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Area forecast discussion 
National Weather Service Pittsburgh PA
231 am EST Thursday Jan 29 2015

Synopsis...
crossing low pressure will bring a wintry mix of precipitation to
the upper Ohio region today and snow early on Friday.

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Near term /through today/...
models are coming into good agreement with timing of
precipitation ahead of front and the strong warm air advection as well. The key
is what will the surface temperatures do. Models are indicating a slow
response to the warm air advection at the surface but I have my doubts that it
will be fast enough to prevent some icing at the beginning of the
event. Because of this...have extended the Freezing Rain Advisory
to cover most of my Ohio zones. There are a lot of factors at
play here which makes this a very difficult forecast. First aloft.
All the models are showing a strong surge of warmer air enveloping
my entire County Warning Area today. This shows up quiet well in the thermal
pattern at 850mb and in the low and middle-level thickness patterns.
I have decided to change very little from previous thinking on
precipitation types. The thing to watch will be how the atmosphere
responds once the precipitation begins and the wet bulbing effects come
into play. Model soundings show a profile very close to the 0
degree line which will keep the difference between liquid or
frozen very close. At the surface. Models do show surface temperatures
rising quickly as the precipitation approaches. However...my concern is
that with such cold dewpoints at the surface...wet bulbing will
allow for a period of freezing rain...at least in Ohio at the
onset of the event. Other factors that need to be considered are
the widespread snow pack and localized effects...mainly sheltered
areas where temperatures will take longer to recover.

Main area of precipitation will exit this afternoon as the surface
front swings into central PA. The onset of cold air advection will be right
behind the exiting front.

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Short term /tonight through Saturday/...
thermal profiles quickly become more unstable tonight. Cold
advection begins in earnest aloft as two short wave troughs are
set to cross the area through Friday morning. This will occur
simultaneously to the drier air coming in...however the dendritic
growth layer looks to crash toward the surface tonight in the
ridges...well before the moisture peels off. Consensus snow to
liquid ratios from the NAM and GFS via BUFKIT suggest around 25 to
1 begin overnight. Given instability and unidirectional
upslope...this should result in very efficient snow growth. Model
quantitative precipitation forecast is generally only a couple of tenths...and given ice cover
upstream on Lake Erie...a ton of quantitative precipitation forecast is very unlikely.
However...even light quantitative precipitation forecast in this case is likely to add up to
several inches of snow. Given that ahead of the system on Thursday
there could be an inch or so in some of the sheltered higher
elevations...and another 2 to 5 seem likely in the upslope through
Friday with additional synoptic help via two additional waves
crossing the area...3 to 6 inch totals seem reasonable in the
ridges by Friday morning. Winter weather advisories were expanded
to accommodate this.

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Long term /Saturday night through Wednesday/...
high levels of uncertainty in the long term particularly with the
system that could affect the area during the Sunday/Monday time
frame. For now...consistency will be the name of the game until
the models provide a better picture. Cold pattern will continue
through the remainder of the long term.

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Aviation /08z Thursday through Monday/...
VFR overnight through sunrise with thickening middle/hi clouds.
Precipitation returns after 14z with low pressure tracking to the
north. Expect a drop to MVFR soon after precipitation onset...with IFR
ceilings to follow at most terminals during the afternoon and evening.
Precipitation types are the main difficulty. Still think best chances of
freezing rain will lie in Ohio and at fkl/duj during the first few hours of
precipitation. Cannot rule out at least a brief period of freezing rain
anywhere save perhaps mgw. Changover to snow will occur after 00z
as a cold front sweeps through. Winds will strengthen out of the
south around midday and remain gusty as they veer towards SW/west
with time. Will continue to monitor for low level wind shear possibilities. Cl

Outlook.../Thursday night through Monday/...
rstrns are likely through Erly Friday as a cold front exits. Wdsrpd rstrns
are likely again with advancing low pressure sun and Monday.

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Pbz watches/warnings/advisories...
Maryland...Winter Weather Advisory from 11 am this morning to 10 am EST
Friday for mdz001.
Ohio...Freezing Rain Advisory from 8 am this morning to noon EST
today for ohz039>041-048>050-057>059.
PA...Winter Weather Advisory from 11 am this morning to 10 am EST
Friday for paz007>009-015-016-023-074-076.
WV...Winter Weather Advisory from 7 PM this evening to 10 am EST
Friday for wvz023-041.

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