Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 AM EST Tue Feb 07 2023

Valid 12Z Tue Feb 07 2023 - 12Z Fri Feb 10 2023

...Pacific Northwest to the Northern Rockies...
Day 1...
An amplifying shortwave trough and its associated frontal band are
expected to bring heavy precipitation, including mountain snow, as
they move across the region beginning later today.  Ample
moisture, along with strong forcing, will help support the
potential for snowfall rates exceeding 2 in/hr along the favored
terrain of the Olympics and the northern Cascades later this
morning before periods of heavy snow develop farther east over the
northern Idaho and northwestern Montana ranges late in the day
into the overnight.  Snow levels around 3000-4000 ft are expected
to drop as the front moves through later today.  Post-frontal
onshore flow, along with some continued upper-level forcing, will
support additional snow showers across western Washington and
Oregon through tonight.  Precipitation is expected to diminish
from west to east as the trough moves east of the Rockies, and a
ridge starts to amplify along the West Coast on Wednesday.  Before
ending, snowfall accumulations may reach an additional 3-4 ft
across the higher peaks of the northern Cascades.  Impactful
amounts are also likely at the pass level, with WPC probabilities
indicating that accumulations of 8 inches or more are likely at
both Stevens and Snoqualmie passes.  Relatively lighter but
impactful amounts are expected farther east, with WPC
probabilities indicating that accumulations of 6 inches or more
are likely for a significant portion of the northern Idaho and
northwestern Montana ranges.

...New Mexico...
Day 1...
A positively-tilted upper trough will slowly move across the state
today.  Moisture associated with this system is limited.  However,
low-to-mid level frontogenesis is expected to help support an
increase in snowfall rates later this morning, with some potential
for locally heavy accumulations across the higher reaches of the
southwestern and south-central mountains.

...Mid Mississippi Valley to the Great Lakes and Northeast...
Days 2-3...
The previously noted trough in the Southwest is expected to swing
across the southern Plains on Wednesday and lift northeast ahead
of the shortwave digging southeast of the northern Rockies.  As
the two features interact and the leading shortwave assumes a
negative tilt, a quickly developing surface low is forecast to
track northeast across the mid Mississippi Valley Wednesday night.
 For the lower Missouri and mid Mississippi Valley, warm air ahead
of the system will support mostly rain.  However, increasing
ascent is expected to generate enough cooling to support a
changeover to snow, with some potential for light accumulations
Wednesday night-Thursday morning. 

For areas farther northeast there is a stronger signal for
accumulating and potentially significant amounts.  As the system
continues to strengthen and track northeast, coupled upper jet
forcing along with low-to-mid level frontogenesis will support
heavy precipitation and cooling within the deformation band on
Thursday.  Overnight guidance shows the better signal for heavier
accumulations centered from southwestern Wisconsin to northern
Michigan, with amounts in northern Michigan bolstered by the
potential for lake enhancement as the system lifts northeast to
the region Thursday night.  WPC probabilities for accumulations of
4 inches or more, which were relatively modest, have increased
across this area with the latest run.

Farther downstream in the warm advection pattern, cold air in
place will support a wintry mix at the onset across portions of
the Northeast, with a few inches of snow possible for interior
Maine and accumulating ice from northeastern Pennsylvania into
Upstate New York and northern New England.  WPC guidance shows at
least a slight risk for ice accumulations of 0.10 inch or more for
parts of the Catskills, Adirondacks, and the northern New England
mountains.

Pereira