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Area forecast discussion 
National Weather Service Pittsburgh PA
606 PM EST Tuesday Nov 25 2014

Synopsis...
low pressure system is expected to develop and ride northward
along the coast on Wednesday...bringing accumulating snow to areas
south and east of Pittsburgh. A second weaker system will usher in
colder temperatures and lingering snow shower chances through the
end of the week.

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Near term /through Wednesday/...
with 545pm update...updated hourly temperature and sky cover
trends...as overcast skies are clipping the northwestern edge of
the forecast area while the rest of the region remains mostly
clear. Previous discussion follows.

Quiet weather in the interim as a surface low pressure system
begins to develop along the southeast coast this evening. This low
will then lift northeastward with the help of a digging upper
level shortwave over the plains and a strengthening upper level
jet....riding along the cold front that crossed the region
yesterday. Despite the expected increase in clouds in advance of
the low...clearing skies early should help to drop temperatures
into the 30's tonight across much of the region.

The general model consensus is that snow will begin to overspread
the region early tomorrow morning from southeast to northwest as
the low is lifted northward. The NAM seems to be the
outlier...with very high Omega values and thus snowfall
accumulations in comparison to all of the other models...and
several runs of the models. With that being said...the quantitative precipitation forecast values
have increased a bit among all of the models...which even with
modest snow ratios leads to warning criteria snow in the higher
elevations of Garrett...Preston...and Tucker counties and just
exceeding warning criteria in the Chestnut and Laurel ridges. A
bit of an expansion westward in the track has also lead to the
issuance of an advisory for 2-4 inches in the counties immediately
adjacent to the warning...with lesser amounts expected further
north and west.

It is expected that the bulk of the snowfall will be over by late
afternoon as the system pulls off to the east. With wetbulbing
expected...high temperatures will struggle to move out of the low
to middle 30's tomorrow.

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Short term /Wednesday night through Friday/...
digging shortwave into the western Great Lakes will bring light snow
showers early Thursday...with a transition to lake enhanced snow
showers later Thursday afternoon and Thursday night. Models still
forecast 850mb temperatures to fall to near -10c by Thursday night and with a
decent lake fetch expect some light accumulations in the ridges and
I-80 corridor. Temperatures will be around 10 degrees below seasonal
average. Dry high pressure will move across the region Friday with
continued cold temperatures.

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Long term /Friday night through Tuesday/...
a stream of moisture from the Canadian rockies will keep clouds and
precipitation chances in through the weekend as northwest flow holds into
early sun. High pressure will slide south into the northern plains over
the latter part of the weekend...dislodging a weak Colorado low with
relatively low moisture into our area. While this means more light
precipitation...the good news is a transition to swerly flow and
overall warming into next week. Models have made remarkable progress
in consistency over the last 24hrs lending to higher confidence in
the extended trends.

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Aviation /23z Tuesday through Sunday/...
VFR conds are expd this evening...except for fkl where persistent MVFR
SC is trapped under a temperature inversion. Otherwise...experimental conds to
deteriorate to MVFR and eventually IFR for most ports near and east of
the Ohio/PA border as low pressure advns up the East Coast and spreads snow
into the region. The snow should exit lt Wednesday as the moves further
northeastward...though MVFR ceilings are expd to continue.

Outlook.../Wednesday night through Sunday/...
periodic rstrns are possible through the weekend with several fast moving
disturbances.

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Pbz watches/warnings/advisories...
Maryland...Winter Storm Warning from 4 am to 6 PM EST Wednesday for
mdz001.
Ohio...none.
PA...Winter Storm Warning from 4 am to 6 PM EST Wednesday for
paz074-076.
Winter Weather Advisory from 4 am to 6 PM EST Wednesday for
paz023-073-075.
WV...Winter Storm Warning from 4 am to 6 PM EST Wednesday for
wvz023-041.
Winter Weather Advisory from 4 am to 6 PM EST Wednesday for
wvz021-022.

&&

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