from previous forecasts.
This includes the Pittsburgh area.

Given the first dominant banding position, while upslope snows
should be fairly minimized tonight into Wednesday for most of
the ridge locations, this many not end up being the case for the
ridges in Westmoreland County. Here again, the evolution of this
band will need to be evaluated, as any persistence of it
combined with upslope flow will necessitate an upgrade of the
current advisory to a warning.

The second band is the shore-parallel band that is currently
extending down the eastern shore of Lake Michigan that models
across suggest will traverse all the way into central Ohio and
WV as the Arctic front surges southeast by tonight. While the
consensus suggests this band will lie just to the south of our
CWA, it seemed worth mentioning here as it may play a role in
the forecast depending upon low-level wind evolution.

While flow starts to turn more southwesterly by Wednesday
afternoon, lake effect bands will quickly transition to the
northeast and out of the area. However, as warm advection begins
in earnest ahead of the next system dropping toward the area for
Wednesday night, isentropic ascent in the 290-300K layer and
strong southwesterly moisture transport will rapidly increase a
field of snowfall from west to east across the area. This will
be in advance of a surface low that the model consensus tracks
southeastward from the western Great Lakes toward the PA/WV
border during Wednesday night. Some disagreement exists on the
strength of the system, with the NAM persisting a closed low-
level center and thus stronger deformation than the
GFS/EC/Canadian, however indications are that at least modest
deformation/frontogenetical light will follow the isentropic
ascent such that a maximized swath of QPF will be likely from
northern Ohio through western PA toward the Alleghenies. Lifting
and moisture profiles suggest a much deeper moist layer and a
far higher dendritic growth region. Thus, snow-to-liquid ratios
will not be as impressive as earlier in the week, however a
gradation from 15 to 10 from N to S is suggested, and as such a
swath of winter weather advisories may be necessary as this
system dives southeastward. These will need to be covered in
future forecast updates, however, as ongoing headlines preclude
issuing them at the moment for the sake of clarity. Fries


Brief cold advection follows behind the departure of Wednesday
night/Thursday`s system. This will be short-lived, however, as a
general retrogression of the upper trough toward the high plains
seems in store for next weekend. This will result in more normal
temperatures returning to the region. However, all signs
continue to point toward the reorganization of the upper trough
over our region sometime early next work week with the return of
below normal conditions shortly after the end of the current
forecast. Fries


VFR conditions will deteriorate late tonight as snow showers
develop associated with crossing low pressure.

Kept similar timing for precip onset and restrictions - around
08-12z with greatest confidence in prolonged IFR at KFKL and
KDUJ but brief IFR visibility conditions possible elsewhere.

Expect some improvement later Tuesday outside of the terminals
impacted by lake-enhanced snow but general MVFR should prevail.

Southerly flow will continue until the passage of the front
overnight/tomorrow with a return to wnw flow with gusts to near

Periodic restrictions in snow showers and northwest flow will
continue through Wednesday. Another system will bring another
round of snow and restrictions to many of the terminals


MD...Winter Weather Advisory until 7 AM EST Wednesday for MDZ001.
     Wind Chill Advisory from 7 PM this evening to 1 PM EST
     Wednesday for MDZ001.
PA...Winter Weather Advisory until 1 PM EST Wednesday for PAZ007-
     Lake Effect Snow Warning until 1 PM EST Wednesday for PAZ008-
     Winter Weather Advisory until 7 AM EST Wednesday for PAZ023-
WV...Winter Weather Advisory until 7 AM EST Wednesday for
     Wind Chill Advisory from 7 PM this evening to 1 PM EST
     Wednesday for WVZ512>514.


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