Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
335 AM EST Mon Jan 07 2019

Valid 12Z Mon Jan 07 2019 - 12Z Thu Jan 10 2019


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

Two phase event will bring heavy snow to northern and central New
England and NY State late Monday night, and again on Wednesday.
The first phase will be a warm advection snow associated with a
warm front lifting northeast from the Mid-Atlantic into New
England. Initially the column is quite cold and dry, but strong
WAA will eventually saturate the column producing snow, sleet, and
freezing rain as the warm nose edges into the region.
This front will lift quickly northeast with dry air filtering in
behind it, so duration of wintry precip will be limited. However,
a brief thump of snow is possible as ascent within the WAA regime
is impressive, so moderate WPC probabilities exist for 4 inches
from the Adirondacks of NY eastward into the Green and White
mountains and interior Maine into Tuesday. 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. The most recent guidance suite has shifted the
primary low a bit north overnight suggesting some warmer air,
especially across southern New England, but intense isallobaric
wind into the low should allow the cold air to collapse back into
the precip producing heavy snow across northern and central New
England. Additionally, NW flow behind the departing system later
on Wednesday will allow for some upslope enhancement into the
terrain as well. 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 MN into southeast Canada into Tuesday. 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 and the eastern portion of the
U.P. of Michigan. Despite a transient system and limited duration
of snowfall, Day 1 WPC probabilities are high for 8 inches with a
tight gradient on the south side. This is because the intense WAA
will produce a warm nose above 0C across central 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 portions of the L.P. of MI and into
western PA. In these areas, day 1 WPC probabilities are moderately
high for more than 0.1 inches, 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 4 inches exist for Tuesday and Wednesday
for Lake Effect snow in typical snow belts in the U.P., upper
L.P., and especially off Lakes Erie and Ontario on Day 3.


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

A broad trough will move east across the Mountain West today, as a
longwave ridge builds across the Pacific Coast. Jet level
diffluence on the left exit region of a Pacific Jet will combine
with moist 700mb flow to produce moderate snowfall in the terrain
from the Northern Rockies southward into the Wasatch of Utah and
Rockies of Colorado. WPC probabilities are moderate for 4 inches
in these ranges.

A significant low pressure across the Pacific will slowly move
eastward towards the coast Tuesday into Wednesday with deep
southerly flow ahead of this feature advecting warm and moist air
into the West. Although moisture will be significant and ascent
will increase within diffluent mid and upper level forcing, snow
levels will climb to 6-7 kft in the Sierras, and 4-5 kft in the
Cascades. While heavy snow is likely, and WPC probabilities for 8
inches are high D2/D3, these are confined to the highest terrain
of 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 have risen to above 50% for 0.1 inches of accretion
for many areas, including the Columbia River Gorge Tuesday into
Wednesday.


Weiss