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

Valid 12Z Sun Jan 06 2019 - 12Z Wed Jan 09 2019


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

Two-phase snow event likely to bring moderate accumulations to
portions of New York and New England this period.

The first portion of this event is slated for late on Day 2 into
Day 3, late Monday night and Tuesday. A warm front lifting
northeast from the Mid-Atlantic will be accompanied by strong WAA
to produce precipitation across the area. Initially, the air mass
is very dry in response to Canadian high pressure shifting east
across New England. However, as the high displaces eastward and
strong WAA commences, the column will saturate, and despite a warm
nose edging in from the SW expect wet-bulb effects to keep the
850mb temps less than 0C, supportive of snow. This front will lift
quickly northeast with dry air filtering in behind it, so duration
of snowfall will be limited. However, a quick burst of moderate
snow is likely, and WPC probabilities are high for 4 inches from
the Adirondacks of NY eastward into the Green and White mountains
and interior Maine.

After a brief respite from precipitation Tuesday, a stronger
surface low is progged to develop in the Gulf of Maine Tuesday
night into Wednesday. As this low deepens in response to height
falls and PVA from a northern stream impulse dropping southeast
through Canada, a narrow but intense deformation band will pivot
northwestward in conjunction with an inverted surface trough
extending into upstate NY. Rapid re-moistening of the 1000-500mb
RH will support snowfall re-developing across the northeast, with
TROWAL development late in the period suggesting the potential for
a burst of heavy snow across Maine. Guidance continues to shift
with the low placement which could enhance, or reduce due to
mixing, snowfall, but trends of the WPC super-ensemble mean
support an increasing likelihood for heavy snow. WPC probabilities
on D3 have increased to show a high risk for 4 inches across
Northern New England, with low chances now existing for 8 inches
in Maine.


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

An upper trough moving out of the northern Rockies Sunday night
will drive a weak surface low towards the Great Lakes 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
southerly low-level flow will feature orographic enhancement into
the Iron Ranges of the Minnesota Arrowhead. Despite a transient
system and limited duration of snowfall, WPC probabilities are
high for 8 inches in the Arrowhead D1 into D2, and 12 inches of
snow is possible. Elsewhere from extreme eastern ND through MN,
northern WI, and the northern portions of MI, WPC probabilities
are high for 4 inches of accumulation. The intense WAA will likely
produce a warm nose above 0C across central MN/WI/MI, and have
edged the southern edge of accumulating snow northward from
previous forecasts to account for this.

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 the greatest accretion
expected across north-central Wisconsin and across north central
portions of the L.P. of MI, where 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.

On Day 3, increasing W/NW flow behind a strengthening low pressure
in the Gulf of Maine will produce CAA and the potential for Lake
Effect snow downwind of all the Lakes. Moderate accumulations are
possible, and WPC probabilities for 4 inches are highest east of
Lake Erie and Ontario.


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

A deep trough moving inland from the coast this morning will push
a cold front into the Mountain West into Monday. This will be
followed by relatively zonal flow Monday before another deep
trough digs towards the West Coast on Tuesday night.

Significant moisture associated with a Pacific Jet maximum will
advect onshore during D1, and intense lift within the left exit
region of this jet combined with increasing 700mb flow will
produce heavy snow. The heaviest snow is likely in the Sierra
Nevada where low level winds will be orthogonal to the ranges to
produce efficient orographic enhancement. Day 1 WPC probabilities
feature a high risk for more than 12 inches above 6000 feet in the
Sierras, with 1-3 feet of accumulation possible through Monday. As
the longwave trough shifts eastward tonight and into D2, moisture
and associated heavy snow will spread into the rest of the
Mountain West, with heavy snow likely as far south as the Mogollon
Rim of AZ, and as far north as the Canadian Rockies. Snow levels
will fall to as low as 4000 feet in AZ/NM, and less than 2000 feet
from Utah and points north. Day 1 WPC probabilities are moderate
for 8 inches in the highest terrain of the Four Corners states,
with widespread high probabilities for 4 inches elsewhere.

After a quieter D2, another storm system will approach Tuesday/D3
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
WPC probabilities are high for 4 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.


Weiss