Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
332 PM EST Sun Jan 06 2019

Valid 00Z Mon Jan 07 2019 - 00Z Thu Jan 10 2019


...Northeast...
Days 2-3...

Successive disturbances will bring snow to northern New England
and NY state Monday night through Tuesday night. The warm conveyor
belt ahead of a deep trough spreads into the northeast Monday
night. This moisture plume will override air from a dry Canadian
high. The strong WAA will saturate the column, with freezing rain
and snow expected as a warm nose edges in from the SW This front
will lift quickly northeast with dry air filtering in behind it,
so duration of wintry precip will be limited. Moderate WPC
probabilities are for 4 inches from the Adirondacks of NY eastward
into the Green and White mountains and interior Maine exist for
Day 2. Farther southwest, two areas of moderate WPC probability
for a tenth inch ice exist over the Tug Hill area of NY and
north-central PA.

After a brief respite from precipitation Tuesday, a stronger
surface low is progged to develop on a reinforcing shortwave
trough over southern New England Tuesday night. As this low
deepens, a narrow but intense deformation band is expected to
pivot northwestward in conjunction with an inverted surface trough
extending into upstate NY. Rapid re-moistening of the 1000-500mb
RH will support snow re-developing across the northeast, with
TROWAL development into Wednesday suggesting a burst of heavy snow
across Maine. Guidance on the low placement has improved among the
12Z GFS/ECMWF/UKMET. Moderate WPC probabilities for 8 inches are
across higher elevations of northern NY, VT, NH, and across
interior ME.


...Upper Midwest and the Great Lakes...
Days 1-3...

An upper trough crossing the Great Plains tonight will drive a
weak surface low from SD to northern WI through Monday. Ahead of
this feature, robust WAA on an anomalously strong LLJ (up to +5
standard deviations above the climo mean) in an extremely moist
airmass (PWATs of up to +4 standard deviations above the climo
mean) will drive moisture and ascent into a marginally cold air
mass. A period of intense lift is likely in response to this WAA
in conjunction with height falls and PVA which supports snowfall
rates of more than 1"/hr. The heaviest snowfall is likely along
the North Shore of Lake Superior where southeasterly low-level
flow will feature orographic enhancement in the Minnesota
Arrowhead. Despite a transient system and limited duration of
snowfall, Day 1 WPC probabilities are high for 8 inches in the
Arrowhead and moderate for 12 inches. WPC probabilities are
moderate for 8 inches over the UP into northern WI with a tight
gradient on the south side. This is because the intense WAA will
produce a warm nose above 0C across central MN/WI/MI. As a result
the southern edge of accumulating snow was constricted northward
again from the previous forecast.

South of the the heaviest snow, the aforementioned warm nose will
climb above 0C while surface temps hover below freezing. This will
create a period of freezing rain, with notable risks for freezing
rain over north-central Wisconsin and across north-central
portions of the L.P. of MI. Day 1 WPC probabilities are moderately
high for more than 0.1 inches over MI, and significant accretion
of 0.25 inches is possible. Heavy precipitation rates and a lack
of dry advection to persist wet-bulb affects should preclude more
widespread heavy freezing rain accretion.

Shortwave trough activity south of the parent low in northern
Ontario brings snow (that will be lake enhanced on the lee sides
of the Lakes) crossing the Great Lakes during the day Tuesday.
Patchy low risk areas of 2 inches are in Day 2 with Lake Effect
snow warranted low risks for 4 inches in typical snow belts in the
UP, upper LP, and especially off Lakes Erie and Ontario on Day 3.


...Western U.S....
Days 1-3...

A deep trough will continue moving east, reaching the Rockies
tonight. A ridge will quiet weather across the west Monday into
Tuesday before a deep low approaches the West Coast on Tuesday
night.

The majority of heavy snow out west tonight is associated with a
surface low on a secondary lobe of the trough coming ashore in CA.
Additional snow over the Sierra Nevada around 18 inches is likely.

After a quieter Day 2 under a trough, another storm system will
approach Tuesday accompanied by a deep 500mb trough. Moisture will
again spread into the Pacific Northwest on warm and moist
advection, and snow levels will rise to 6000-7000 feet in
California, but remain below 3000 feet in the Washington Cascades.
Heavy snow is likely, and Day 3 WPC probabilities are moderate to
high for 8 inches in these ranges.

Additionally, there is an increasing threat for freezing rain
across Washington and Oregon on Tuesday. Cold surface temperatures
will be reinforced by easterly winds banking against the Cascades,
which will support freezing rain as WAA aloft produces widespread
precipitation. The current model suite is in good agreement for
significant accretion in the lee of the Cascades, and WPC
probabilities are moderate for 0.1 inches of accretion for many
areas, including the Columbia River Gorge on both Days 2 and 3.


Jackson