Area Forecast Discussion
Issued by NWS -- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS61 KPBZ 082306
Area Forecast Discussion
National Weather Service Pittsburgh PA
606 PM EST Thu Dec 8 2016
Periodic snow showers are expected into the weekend, especially
north of Pittsburgh. All locations are likely to receive
accumulating snow on Sunday.
.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
Flurries / very light snow associated with the upper trough
transitioning over the region will taper off this evening.
850mb flow will veer northwest overnight on the backside of the
trough but without good wind alignment vertically, inversions
remaining below 5000 ft, and best lift outside of the dendritic
growth zone, confidence remains high that accumulations overnight
should not be much more than an inch or so north of Interstate
80 and lesser totals to the south.
.SHORT TERM /6 AM FRIDAY MORNING THROUGH SUNDAY/...
Surface winds finally take on a northerly component on Friday,
particularly during the late morning and early afternoon. This is
expected to be the period when snow accumulations would be the
highest. Continuing the trend of the previous forecast shift, only
minor changes were made to snowfall accumulations. While there
will be an extended period of snowfall, there is no period with
especially high snowfall expected. Although accumulations across
northern portions of Mercer, Venango, and Forest counties could
reach advisory levels of three inches per 12 hours on Friday, do
not think that county averages will be high enough to justify any
advisories. Perhaps the one exception might be in Forest county,
with the 18Z RAP suggesting that a dominant band may develop to
the east and move into the county by 15Z, a forecast that is also
suggested by the 12Z GFS. This will bear watching by future
shifts. With the arrival of today`s trough, an extended period of
below normal temperatures has begun, with temperatures about ten
degrees below normal.
As winds begin to develop a southerly component once again Friday
night, snowfall accumulations should begin to diminish. As high
pressure builds in from the southwest Saturday, coverage of snow
showers should really decrease, with any minor accumulations limited
to locations north of Interstate 80. The break in precipitation will
be short lived as more precipitation will lift across the region
Saturday night into Sunday. This is expected to bring snow to all
locations, although models are still differing in how far south
precipitation will extend. So far it appears that snowfall totals
would be highest in the north, with minimal snowfall accumulations
to the south where rain may mix in. Below normal temperatures
will continue through this period.
.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
12Z GFS/ECMWF are in relatively good agreement that a warm front
will lift across the region Sunday night into Monday, and until
the entire region is in the warm sector, a period of freezing rain
could be possible before snow changes over to rain. Did not have
enough confidence in this to include freezing rain in the forecast
at this time. The surge of warm air should bring temperatures back
towards normal on Monday. Model agreement drops after the cold
front moves through Monday night, and while models both show high
pressure developing in the Plains and a storm system moving up the
East Coast, the differences in where those systems track result in
little agreement on precipitation, and have stayed close to the
SuperBlend with chance pops through much of the rest of the
forecast. After Monday`s near normal temperatures, temperatures
will drop back below normal, with another reinforcing shot of cold
air likely to drop many locations into the single digits Wednesday
night and keep some locations in the teens on Thursday.
.AVIATION /23Z THURSDAY THROUGH TUESDAY/...
MVFR to low VFR stratocu ceilings are expected through Friday
under broad upper troughing and cold W-NW flow. MVFR to local IFR
snow showers are expected overnight and Friday as a weak surface
trough crosses. This trough should veer boundary layer winds to
the WNW, resulting in some lake enhancement and localized IFR
restrictions for ports N of I 80.
A tight pressure gradient should result in gusty west winds
through the period.
Restrictions are likely through early Saturday under upper
troughing, and again Sunday and Monday with crossing low pressure.