Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 PM EST Sun Feb 24 2019

Valid 00Z Mon Feb 25 2019 - 00Z Thu Feb 28 2019


...Pacific Northwest into the Northern Rockies...
Days 1-3...
A longwave trough will drift slowly southward while retrograding
to the west, while at the same time shedding energy to the
northeast and pushing nearby shortwave impulses onshore. This
feature will move very little during the 3 days, and two distinct
surface lows will move onto the CA/OR coast through the middle of
the week.

On Day 1, confluent flow through a deep layer leads to an enhanced
jet stream, with the RRQ of a 100 kt jet maxima impinging on OR
and then into ID/MT/WY. Enhanced moisture and and synoptic ascent
through the jet, combined with upslope lift and modest id-level
frontogenesis will produce rounds of heavy snow from the Oregon
Cascades through the ranges of NW Wyoming, and WPC probabilities
are high for 12 inches, with 2 feet or more possible in the
highest terrain. The maximum snow Monday will likely be in the
Oregon Cascades as a surface low moves onshore near the OR/CA
border, but with snow levels falling across OR, the Willamette
Valley is also expected to see significant snow, with
accumulations also likely into the Columbia River Gorge, as well
as the city of Portland, where WPC probabilities are low for 4
inches.

Late Monday and through Tuesday, jet energy will drift south into
northern CA allows heavy snows to extend south into the northern
CA Sierra, with drier air advecting into OR bringing a reduction
in snowfall there. However, continued moist 700mb flow from the
W/SW and jet level diffluence will allow heavy snow to continue
into the mountains of ID and WY, with another 1 foot or more
possible in the Sawtooth, NW WY ranges, and Sierra. The synoptic
forcing will remain in place into Day 3 as the confluent flow
aloft angles into the California coast, with only subtle southern
shift in the main moisture plume. A second surface low will move
onshore beneath shortwave energy rounding the trough enhancing
snowfall once again. On Wednesday, the heaviest snow will likely
be in the favored upslope region of the Sierra where 2 or more
feet of accumulation is likely. Elsewhere, WPC probabilities are
high for 12 inches in the familiar ranges north of 40 N latitude
into ID and WY, with lesser amounts down to the snow levels of
around 3000-4000 feet.


...Great Lakes...
Days 1 and 3...
Intense low pressure moving through southern Canada will push
eastward into the Canadian Maritimes Monday. Behind this system,
very strong winds and cold advection will develop, producing a
favorable setup for Lake Effect snows south and east of the Lakes.
This late in the season, much of Lake Superior and all of Lake
Erie are frozen over, but any open water should be enough to
produce snow as lapse rates steepen on the CAA. Forecast profiles
depict a very favorable sounding, with an EL up above 800mb, and
strong nearly unidirectional shear from the W/NW. Increasing CAPE
over the waters and intense forcing into a low-level DGZ suggests
intense snow rates are likely as bands develop. However, the
anomalously strong wind speeds will limit residence times across
the lakes, somewhat limiting what could otherwise be a very
impressive LES event.

The heaviest snow is likely downwind of Lake Ontario where a
secondary connection from Lake Huron may offset some of the
reduction in potential due to the higher wind speeds. A prolonged
period of heavy snow is likely into the Tug Hill Plateau as a
single west to east band of snow persists through much of Monday.
As flow becomes more NW during the late aftn/eve, this band will
begin to weaken and drift south. Additionally, with the very
strong winds in place rising above 60 kts at 900mb, the bands are
likely to push well inland, and light accumulations are possible
into VT and MA. However, the heaviest snow is likely into the Tug
Hill where significant upslope enhancement is likely, and WPC
probabilities are over 50% for 8 inches. Downwind of the other
Lakes, much lighter accumulations are forecast with high
probabilities for 4 inches across the U.P. and NW L.P. of
Michigan, although isolated higher amounts are possible where
frictional convergence occurs near the lake shores. 1-2 inches is
possible downwind of Lake Erie.

On Wednesday, a strung out vorticity lobe ejecting from a west
coast trough will race across the Plains towards the Great Lakes.
This feature is accompanied by modest synoptic ascent within a
Pacific Jet max, and a few inches of snow are possible from
eastern MN through WI and into MI. Accumulations are expected to
remain light however, as the system is transient and 1000-500mb RH
saturates only briefly.


...New England and New York...
Day 1...
Upper trough and associated surface low will move just north of
New England through Monday, with warm advection giving way to cold
advection across the area. In the warm advection regime, southerly
winds will spread moisture in the form of snow across northern NH
and ME, with some light accumulations possible. More widespread
snow into the terrain of the Adirondacks and northern New England
will develop as CAA sets up behind the departing system, producing
upslope flow and snow in a saturating column. This will likely
produce additional light accumulations. The heaviest snow tonight
and Monday is expected across northern Maine where high
probabilities exist for 4 inches, with low probabilities for 8.
Small probabilities for 4 inches exist elsewhere in Northern New
England from the Adirondacks eastward across the terrain of
extreme northern VT and NH.


The probability of a quarter inch of freezing rain is less than 10
percent days 1-3.


Weiss