Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 PM EST Fri Jan 20 2023

Valid 00Z Sat Jan 21 2023 - 00Z Tue Jan 24 2023

...Central Rockies and High Plains...
Days 1 & 3...
An upper low is forecast to track east from the Four Corners
region into the Rockies this evening.  As the low moves east,
models continue to show a strong signal for locally heavy snow
amounts developing tonight initially over far northeastern New
Mexico and southeastern Colorado before shifting east into western
Kansas overnight into early Saturday.  A strengthening low-to-mid
level center is forecast to draw moisture into an area of strong
lift supported in part by upper jet streak forcing and
well-defined low-to-mid level frontogenesis.  This will likely
support the development of heavy snowfall, with rates exceeding 1
in/hr.  WPC probabilistic guidance shows accumulations of 4 inches
or more likely across the region, with locally heavier amounts of
a foot or more possible, especially along the eastern Colorado-New
Mexico border near the Raton Mesa.

Dry conditions are expected for much of the region during the day
on Sunday, before snow returns beginning Sunday evening and
continuing into Monday for parts of the central Rockies and High
Plains.  Snows will develop as a second upper low drops south
through the Intermountain West and northern Rockies.  Snow is
expected to develop initially over the central Rockies late
Sunday, before spreading into the Plains as another low-to-mid
level center begins to direct moisture into the region. 
Probabilistic guidance indicates that southeastern Colorado may
once again be a focus for heavier amounts, with WPC guidance also
showing higher probabilities farther to the north centered near
the Denver Metro.  While there are certainly differences in the
details, the models are fairly well-clustered with the larger
scale aspects of this system.  A notable exception at this point
is the 12Z NAM, which is less amplified and much more progressive
than the other deterministic models.

...Pacific Northwest to the Northern Rockies...
Days 1-3...
An amplifying, but fast-moving shortwave trough is forecast to
move across the Gulf of Alaska and northeastern Pacific, bringing
mountain snow into the Olympics and northern Cascades Saturday
into Saturday night.  Any heavy accumulations will likely remain
confined to the higher peaks.  Models show the system continuing
to amplify as it moves east and then southeast from Pacific
Northwest into the Rockies.  While widespread heavy amounts are
not expected, locally heavy accumulations are possible from the
northern Idaho to the western Montana ranges.  A period of
northerly flow in the wake of a low level front moving east from
the Rockies into the High Plains is expected to support some
higher totals over the central Montana mountains late Sunday into
early Monday. 

...Northeast...
Day 3...
The previously noted upper low moving east of the central Rockies
tonight is forecast to lift northeast and phase with a northern
stream trough digging across the northern Plains and Upper Midwest
on Saturday.  This will result in a highly-amplified trough moving
east of the Mississippi Valley Sunday evening.  Surface low
pressure is forecast to develop along the right-entrance region of
a strong upper jet and track north and strengthen along the Mid
Atlantic to Northeast coast Sunday night.  Additional
strengthening is expected as the low moves along the New England
coast into Atlantic Canada on Monday.  The potential for heavy
snow with this system remains high, however confidence in its
placement is still limited by model spread and run-to-run
discontinuities.  The general trend of the 12Z guidance was toward
a more a more easterly track -- supporting lighter QPF and a
lesser threat for heavy snow from western to far northern New York
and far northern New England.  At this point, even with the more
easterly track this still appears to a mostly to all rain event
for the I-95 corridor up through Boston.  Probabilities for snow
accumulations of 8 inches or more are currently highest from the
Adirondacks through interior northern New England, however with
the shifting track, anticipate some changes of this axis of higher
probabilities as well.

Pereira