Area Forecast Discussion
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FXUS61 KPBZ 110854

Area Forecast Discussion
National Weather Service Pittsburgh PA
354 AM EST Sun Dec 11 2016

Snow will overspread the area through Sunday with many areas
changing to rain or a wintry mix by the evening. Much colder air
arrives late in the week.


Earlier virga on the regional radar mosaics has managed to moisten
the boundary layer sufficiently to start to fall to the surface as
snow across basically all of the CWA between I-80 and I-68 as of 3
AM. Wet bulb effects have overcome modest surface warm advection
as of this hour and been manifest as largely a degree or so of
surface temperature falls amongst the observations that have
started to snow in the past couple of hours. The multi-model
consensus indicates decent moisture transport should increase
through the next several hours above the boundary layer in the
285-295K layer with condensation pressure deficits crashing
through the floor largely CWA-wide and especially north of I-68.
As such, not only will snow develop, but it should become
progressively more efficient as the early morning progresses.

Snow to liquid ratios should quickly increase to 13-15 to 1 given
a very impressive depth of saturation in the dendritic growth
layer north of the advancing warm front. This should lead to
several hours with deep layer isentropic ascent and strong ice
crystal growth potential. While QPF from all models is fairly
modest, efficient growth should allow most areas in the CWA north
of I-70 to manage at least an inch through the morning. However,
the cutoff in precipitation should be pretty stark south of I-68,
so those areas may see little/no snowfall with the warm front.

As the warm front rides northward during the daytime hours today,
dry air in the boundary layer will accompany the northward
migration of the pre-frontal isentropic ascent. Thus, most areas
will simply stop snowing behind the warm front with a seemingly
lower chance of an immediate changeover to rain compared to what
the forecast implied this time yesterday. Once in the warm sector,
PoPs were thus reduced decidedly generally south of Pittsburgh
this afternoon as temperatures warm toward/above freezing in these

Warm frontal northward progression largely stalls across the
northern periphery of the CWA tonight, however warm air aloft
looks to be marginally sufficient to melt some/all hydrometeors
for a period across the northern zones. The mention of freezing
rain/sleet was thus maintained in the forecast. However, there is
some trend toward suggesting the surface itself may challenge or
rise above freezing toward Monday morning even there. Thus,
accumulation expectations were tempered during this time frame,
particularly over Armstrong and Indiana County.

As the cold front begins to sweep eastward tonight, much stronger
northward moisture transport moves over the area. This will bring
showers back into the warm sector of the system and allow for rain
and snow to again traverse the entirety of the CWA ahead of the
cold front. Most areas from Pittsburgh southward stand little/no
chance of accumulation due to warm boundary layer temperatures
even if snow does fall. A different problem arises for the ridges,

In the trapped valleys of the higher terrain, SE surface flow
looks to persist in the pre-cold frontal environment tonight. This
should hold surface cold air into especially Garrett County until
the cold frontal passage and a turn of surface flow to the SW by 4
AM. Until that time, incoming pre-frontal showers will likely
result in a period of freezing rain in Garrett County as well as
the ridges of Fayette and Westmoreland County. A freezing rain
advisory was thus issued for these areas for this. Even these
areas, however, will transition to mostly plain rain by Monday
morning as SW surface flow is efficient at eroding surface cold
air even in Garrett County.

Westerly flow follows the frontal passage on Monday with drier air
in the column likely to shut off showers everywhere except the
ridges quickly. Without much post-frontal cold advection and deep
layer westerly flow, lake effect showers should not affect our
area. Fries


The cold front will be well to our east Monday night. Any
remaining cold advection showers that exist will largely be
confined to the ridges and north of I-80. These shouldn`t linger,
however, as the post-front flow is westerly which is unfavorable
for lake-effect or upslope production. Mid and upper-level
moisture evacuates, leaving only saturation below about 900mb.
This could leave patchy drizzle in addition to whatever light
showers remain.

Tuesday will begin dry as high pressure briefly settles into the
region. By Tuesday evening, a relatively dry cold front will sweep
through, bringing only a chance of snow, mainly north of
Pittsburgh. This cold front will effectively open the door to the
a cold continental air mass to stream in from the west-northwest.
Snow showers will again ramp up downwind of the Great Lakes
aggregate moisture plume Wednesday into Wednesday night as the
primary upper trough axis sweeps through, bottoming out the mid-
level temperatures.

Temperatures will begin near averages, and then steadily fall
through mid-week to well-below averages.


The persistent upper trough and re-enforcement of cold air will
continue the potential for lake-effect snow showers and below
average temperatures through Friday. Thereafter, operational
model guidance and ensembles continue to show a digging trough
through the four corners region of the southwest CONUS. This will
aid in building heights across our region, shutting down the lake-
effect snow, and leading to a general warming trend. The digging
trough will then eject from the Rockies and impact our area over
next weekend. Utilized SuperBlend for the increase of likely PoPs
Saturday into Sunday. Will continue to carry a relatively vague
rain/snow mix through the warming transition, but will need to
monitor the potential for icing as this system nears.


Current VFR conditions will begin to deteriorate this morning as
widespread light snow commences along the warm front. Reduction in
visibilities and ceilings will spread northward, engulfing the area
by about 12z. The southern edge will quickly lift northward later
this morning, meaning southern ports of KMGW, and to a lesser extent
KLBE/KHLG will see improvement by mid morning. Along the north, the
snow will reside for most of the day, keeping low MVFR or IFR
restrictions in through the period. Between these areas, some
improvement from MVFR will be seen, but to what extent remains in

Cold front approaches late Sunday bringing at least a mix of rain
and snow. There is at least some potential for freezing rain along
the leading edge of the rain/snow mix, but confidence is too low for
TAF inclusion at this time. Best chances for freezing rain would be
for KDUJ/KFKL 03-09Z Monday.

Restrictions are likely to continue through Monday, returning again
Wednesday, with crossing areas of low pressure.


MD...Freezing Rain Advisory from 10 PM this evening to 4 AM EST
     Monday FOR MDZ001.
PA...Winter Weather Advisory until 4 AM EST Monday FOR PAZ007>009-
     Freezing Rain Advisory from 10 PM this evening to 4 AM EST
     Monday FOR PAZ074-076.


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