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fxus61 kpbz 111417 

Area forecast discussion
National Weather Service Pittsburgh PA
917 am EST Mon Dec 11 2017

much colder temperatures will arrive on Tuesday along with snow.
Heavy lake effect snow showers will be possible through mid-
week. Some warmer temperatures temporarily arrive this


Near term /through tonight/...
morning update to make only minor changes based on latest hi res
guidance and observations. Otherwise no changes needed.

modest warm-air advection will increase ahead of a
second shortwave trough that will advance ewd from the nrn
plains today. Despite the advection, temperature will remain
below the seasonal average, but will be just a bit warmer than
those seen yesterday.

Any light snow that develops this afternoon in the warm
advection pattern will be confined to the far nrn zones, where
cold air will linger the longest.

Conditions start to get more interesting going into the
overnight hours as warm advection isentropic ascent starts to
become sufficient to saturate the 290-295k layer such that light
snow chances start to increase in the northern zones. Given the
height at which the maximum lift occurs, a marginal boundary
layer, and a less than convincingly saturated dendritic layer
with this activity, though quantitative precipitation forecast projections suggest a few tenths
will be possible, an efficient snow is not expected. As a
result, just a couple of inches will be possible tonight over
the northern reaches of the County Warning Area in advance of a strong cold
front that will be following the first of a few potent short
wave disturbances through the area over the next several days.
This will cause the column to cool rapidly aloft, dropping the
dendritic growth layer toward the surface, destabilize the
column, and cause far more efficient snow growth. Thus, snow
chances rapidly increase along the front and also snow
accumulation quickly become more efficient.


Short term /Tuesday through Thursday night/...
the cold front completes its passage through the area by early
Tuesday, and much more favorable lapse rates in its wake
combined with remnant boundary layer moisture and a dendritic
growth zone dropping toward the ridgetops will more than likely
mean a short interval of very efficient snowfall across the
terrain. Tuesday morning was really hit quite hard in the
forecast in the terrain, however given the amplitude of cold
advection and an unfavorable fetch for lake moisture to
intersect the ridges, while snow will continue there, the lack
of moisture will mean quickly decreasing snow amounts beyond
Tuesday for the ridges even though light snow showers will
probably continue.

For the ridges, snow will not necessarily be the biggest issue.
In the wake of the front, while most of the area will be likely
to see gusts in the range of 30 kts, the potential momentum
increases rapidly off the deck, thus in the terrain, gusts in
excess of 40 kts will be possible. With new and falling snow,
this will lead to issues with blowing and drifting. Furthermore,
as 850 mb temperatures fall off a cliff toward -19c by Tuesday
and Tuesday night, much colder air temperatures will combine
with the wind to cause sub-zero wind chill values across the
area and sub -10 degree wind chills in the terrain. The
combination of snow, wind chill, blowing snow, and wind
potential in the ridges will likely lead to an advisory or
potential warning in a subsequent forecast issuance.

Elsewhere, a burst of heavy snow seems likely with the frontal
passage. For most areas this will be rather brief, however with
the dipping dendritic growth layer, it would seem all portions
of the area could see at least some light accumulation as the
front passes. The biggest impacts seem poised to occur as the
lake effect starts to increase behind the front. This will be
especially true across the areas north of I-80.

While the immediate Post-front environmental fetch doesn't turn
horrible northwesterly over the snowbelts, a secondary wave
passage aloft of Tuesday night will give another trend toward
more northerly component. This combined with a forecast dual-
lake band setup from Lake Huron over Lake Erie and into northwest PA
will mean snow rates in the snowbelts should rapidly increase
Tuesday night as inversion heights rise yet again over 11 kft,
flow becomes better aligned through the deepening boundary
layer, and frictional convergence increases farther inland as
lake induced troughing wanders toward the I-80 corridor Tuesday
night. Not only does moisture flow increase into the area while
the boundary layer deepens during this time, however, but so too
does Crystal efficiency. Thus, snow-to-liquid ratios rapidly
increase toward 22 or higher to 1.

With all of these factors in mind, the Winter Storm Watch for
the Lake Erie snowbelts remains in effect from Monday night
through Wednesday, though the heaviest lake effect potential
during the interval is Tuesday night into early Wednesday.
Further, while no headlines will be issued at the moment for the
ridges, it would seem winter weather advisories at least will be
needed early Tuesday for snow, possibly extending thereafter for
blowing snow or wind chill concerns. Please refer to future
statements from the National Weather Service regarding this.
Elsewhere, depending upon how one fetch downwind from Southern
Lake Michigan into Ohio and a second fetch that extends across
Lake Superior/Northern Lake Michigan/Western Lake Erie into
western PA evolve, accumulations from additional less easily
forecastable snow bands will be possible. As higher resolution
datasets start to resolve this period, we will likely get a
better idea as to how these potential bands may develop and
evolve with some refining of the snow forecast likely centering
around these bands.

Everything should start to wind down by late Wednesday afternoon
as flow turns more westerly and eventually southwesterly. Warmer
air will quickly start to move into the region on Thursday,
which given how cold the air it will be displacing is, the
chance of light snow will increase yet again. At the moment, qpf
looks fairly meager, so amounts of snow should be fairly light
on Thursday with this as temperatures rebound toward freezing.


Long term /Friday through Sunday/...
the cold upper trough that has been in residence in the ern
Continental U.S. Will be ejected newd and supplanted by progressive flow
aloft, leading to the passage of a series of shortwave troughs
for much of the medium-range forecast period.

Return flow around Gulf Coast high pressure will lead to
temperature moderation in the Sat-Mon/Tue timeframe, with maxima
and minima around climatological average and periodic chances
for precipitation.

Thereafter, models suggest that a return to a colder pattern may
be in the cards, as a 500 mb ridge builds again into Alaska, and a
shortwave trough dislodges cold air into the ern Continental U.S..


Aviation /14z Monday through Friday/...
residual light snow this morning in association with a now-
departing shortwave trough will come to an end, returning all
terminals to VFR (or worst-case high-end mvfr) by mid-morning.
Highest chance for IFR conditions through early morning will be
from kfkl-kduj.

Warm air advection will strengthen by this afternoon as a second
trough approaches from the nrn plains. Cloud height will lower
gradually this evening, with restrictions and light,
accumulating snow expected late tonight as a cold front
approaches and crosses the region 12/0800-1200 UTC timeframe.

a prolonged period of lake-effect snow showers is expected in
the region, especially in the vicinity of kfkl/kduj. Widespread
restrictions can be expected Tue-Wed before improving Thu.


Pbz watches/warnings/advisories...
PA...Winter Storm Watch from 7 PM EST this evening through
Wednesday afternoon for paz007>009-015-016.

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