Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
420 PM EST Tue Jan 15 2019

Valid 00Z Wed Jan 16 2019 - 00Z Sat Jan 19 2019


...California/Pacific Northwest/Great Basin/Rockies...

Days 1-3...
Rounds of heavy snow are expected in the ranges of central to
northern California on days 1-2 with multi-day totals of 4-5 feet
possible in the CA Sierra Nevada mountains of CA.
Secondary maxima of 1-2 feet are forecast across the Wasatch
Mountains of UT and ranges of western CO.

The active weather pattern will continue through Thursday night
across the West Coast bringing multiple rounds of heavy snow,
particularly for the Sierra Nevada.
The initial trough crosses onshore and inland on day 1, a well
defined upper divergence maxima combining with upslope flow to
produce heavy snow in the CA Sierra Nevada mountains.

The probability of day2 heavy snow is high across the ranges of
central to northern CA as the upper trough approaches and moves
onshore, driving the upper jet into the CA mountains, with
sustained upper divergence and bands of 700 mb convergence aiding
upslope flow in producing heavy snow. The jet continues into
central NV with terrain aided maxima expected in windward portions
of the NV ranges, with several inches expected.

As the upper trough moves further inland from Nevada into Utah and
then CO, the upper level jet and 30 mb divergence maxima cross the
ranges of UT and CO, resulting in heavy snow there.  The highest
probabilities are in the ranges of western CO Fri due to the
longer duration snow.

Furthermore, surface high pressure shifting south over the
Canadian Prairies provides MT with upsloping southeasterly flow
with upper level divergence downstream from the northern upper
trough aiding in producing ascent. This brings moderate
probabilities for 4 inches to the ranges of western to central MT.
Moderate to high probabilities are also indicated where a higher
magnitude 300 mb divergence maxima crosses the ranges of ID to
western WY.

...Midwest/Northeast...
Days 2-3...

A mid level shortwave trough crosses the central Plains to the mid
MS Valley Wednesday night into early Thu and then up the OH valley
Thursday.  700 mb warm/moist advection produces a stripe of light
mixed QPF starting in KS and then extending across northern
MO/Iowa/northern to central IL and much of IN/OH. The northern
portions of this area should have light snow and the southern
portions have snow changes to sleet and freezing rain. 
Most models have light icing accumulations with low QPF while
soundings are favorable for freezing drizzle, which is a result of
moist low level conditions with dry air aloft in the snow crystal
growth region aloft.

On Thursday night/Fri, the mid level warm/moisture advection
progresses steadily downstream across the mid Atlantic and New
York and New England.  The low amplitude of the way and fast
motion of the wave keeps QPF and resultant snow showers limited in
intensity and duration, resulting in light amounts.

In the central Appalachians and adjacent valley areas of the mid
Atlantic, light icing is possible possible as the mid level wave
crosses as the initial bust of light precip is followed by drying
aloft with low level moisture and lift continuing, allowing for
light freezing drizzle to occur.  A wintry mix is likely over the
central Mid-Atlantic with potential for light snow to transition
to mixed sleet and freezing rain, keeping amounts of each precip
type low.

...Great Lakes...
Days 1-2...

A shortwave trough/low crosses Ontario through tonight and Quebec
Wednesday.  A trailing cold front crossing the upper Great Lakes
tonight results in an enhanced band of low level convergence with
the front and then followed by veering post frontal flow that
produces cross Lake Superior trajectories into the UP of MI, where
near shore areas should receive numerous snow showers and light
snow accumulations.  On Wed dry advection aloft reduces snow
coverage/intensity in the upper lakes, with southwest flow
crossing lakes Erie and Ontario helping to generate snow showers
on the eastern end of the lakes, in conjunction with the cold
frontal passage over the eastern Great Lakes Wednesday.  The cold
frontal passage is followed by drying aloft in the lower lakes Wed
night, with snow decreasing afterward.

The probability of a quarter inch of freezing rain is less than 10
percent days 1-3.

Petersen