Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 PM EST Sun Jan 05 2020

Valid 00Z Mon Jan 06 2020 - 00Z Thu Jan 09 2020


Days 1-3...

...Pacific Northwest to the Northern Rockies...
An active period in the Northwest continues as the next in a
series of shortwave troughs and its associated frontal bands move
inland Sunday afternoon.  A low amplitude ridge is forecast to
build in its wake, with a surface warm front lifting into the
Northwest on Monday.  Models show the front remaining in place as
fast flow aloft ushers a stream of deeper moisture and mid level
energy across the region.  WPC guidance indicates high
probabilities for additional accumulations of a foot or more
across of a good portion of the Washington Cascades on Day 1
(ending 00Z Tue).  Residual moisture spilling east is expected to
support locally heavy amounts across the northern and central
Idaho ranges as well.  Meanwhile, expect increasing low level
convergence along a front dropping south along the Rockies to
support locally heavy accumulations along the northwestern Montana
ranges by late Monday. 

Southwesterly flow aloft into a stalled frontal boundary will
continue to support heavy snows across these regions into Tuesday.
 While building heights ahead of the next approaching system is
expected to momentarily limit the extent of heavy snows, WPC
guidance shows high probabilities for additional accumulations of
a foot or more across portions of the northern Cascades and the
northern Idaho and northwestern Montana ranges on Day 2 (ending
00Z Wed). 

The next shortwave trough is forecast to move through the
northwestern U.S. late Tuesday and Wednesday, sending snow levels
downward once again and the threat for further additional heavy
amounts for portions of the Cascades and northern Rockies.

...Northeast...
An elongated mid-level shortwave is expected to bring widespread,
but mainly light snows as it moves across the Northeast on Monday.
 Following the initial synoptic snows, westerly winds will support
lake effect snow showers east of the Lower Great Lakes, with
several more inches possible, especially east of Lake Ontario into
the Tug Hill Region, before diminishing Monday night.  A strong
cold front will bring additional snow showers across the Northeast
on Wednesday, with strong cold air advection and northwesterly
flow supporting lake effect snows in its wake. 

...Central Appalachians to the Northeast...
A shortwave trough is expected to bring accumulating snow to
portions of the central Appalachians Tuesday morning.  Guidance
shows little threat for widespread heavy accumulations as this
system moves quickly east.  The fast movement of the system, along
with marginal boundary layer temperatures are expected to limit
the threat for significant accumulations as it moves through the
Mid Atlantic region during the day Tuesday.  Impacts for the
Northeast are less certain as the models show a fair amount of
spread with the track of the low as it moves offshore Tuesday
night into Wednesday.  While cannot rule out more significant
impacts, the more recent trends of the models support a more
southerly low track off of the Mid Atlantic coast, with only light
snow accumulations for the Northeast.

The probability of significant icing (0.25 inch or more) is less
than 10 percent.

Pereira