Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 PM EST Sun Jan 03 2021

Valid 00Z Mon Jan 04 2021 - 00Z Thu Jan 07 2021

...Northeast...
Days 1-3...

A weak area of low pressure moving up the East Coast will continue
to spread precipitation across the Northeast through tonight, and
across portions of Maine through Tuesday, as the low stalls or
slowly retrogrades back to the west. For this cycle, much of the
12Z guidance has trended less with the QPF given the relatively
weak forcing and lack of a deepening low that strays away from the
coast. As such, the snow amounts have decreased tonight/early
Monday with a few inches now expected across the interior
Northeast. With the low stalling or near the Gulf of Maine, some
interaction with the inverted trough along the coast could favor
some light snowfall across downeast coastal Maine.


...Pacific Northwest/California/Northern Rockies...
Days 1-3...

A couple waves of heavier precipitation will move onshore the West
Coast through Wednesday, one of which will come tonight through
Monday followed another surge late Tuesday into Wednesday. Copious
amounts of moisture and large scale forcing for ascent will bring
heavy mountain snow totals to the WA Cascades south through the
Sierra Nevada during day 1, followed by higher totals on day 2
across the Northern Rockies, Blue Mountains, Sawtooth, and
Bitterroots. By day 3, the second system will again favor the
highest snow totals for the WA Cascades. WPC probabilities for
exceeding 6 inches are high during Day 1 across the WA Cascades
then those high probabilities shift to portions of the northern
Rockies day 2. By day 3, the highest snowfall probabilities are
confined to the WA Cascades.

The probability of significant icing (0.25" or more) is less than
10 percent all 3 days.

Taylor