Scientific Forecaster Discussion
fxus61 kpbz 110553
Area forecast discussion
National Weather Service Pittsburgh PA
1242 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.
Near term /through today/...
widespread virga is quickly encompassing the area early this
morning as evidenced by the regional radar mosaic. Model soundings
indicate a fair amount of dry air below 5 kft will need to be
overcome, however the multi-model consensus indicates decent
moisture transport above this layer in the 285-295k layer with
condensation pressure deficits crashing through the floor. As
such, snow should quickly start to develop in Ohio and move
eastward early this morning.
Once the layer below 5 kft becomes saturated, 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 quantitative precipitation forecast from all models is fairly modest, efficient
growth should allow most areas in the County Warning Area north of I-70 to manage
at least an inch by daybreak. However, the cutoff in precipitation
should be pretty start should 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
Short term /tonight through Tuesday/...
highest snow totals were placed across the northern counties
where best lift and moisture will reside into Sunday evening and
the Winter Weather Advisory will continue. As surface low shifts
north expect enough warming aloft to change precip over to rain
across the south, with a period of freezing rain possible north of
Pittsburgh and in the ridges. With less snow expected across the
southern ridges, no headline was issued at this time, with any
freezing precip not expected until Sunday night.
Still expect all precip to mix with or change to plain rain before
sunrise Monday, as southwest flow ahead of an approaching cold
front warms the column. Temperatures will then level off Monday as
the front sweeps east with rain and or snow showers ending.
An Arctic front will then be approaching Tuesday with low chance
pops for snow showers ahead of the front.
Long term /Tuesday night through Saturday/...
upper troughing and cold west-west-southwest flow should persist across the
eastern Continental U.S. Through Friday maintaining the chance for lake
enhanced and upslope snow showers. Some accumulation is likely with
a 13+ degree temperature difference between the lake and 850hpa.
With uncertainty in band placement, have just upped pops for the
likely locations through Saturday morning. A less amplified
pattern sets up for the weekend through early next week, with
several crossing shortwaves progged. Differences in the evolution
of these systems prompted the use of the superblend for much of
this timeframe. Much below average temperatures are expected to
moderate by early next week.
Aviation /06z Sunday through Thursday/...
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.
PA...Winter Weather Advisory until 4 am EST Monday for paz007>009-