Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
301 PM EST Fri Feb 08 2019

Valid 00Z Sat Feb 09 2019 - 00Z Tue Feb 12 2019


...Pacific Northwest/California/Range of Great Basin and northern
to central Rockies/northern Plains...

A pair of systems impacting the western U.S. are expected to
produce widespread snows across the Pacific Northwest and Northern
California, including lowland snows over western Washington and
Oregon as well as the North Coast of California.

An anomalously deep upper low is forecast develop over southern
British Columbia and drop south along the Pacific Northwest coast
tonight into Saturday,a and then near the northwest coast of CA.
The colder than normal temperatures aloft will support significant
snow accumulations for portions of western Washington, including
the lowland areas like Seattle-Tacoma. Winter Storm Warnings are
in effect for 6-8 inches of snow in the Seattle area.

As the low continues to drop south, snows are expected to extend
south along the coastal ranges along the Northwest Coast of
California on Saturday, with snow levels dropping by Sunday
morning.  Two-day totals have potential to produce three to four
feet of snow for portions of the Sierra Nevada.

Further inland, the trough progresses across ID and NV on day 2,
producing several inches of snow then, followed by the ranges of
UT and western CO the end of day 2 into day 3. 
The steady forward progression of the trough and front leads to a
6-10 hour window of enhanced moisture and lift, with the short
duration limiting heavy snow potential.

On day 3 (Mon), as the low-mid level trough crosses the northern
Plains, snow breaks out in North Dakota, with a few inches
expected, as warm advection and pre-frontal convergence produces
lift.

...Great Lakes/Upper MS Valley...

Westerly flow persists across the Great Lakes in the wake of the
low departing this morning through tonight which will maintain
lake effect snow, with the heaviest southeast of Lake Ontario,
starting in the Tug Hill, and then drifting south along the
southeast shoreline of Lake Ontario as boundary layer winds
gradually veer to the west northwest late tonight and Saturday. An
additional 6 to 10 inches is possible in the bands f heavy snow
tonight into early Sat.  The lake effect snow winds down as a low
level ridge crosses and drier air advects across the region.

Light snow develops in portions of the mid-upper MS Valley Sun and
spreads to the upper Great Lakes as confluent flow leads to a 700
mb jet maxima, with associated warm/moisture advection and
convergence leading to snow developing and moving southwest to
northeast across the region.

...Northeast TX/ARKLATEX...
The strong high over the central US has driven low level
subfreezing cold air across northeast TX.  An upper level jet
maxima from the central plains and MS Valley support ascent in the
right entrance region over northeast TX into the ARKLAREX.  Moist
southwest flow aloft allows precip to break out, but warm air
aloft leads to a likely mix of precip types, including sleet and
freezing rain. The NAM and a few high res models forecast a tenth
to two tenths of an inch liquid equivalent in the form of freezing
rain Saturday.

...Central and Southern Appalachians and mid Atlantic States...

Canadian high pressure shifts off Cape Hatteras Sunday with return
flow on the back side of the high and ahead of an approaching
trough over the central Great Plains will allow for mid level warm
advection-induced precip. Enough cold air is expected to be dammed
that pockets of freezing rain around a tenth inch can be expected
on the eastern slopes of the Appalachians on Day 3. Moderate Day 3
WPC probabilities for a tenth inch of ice are over western VA.
North of the area of ice, a band of low level frontogenesis
extends across southern PA and MD on Mon. Light snow may develop
in northern WV/MD/southern PA/northern DE/southern NJ, with a few
inches possible.

Petersen