Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 AM EST Mon Jan 13 2020

Valid 12Z Mon Jan 13 2020 - 12Z Thu Jan 16 2020

...The West...

Days 1-3...
A potent shortwave trough over WA is rounding a deep cyclone
centered over the BC/Alberta border. Locally heavy mountain snow
is ongoing over the Cascades and northern Rockies in broad onshore
flow with abundant Pacific moisture. Low snow levels will persist
as the trough shifts west from western Canada into the
northeastern Pacific through the midweek. The next system in a
long stretch of shortwave troughs arrives into the OR coast today
with further onshore flow continuing through Tuesday. In addition
to heavy snows along the southern Cascades and coastal ranges of
southwestern Oregon and northwestern California, significant
accumulations are also possible across the lower elevations of
western Oregon as well. Fast, zonal flow with embedded mid-level
energy will support snow showers across the region through Tuesday
into early Wednesday. As an amplifying shortwave approaches the
coast, organized heavier precipitation is forecast to return to
western Oregon and northwestern California by late Wednesday. This
is expected to bring additional heavy accumulations to the coastal
ranges and Cascades. WPC probabilities show that three-day totals
will likely exceed two to three feet across a large extent of the
Oregon Cascades, as well as for portions of the southwestern
Oregon and northwestern California coastal ranges.


...Upper Midwest and Great Lakes...

Day 1...
An upper trough currently over the Intermountain West is rounding
a cold-core low centered over western Canada. This trough will
take of a negative tilt as it spills onto the northern Great
Plains. The associated surface low will tap into gulf sourced
moisture tonight with warm air advection precipitation breaking
out across MN/IA which are currently quite cold. Expect a mix of
snow and freezing rain (more snow farther north) tonight. Low
probabilities for four inches or more are limited to the northern
shore in the MN Arrowhead where southeasterly flow is enhanced by
western Lake Superior and the steep terrain there. Freezing rain
is most likely over northern IA where there is a 20 to 30 percent
Day 1 probability for a tenth of an inch tonight.


Day 3...
A trough currently over the Gulf of Alaska shifts across the
northern Rockies Tuesday night with its surface low tracking
farther south and more zonally than the Day 1 trough. Gulf
moisture is tapped around IA again where precipitation breaks out.
This low tracks east-northeast across the Great lakes Wednesday
into Wednesday night with deep high pressure to the north in
Canada aiding convergence with the potential for decent snowbands
across the northern Great Lakes. As of now the snow axis looks
best from northern MN, far northern WI, and across the UP of MI.
As of now there is a 10 to 20 percent probability for six or more
inches across the UP. There is a freezing rain risk over the
southern Great Lakes, though this looks limited to light icing at
this time.


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

Jackson