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

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


...WA/OR to the Northern Rockies...
On Sunday night and Monday low-amplitude shortwaves embedded with
fast northwest flow are expected to increase layer relative
humidity with corresponding ascent, resulting in several inches of
snow across the higher peaks of the Wa/Or Cascades. Generally
lighter amounts are expected farther east over the Blue Mountains
and northern Rockies, due to limited duration of combined moisture
and lift.

On Monday night and Tuesday, the departure of the initial
mid/upper trough leads to a respite until the next east Pacific
system develops and moves towards the OR/CA border Tue afternoon
and night and inland Wed.  Snow will redevelop in the OR cascades,
with potential for mid level deformation and upper divergence to
produce heavy snow in western OR as the storm approaches and then
the higher elevations further inland in eastern OR to southern ID
Wed. There are significant QPF and snowfall accumulation
differences to be resolved.  There is potential for a couple feet
of snow in the OR Cascades with several inches in areas in south
central OR where heavy snow is uncommon. 

On Wed., snow redevelops in the northwest MT Rockies as moisture
wrapping around the cyclonic circulation in the northwest impinges
on the mountains, with sporadic 700 mb ascent occurring.  This
lift combines with windward upslope flow to produce potential for
several inches of snow.  The SREF and GFS/ECMWF indicate snow
focusing on the Lewis range on Wed, including parts of Glacier
National Park.  

...California into the Great Basin...
The models are increasingly in agreement in showing a rapidly
deepening cyclone dropping southeast toward the Oregon/California
border on Tuesday night and moving inland near the CA/NV border
with OR/ID.  Beginning late Tuesday and continuing into early
Wednesday, powerful onshore flow will support widespread moderate
to heavy precipitation across northern and then central
California.  Widespread heavy snowfall accumulations are expected
across the coastal ranges into the CA Cascades, Shasta/Siskiyou,
and the Sierra Nevada mountains.  Widespread snowfall
accumulations of a couple of feet are expected, with locally
heavier amounts across these areas.  Lighter accumulations will
extend farther east into the ranges of Nevada.

...Central Rockies to the High Plains...
As the progressive shortwave dives southeast across the Great
Basin, expect a period of snows across the Uintas and northern
Wasatch Monday morning.  As the wave moves further east southeast,
snows will develop farther east across northern Colorado,
southeastern Wyoming and western Nebraska.  As low pressure
develops east of the Rockies, increasing upslope flow north of the
low, along with low-to-mid level frontogenesis, is expected to
support heavy snowfall accumulations from the Laramie and Medicine
Bow Mountains and the Front Range of northern CO and southeast WY
eastward into High Plains, extending into the southwestern
sections of the Nebraska Panhandle Tuesday as low pressure moves
east out of CO into the central Plains.  There is potential for
amounts locally as high as a foot.

...Central Plains to the Upper Great Lakes...
Models show the upper system continuing east northeast with the
700 mb low tracking from the CO Rockies across the central Plains,
lower Missouri valley late Tuesday, and then the Great Lakes
Wednesday .  In addition to the strong upper forcing, low-to-mid
level frontogenesis will likely support a stripe of moderate to
locally heavy snows northwest of the sfc low center.  The general
consensus of the overnight models places the axis of heavier
amounts from northeastern Colorado and southeast Wyoming across
Nebraska, northwest IA, southeast SD, southern MN, northern WI,
and the U.P. of Michigan.  Along the sfc low track, temps are
likely to be too warm for significant snow accumulations in
eastern KS, northern MO, and eastern IA if the ECMWF, GFS, and NAM
forecasts are correct.  

...Northeast...
A band of wet snow is forecast to fall in interior Maine northwest
of a deepening surface low tracking across the Gulf of Maine early
tonight and ending as the low moves up into Canadian Maritimes
Monday. Low-to-mid level frontogenesis in combination with a well
defined 300 mb divergence maxima is expected to support enhanced
precipitation rates and the threat for locally heavier snowfall
accumulations from northern New Hampshire across northern Maine. 
The threat subsides and ends Mon as the low level
frontogenesis/upper divergence move north into Canada and out of
northern New England.

For Days 1-3, the probability of significant icing (0.25-inch) is
less than 10 percent.

Petersen