Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
416 AM EST Wed Jan 08 2020

Valid 12Z Wed Jan 08 2020 - 12Z Sat Jan 11 2020


Days 1-3...

...Western U.S....
An active pattern is expected to continue across the West as a
series of shortwave troughs move across the region.  The first
shortwave and associated frontal boundary are moving onshore this
morning and will progress eastward into the northern/central
Rockies tonight. A weakening cold front will translate beneath
this trough, which will drop snow levels to the surface east of
the Cascades tonight, and as low as 1000-2000 ft elsewhere from
Oregon eastward to Wyoming. The lowering snow levels, continued
onshore moist advection, and LFQ jet level diffluence will induce
snowfall across the region, with more than 8 inches of
accumulation likely in the Cascades, Blues, Northern Rockies,
Sawtooth, and ranges of NW WY, as well as northern CA. The highest
snowfall may exceed 18 inches in the Oregon Cascades and Tetons of
WY. Elsewhere, widespread probabilities for 4 inches exist in most
of the terrain of the northwest.

This first impulse will continue to dig southeastward on D2, while
weakening and outrunning the jet level aid to ascent. However,
another shortwave and weak surface low is progged to race
southward along the CA coast, lowering snow levels and creating at
least modest forcing. Snow accumulations are likely across most of
the West on Thursday as far east as the Wind River range of WY,
and as far south as the Mogollon Rim and Sacramento Mountains. WPC
probabilities for 6 inches are high only in the Tetons however.

The next in this train of shortwaves is expected to dig into the
Pacific Northwest late Friday and Friday night. This feature will
be accompanied by another surface low and frontal boundary, moist
advection, and intense mid-level ascent. Snow levels will remain
low as persistent cold air re-energizes across the West, and snow
levels Friday are forecast to fall below 2000ft everywhere west of
105W longitude except the OR and CA coasts. The most robust
forcing on D3 will be confined to WA/OR/ID, and WPC probabilities
for 12 inches are high for the Olympics and Cascades, while only
slightly lower into the Blues and Northern Rockies. Lighter snows
will extend further south, but most of the snowfall into those
areas will occur beyond D3.


...Northeast...
A strong cold front associated with an amplifying upper trough is
forecast to move from the lower Great Lakes to off the Atlantic
coast Wednesday afternoon.  Northwesterly flow and cold air
advection across the upper lakes will support lake effect snow,
with several inches possible along the favored locations downwind
of Lake Erie and Ontario, with the highest WPC probabilities for 4
inches existing in the Tug Hill Plateau.

Additionally today, potent snow squalls are likely behind the
strong front. While these squalls will produce little accumulation
overall, briefly very heavy snow rates combined with gusty winds
could create hazardous travel at times. The highest likelihood for
snow squalls will be across upstate NY, PA, and into Western New
England.


...Central Plains into the Great Lakes...
A slowly amplifying southern stream trough will begin to interact
with northern stream energy on Friday to produce surface low
pressure over the Southern Plains. Strong moist advection ahead of
this feature will interact with a poleward sharpening and
intensifying jet streak to produce significant ascent and heavy
precipitation spreading northward ahead of the low. While there
continues considerable spread in the guidance as to how this
system will evolve, and much of the precipitation may be just
beyond this forecast period, there is likely to be both a swath of
heavy snow, and moderate to heavy freezing rain from Kansas
northeast into Michigan. WPC probabilities for 4 inches are
highest in eastern Nebraska and central Iowa, while the highest
probabilities for 0.25" of freezing rain are across northern
Missouri and southeast Iowa. Additional snow and ice is possible
into the medium range, and the evolution of this system will need
to be monitored.


Weiss