.DISCUSSION...
KEY MESSAGE 1: Heavy snowfall is expected today into Monday
over practically all of the Southeast half of Pennsylvania.
Confidence continues to increase in an area of low pressure
rapidly developing off the east coast today as a very dynamic upper
trough digs southeast from the Great Lakes region and amplifies
across the Central Appalachians and Mid Atlantic Coast. The
latest 00z Sunday guidance continues to come into good agreement
on the position and strength of the coastal low as it
strengthens/bombs out as its heads from the Outer Banks late
this morning NNE to about 125NM east of KSBY this evening.
GEFS Ensemble Plumes and their respective anomalies of various
parameters show a well-defined inverted trough over Central PA
today, extending NWWD from the aforementieond coastal low.
This trough is at the western edge of a highly anomalous East
to NErly LLJ (falling into the minus 4-5 sigma u-component of
the wind at 925 and 850 mb). Increasing upper level diffluence
ahead of the digging upper trough will support gradual
enhancement of snowfall rates across all of Central PA today
with snowfall rates within these quasi north/south bands of snow
exceeding 1 inch per hour (possibly over 2 inches at times in
heavy CSI bands across our far eastern CWA.
A complicating feature that will enhance and extend significant
impacts from this storm will be the increasingly strong NNE to
NNW wind - that will gust over 30 mph tonight (and likely over
40 mph) on Monday. This will lead to significant/extensive
blowing and drifting snow with localized near whiteout
conditions possible.
KEY MESSAGE 2: A high degree of uncertainty remains with
expected snowfall over the rest of northern PA.
Significant uncertainty still remains regarding snowfall over
the rest of North-Central, NW PA and the Middle Susq Valley
considering the likelihood for a quasi-stationary nearly
north/south band of heavy snow (lasting several hours or more)
along the axis of an inverted llvl trough and leading edge of
the lower/deeper DGZ that will be situated from the Scent Mtns
to somewhere between KUNV/KSEG and KIPT later Sunday
afternoon/Sunday night.
A couple of things to keep in mind regarding this snowfall
forecast:
* As the low quickly deepens off the east coast, expect areas of
enhanced low to mid level frontogenesis and UVVEL to develop
in the western periphery of the system. Much of this banding
will likely set up just east of the CWA, but could still
develop across parts of our central and eastern zones.
* Multiple models continue to show a snow band associated with
an area of low level convergence within an inverted trough
extending northwestward from the coastal low. While this type
of band is more likely to set up somewhere in central PA, it
will likely be narrow with a widely varying snowfall totals
on either side of the band.