Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 AM EST Fri Mar 01 2019

Valid 12Z Fri Mar 01 2019 - 12Z Mon Mar 04 2019

...Northeast...
Days 1-3...

Surface low moving up the coast from the Tidewater of VA area will
lift northeast to near the Benchmark Saturday evening. This occurs
in conjunction with an upper level jet streak lifting out to the
northeast ahead of the system, while a kicker shortwave races
eastward across the Great Lakes. The guidance has trended further
NW overnight, likely due to better interaction with the kicker
shortwave, and also stronger with the surface low in response to
better height falls and jet diffluence. This setup will spread
snowfall from eastern PA through southern New England and into
coastal Maine during Saturday. As the surface low deepens a
deformation band will develop NW of the mid-level center which may
track along or just inland the SNE coast. This will enhance
snowfall, and WPC probabilities have increased to more than 50%
for 4 inches from NE CT and points east towards Boston, with low
probabilities for 8 inches just SW of Boston. Some low
probabilities for 4 inches also exist along the immediate coast of
Maine, but this will be highly dependent on the exact track of the
surface low, and how much interaction occurs with the kicker
shortwave moving in from the west. The GFS remains a strong and NW
outlier, but the trend in the ECMWF/CMC/UKMET has been for a
further NW and slightly stronger solution, so isolated heavier
snowfall in a banded structure is possible anywhere NW of the
surface track.

A potentially more significant system will move up the coast
quickly behind the first, bringing renewed snow to the northeast.
Much uncertainty still revolves around this second system. While
it is likely to be a more potent low, the track will be heavily
dependent on the first system from Saturday, which will impact the
position of the strongest baroclinic gradient. A stronger system
Day 2 would likely produce a further SE track on Day 3, and the
guidance has begun to trend that way overnight. While confidence
is below average, current WPC probabilities suggest a better than
50% chance for 6 inches from the mountains of WV through central
PA and into interior New England. Lower amounts are likely NW of
this line, with some freezing rain south of this line keeping snow
amounts lower near the coast as well.


...Central Plains to Ohio Valley...
Days 2-3...

A shortwave ejecting eastward from CA will ride a 120+kt pacific
Jet across the Four Corners Saturday night and into the Central
Plains on Sunday. At the same time, a cold front will be dropping
through the Plains producing a tightening baroclinic gradient.
This baroclinic gradient will be acted upon by the aforementioned
upper forcing and an increasingly coupled jet structure to produce
surface cyclogenesis which will begin late Saturday night over the
TX Panhandle.

Mid-level confluence will drive Pacific Moisture into the region,
which will be wrung out as precipitation in response to increasing
synoptic lift, aided by isentropic ascent on low-level warm
advection and intensifying mid-level frontogenesis. Although the
mid-level impulse is expected to traverse quickly to the east with
minimal surface reflection until Sunday evening in the TN VLY,
cold temperatures promoting high SLRs and a deep DGZ with ample
moisture suggests a band of heavy snow will spread west to east
from eastern CO through KS/MO and up through the OH Valley Sunday.
WPC probabilities are high for 4 inches, with a moderate risk for
8 inches across eastern KS into MO.

On the southern fringe of this snow swath, isentropic lift atop
the cold surface high will likely produce freezing rain. Day 3
probabilities are low for a tenth inch from OK and across southern
MO.


...California across Great Basin to Central Rockies...
Days 1-3...

A filling mid-level low will move onshore Oregon this afternoon,
with a secondary shortwave and associated surface reflection
lifting onto the California coast Saturday. Along and south of
these features, Pacific Jet energy will drive significant moisture
onto the coast, before spilling over into the Mountain West,
especially on Day 2. Snow levels will remain fairly high through
the period, 4000-7000 feet south of 40N latitude, but heavy snow
is expected above these levels.

On day 1, heavy QPF in response to shortwave energy and modest
synoptic ascent within the diffluent region of an upper jet will
produce heavy snow, mostly in the Wasatch of Utah and the Rockies
of Colorado. Here, WPC probabilities are moderate for 8 inches of
snowfall. By Day 2 /Saturday/ a more powerful Pacific Jet
associated with the secondary shortwave will spread copious
moisture inland from the Sierra Nevada across NV/UT/CO with snow
elevations 5000 to 7000ft north to south across the zone. The
fairly tight moisture corridor along with jet diffluence will
spread heavy snow across this area, with impressive two day totals
featuring high probabilities for 18 inches in the southern Sierra
Nevada and for the central Rockies. Lighter accumulations are
expected in the lower mountains along this swath with moderate to
high probabilities for 6 inches. By the end of the period a brief
respite in snowfall is likely as ridging blossoms from the
Southwest.


...Northern Plains to the Great Lakes...
Days 1-2...

A piece of energy ejecting from a closed upper low off the OR
coast will advect eastward as a shortwave tailed by a modest jet
streak through the Northern Plains and into the Great Lakes by
Saturday morning. This feature will be somewhat moisture starved
and transient as it gets embedded in the fast northern stream, but
ascent due to height falls and weak upper diffluence will be
enough to produce a swath of light to moderate snow from the
eastern Dakotas through MN/WI and into northern MI. Only a few
inches of snowfall is expected, and WPC probabilities are less
than 20 percent for 4 inches, highest across portions of WI.


Weiss