Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
338 PM EST Fri Jan 18 2019

Valid 00Z Sat Jan 19 2019 - 00Z Tue Jan 22 2019


...Great Lakes/Mid-Upper Mississippi valley/Ohio Valley to the
northern Mid Atlantic and Northeast...

...Day 1...
Recent radar imagery indicates a band of moderate to locally heavy
snow occurring in conjunction with a low-mid level front is
forecast to cross from Iowa across southern WI/northern IL and
then southern lower MO and adjacent northern IN.   Several inches
of snow are expected in this corridor.  Locally higher amounts are
expected along the Lake Michigan shoreline with enhancement due to
cross lake flow picking up additional moisture.

The models continue to show vertical motions pickling up within a
developing deformation across northern Missouri this afternoon and
evening.  The mid level developing frontogenesis and and
associated deformation band north of the surface low track are
forecast to produce snows downstream across the northern Ohio
Valley into Saturday.
The precip type transition zone across central MO through southern
IL/IN/OH/northern WV is expected to see warm advection change
precip over from snow to sleet and freezing rain.
The duration of each precip type is the primary mode of
uncertainty in these areas into southwest PA/northern WV.

...Day 2/3...

On Saturday night-Sunday, the low moves towards the mid Atlantic
coast and then turns north to just off the New England coast.
Between the low and high over Canada, powerful low-mid level
frontogenesis and deformation cross from Ohio across PA/NJ/New
York/New England.  The models continue to show heavy snows across
interior New York and central to northern New England, with
potential for a continuous area of a foot and mesoscale bands of
18-24 inches. WPC Day 2 probabilities show a High Risk (greater
than 70 percent chance) for amounts exceeding 8-inches from
northeast Ohio to New Hampshire and Maine.
With the 700 mb low forming over ME on day 3, additional snow
accumulations are possible in northern ME before the lift departs
into Canada.

An icy transition zone is expected further to the south, with
guidance continuing to signal some potential for significant ice
accumulations along the southern periphery of the heavier snow
axis from the Ohio valley to southern PA/northern NJ, southeast
NY/southern New England on Day 2. 
The NAM forecast sounding show an impressive bank of low level
cold air, with forecast soundings in this band indicating a layer
of -4 to -6c cold air, likely resulting in an extended period of
sleet. If correct, this would lead to lighter freezing rain
amounts than depicted by the NAM, and more sleet instead.

...California, Pacific Northwest, Great Basin, Central to Northern
Rockies...

...Days 1/2...

Low-mid Warm and moisture advection/convergence ahead of a wave
tracking northeast across the eastern Pacific toward British
Columbia is expected to produce some locally heavy snow
accumulations with locally up to a foot across portions of the
northern WA Cascades and Blue Mountains of northeast Oregon
tonight into early Saturday.  Locally heavy amounts are expected
to extend further to the east into the central Idaho and western
Wyoming ranges along a low to mid level frontal band and 700 mb
convergence bands dropping southeast across the region Saturday
into early Sunday.  

On Sunday low pressure over the northeast Pacific moving northeast
towards OR
produces the next wave of enhanced 700 mb convergence and
warm/moist advection Sunday that extends along the OR Cascades
south into the ranges of northern CA, leading to potential for
locally heavy snow in parts of the CA Sierra Nevada range.

...Day 3...
The amplifying and east motion of the upper trough and mid level
frontogenesis across NV and UT/western WY leads to snow spreading
across the ranges of NV/UT/western WY and then into western CO by
Monday afternoon.  Locally heavy snows depart the Sierra Nevada
range and across the aforementioned ranges, with probabilities
favoring the UT Wasatch/WY Tetons as strong 700 vertical velocity
and layer average 90 percent relative humidity accompany the
ascent. 

The probability of a quarter inch of freezing rain is less than 10
percent Day 3.

Petersen