Area Forecast Discussion
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FXUS61 KPBZ 081033

Area Forecast Discussion
National Weather Service Pittsburgh PA
533 AM EST Thu Dec 8 2016

A trough of low pressure will result in a return to cold
temperatures and periodic snow showers through the end of the
week. More widespread precipitation is likely Sunday with
crossing low pressure.


530 AM...Little change needed at the present time. Clouds will
continue to lower this morning as cold advection kicks into gear.
Returns on radar remain mostly virga at this time. Temperatures
are in decent shape.

Previous discussion...

Mid-level clouds continue to stream across the area ahead of a
trough in the upper levels. Some virga is likely falling from some
of these, with the low levels remaining rather dry. The trough,
currently swinging across the Great Lakes, will cross later today.
Cold advection will be ongoing, picking up in intensity this
afternoon. Temperatures may reach their maximum values during
midday/early afternoon before dropping off thereafter. Snow shower
activity this afternoon should be mainly confined to areas north
of I-80, as low level W/WSW flow keeps most of the lake enhanced
activity to our north initially. Even here, the dendritic growth
zone is slow to saturate, so kept PoPs in the chance range for the
most part.

Low-level flow starts to take on a more northerly component
tonight behind a weak surface-based trough. Snow shower chances
pick up north of Pittsburgh, especially in the favored lake-effect
area north of I-80. Increasing inversion heights and moisture
point to better potential for accumulation, but the best setup
will wait until towards 12Z Friday and thereafter. An inch or two
of accumulation seems likely here. More scattered activity will
occur elsewhere, driven by continued cold advection. Some modest
upslope enhancement may allow for up to an inch or so of
accumulation along the ridges in spots, with lighter
accumulations elsewhere. Low temperatures will run some 5-10
degrees below climatology.


Ongoing snow showers north of I-80 Friday morning will spread
southward through the day as the wind shifts slightly behind the
passage of a surface trough. Boundary layer flow will veer to
northwesterly as moisture depth increases and lapse rates steepen.
For this reason, will keep the southward spread of likelies nearing
the Pittsburgh metro by Friday afternoon. All areas could see at
least some snow flakes, but areas south and west of Pittsburgh will
see little to no accumulation.

High pressure and backing boundary flow will quickly taper down snow
showers southwest to northeast Friday night into Saturday morning.
By daybreak Saturday, only the extreme northern edge of the forecast
area will remain near the downlake moisture plume. Will linger
chance PoPs in this area for now, but additional accumulation is not
expected to be significant during the day Saturday.

All told, total snow accumulation beginning tonight through Saturday
morning has changed very little from the previous forecast. With the
extended duration of the snowfall, will continue to hold off on any
winter weather headlines. The re-enforcing cold air will hold
temperatures 5-10 degrees below the averages.


Operational models continue to stream a clipper type
system through the Midwest, approaching our western fringes by
Sunday. Despite this weak disturbance dampening upon arrival as the
upper-level flow goes zonal, snow accumulation is possible Sunday

With significant guidance differences extending into next
week, the latter part of the long term forecast is offered with
lower confidence. Models continue to slide a deepening low out of
the northern Rockies early next week. Ensemble guidance fairly
consistent in the depiction of an anomalously deep upper-level
low tracking through the northern Plains. Their operational
brethren carry this low much further north. These differences
impact the track and intensity of a system arriving Monday. Opted
to utilize the Superblend, siding closer to the 00z ECMWF guidance
for this system.

As the bowling ball low pinwheels east across North America next
week, the door to much colder air swings open. The middle of next
week should feature temperatures that are well-below average.


VFR conditions will be in place at the very start of the TAF
period for most areas. However, MVFR stratocumulus will arrive
during the morning hours as colder air moves in from the
northwest. These will persist into the afternoon, perhaps lifting
to VFR again for a period, before MVFR is reestablished tonight
with an approaching weak surface trough. Any snow showers will be
confined to mainly north of Pittsburgh through this afternoon,
before becoming a bit more widespread tonight. Visibility
restrictions are expected to be confined to mainly north of I-80
through 12Z. Light southwest winds this morning will pick up out
of the west and gust to between 20 and 25 knots this afternoon and

Occasional, spotty restrictions in snow showers are expected to
continue into Saturday. More widespread IFR possibilities will
arrive with snow on Sunday.




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