Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
426 AM EST Mon Feb 03 2020

Valid 12Z Mon Feb 03 2020 - 12Z Thu Feb 06 2020

...Northern Rockies through the Southern Plains...
Days 1-3...

A large positively tilted trough will amplify and drift E/SE
through mid-week, while pieces of vorticity energy rotate around
and through the trough. While differences in the evolution and
thermal structure of this trough persist, there is general
consensus that the axis will shift from the Great Basin this
morning, to a full latitude trough axis from MN into eastern TX by
Thursday morning.

As this feature shifts eastward, it will be accompanied by several
reinforcing shortwaves which will combine with height falls to
provide ascent. At the same time, a strong jet streak downstream
of the main trough axis will intensify and become increasingly
anticyclonic, especially on Tuesday, with secondary jet streak
development occurring Wednesday over the Southern Plains. This
forcing will all interact with a moistening column provided by
tropical Pacific moisture, and an expansive area of moderate to
heavy snow is likely.

The heaviest snow is expected on D1 /Monday/ across WY and CO
where enhanced moisture will be wrung out by strong deep layer
ascent, aided by upslope flow on the north side of a developing
lee low in CO. WPC probabilities are high for 8 inches across much
of eastern and central WY, as well as in parts of the UT Wasatch
and CO Rockies. Although snow levels will initially be as high as
5000ft in CO, they will fall quickly today, so even the valleys
are expected to see accumulating light snow today as far south as
SE Utah. The highest amounts will remain in the terrain of WY
however, where isolated amounts of 18" are possible.

On Tuesday into Wednesday, energy will rotate into the base of the
positively tilted trough, sharpening it across northern Mexico. At
the same time high pressure will ridge down from the Dakotas into
the Southern Plains, and a period of strong and moist isentropic
lift will spread snowfall into eastern NM, much of W/Central TX,
and into Oklahoma. Initially the column will feature marginal
thermal profiles for snow, and rain is likely to be the initial
precipitation type. However, as the front digs southward ahead of
the high to the north, the combination of cooling temperatures,
strong jet dynamics, and increasing mid-level fgen will change
precipitation over to snow. Snowfall could be heavy at times in a
stripe across northern TX into OK, and WPC probabilities have
increased to feature a high risk for 4 inches of accumulation with
locally higher amounts. Heavy snow is also likely in the
Sacramento and Guadalupe ranges of New Mexico where more than 8
inches of snowfall is possible.

As the trough begins to lift northeast out of Mexico and across
TX, renewed surface low development is likely near the Gulf of
Mexico, shifting northeast towards the Ohio Valley by the end of
the forecast period. Moisture on the NW side of this low track
will produce a swath of snow, as ascent is provided by continuing
intense jet dynamics. The system moves quickly, so WPC
probabilities for 4 inches are less than 20 percent from Oklahoma
into the Missouri River Valley.


...Pacific Northwest...
Days 2-3...

The next atmospheric river reaches western WA Tuesday night on the
back side of a broad northeastern Pacific Ridge. Snow levels will
rapidly rise from near the surface early Wednesday to as high as
6000 ft by Wednesday night as a warm front and height rises occur.
Above these levels, prolonged moist advection will produce heavy
snow in the Olympics and Cascades of Washington, spreading into
the Northern Rockies and into the ranges of ID/MT/WY on day 3. WPC
probabilities are high for 8 inches across these areas.

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

Weiss