Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
344 AM EST Sat Jan 11 2020

Valid 12Z Sat Jan 11 2020 - 12Z Tue Jan 14 2020

...Pacific Northwest to the Northern Rockies...
A active period continues with very heavy snow accumulations
expected for the mountains of the Northwest and the northern
Rockies.

An occluded low will swing through western WA today, quickly
followed by another low tracking from Vancouver Island to western
WA tonight into Sunday. These two systems will combine to produce
2 to 5 inches of precip in the Cascades through Sunday night. Snow
levels drop from around 3000ft this morning to below 2000ft
tonight behind the first system. Two day snow totals look to be
from 1 to 5 ft in the WA and OR Cascades. Snow of one to three
feet are expected for the Blue Mtns in OR and the northern Rockies
of ID/MT and WY.

Snow levels crash Sunday night behind the second, supporting an
increasing threat for lowland light snow in Seattle and Portland
starting Sunday afternoon and continuing well into next week as
cold air grips the region under a deep low over southwest Canada
that directs the next low farther south into OR Monday.

The third system pushing into OR Monday has snow levels around
3000ft in southern OR/northern CA with Day 3 probabilities
moderate to high for a foot or more of snow for the OR Cascades,
Shasta/Trinity Mtns and northern Sierra Nevada.


...Southern Plains and Midwest...
Thick ice accumulations are expected in a stripe across the
Midwest with damaging ice accumulations expected over portions of
Lower Michigan. Heavy snow is expected to the northwest of the ice
from northern Illinois across southern Wisconsin and northern
Michigan.

Increasing southerly flow ahead of an amplifying southern stream
trough will continue to move moisture north of a slow moving
frontal boundary extending across the eastern Midwest. As low
pressure ejects northeast from the Mid-South states this morning
it takes on a classic mid-latitude cyclone appearance with an axis
of freezing rain just left of the track and accumulating snow
northwest of the ice in the comma head. Cold air will remain in
place across the Great Lakes with high pressure sliding northeast
and remaining across Ontario through Sunday. This will set the
stage for potentially damaging ice accumulations across portions
of southern Lower Michigan today into tonight. However, movement
of the cold air may keep a worst case scenario at bay as the ice
axis shifts a bit. Still, Day 1 WPC probabilities for greater than
half an inch are moderately high particularly along the I-69 and
I-94 corridor in southern MI, Meanwhile, as the mid-to-upper level
shortwave assumes a negative tilt, low-to-mid level frontogenesis
northwest of the low along with strong upper forcing will support
a swath of moderate to heavy snows from southern Wisconsin to
northern Lower Michigan through tonight with Day 1 probabilities
for exceeding one foot 10 to 20 percent from the eastern WI shore
of Lake Michigan to the northeast LP of MI shore of Lake Huron -
both areas are expected to have lake enhanced snow.

...Northern Northeast...
Significant ice accumulations are expected in a narrow stripe
across far northern NY State into VT and across central and
eastern Maine.

A cold front over eastern Canada will sag southeast through the St
Lawrence Valley into New England today, with high pressure over
Ontario extending east into Quebec. Moisture spreading north ahead
of the low in the Great Lakes and into the shallow low level cold
air north of the boundary will support freezing rain, with
accumulating ice expected in a narrow stripe beginning tonight. As
high pressure remains anchored over Ontario and Quebec, the low
over the Great Lakes will slide east across the Northeast, with
freezing rain continuing across portions of the region on Sunday.
A narrow stripe of 30 to 50 percent Day 1.5 probabilities for 0.5
inches of ice arc across the north side of the Adirondacks, and
across central/eastern Maine. Meanwhile deeper within the cold
air, a swath of heavy snow is expected to cross far northern Maine
where Day 1.5 probabilities for one foot are 20 to 30 percent.

Jackson