Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
321 AM EST Sun Feb 05 2023

Valid 12Z Sun Feb 05 2023 - 12Z Wed Feb 08 2023


...Western U.S....
Days 1-3...
An amplified trough moving onshore this morning is expected to
elongate over the western U.S. as a southern stream shortwave digs
southeast across California into the Southwest beginning later
today.  This positively-tilted wave is expected to gradually move
across the Southwest early this week, with some models showing a
closed low developing over Arizona and New Mexico on Tuesday. 
Meanwhile, a much more progressive northern stream trough is
forecast to sweep across the Pacific Northwest and the northern
Rockies, reaching the northern High Plains by early Monday.  In
the wake of these systems, a pronounced upper ridge will move
across the West Coast late Monday into early Tuesday before
another shortwave trough and its associated frontal band begin to
impact the Northwest later in the day.

There remains a strong signal for additional heavy snow today
along the Sierra, where orthogonal flow and steep lapse rates are
expected to support rates as high as 3 in/hr before waning later
this evening.  Snow levels are expected to drop to around 4000 ft
as the upper trough and the associated front moves across the
region.  Widespread additional accumulations of 8 inches or more
are likely, with heavier totals expected across the higher terrain.

Meanwhile, the signal for heavy amounts has increased across the
southern Cascades.  A well-defined, compact low continues to track
northeast along the Oregon coast this morning.  This system is
expected to move inland later this morning near the
Washington-Oregon border, with enhanced onshore flow bolstering
the potential for heavy, orographically-enhanced precipitation to
its south.  Snowfall rates may reach 2 in/hr in parts of the
Oregon Cascades.  Even as the low weakens and moves east,
persistent onshore flow in its wake is forecast to promote
additional periods of moderate to heavy snow before snow
diminishes tonight.

As the trough translates east, widespread high elevation snow is
expected to spread across the remainder of the Intermountain West
into the central and northern Rockies, with areas of heavy snow
possible.  Areas impacted include the Blue Mountains and the
central Idaho ranges, where WPC probabilities indicate that
localized accumulations of 6 inches or more are likely.

Generally light snow is expected to continue across the northern
and central Rockies into Monday, with dry conditions likely
farther west.  One primary exception will be western Washington,
where warm advection precipitation ahead of the next system is
expected to impact the region on Monday. The threat for heavy snow
is forecast to remain largely confined to the higher peaks of the
northern Cascades before the cold front moves onshore, bringing
lower snow levels and a more widespread threat for heavy snow
across the northern Cascades into the northern Rockies on Tuesday
into early Wednesday.

The probability of significant icing is less than 10 percent.

Pereira