Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
351 PM EST Wed Mar 03 2021

Valid 00Z Thu Mar 04 2021 - 00Z Sun Mar 07 2021

...Southwest/Southern Rockies...
Days 1-2...

A 700 mb closed low in California this evening will move east
northeast across southern Nevada, Utah, and Colorado before
turning southeast on the Colorado high Plains and then Kansas by
tomorrow night. Enhanced upper-level divergence, low level
convergence, and warm/moisture advection ahead of the 700 mb low
leads to ascent starting in the Mogollon Rim of AZ to the southern
UT Wasatch, continuing in tandem with the low across the ranges of
CO, and then departing with the low Thu as it moves east across
the Plains.

The heaviest snow through Thursday is likely to be in the San
Juans of southern CO/adjacent northern NM which will benefit from
upslope enhancement in windward terrain. WPC probabilities are
high for 4 inches and moderate for 8 inches on Day one. Otherwise,
WPC probabilities are moderate for 4 inches on D1 in the southern
Wasatch, extending into most of the CO Rockies. Lighter snows
accumulating to a few inches are likely Thu across the Palmer
Divide in CO.


...Pacific Northwest/California...
Days 2-3...

A closed mid-level low is likely to move slowly northeast off the
OR and WA coasts Thursday night through Friday night, with
pronounced southerly flow advecting warm air and moisture
northward along the coast of northern CA and across WA/OR. As the
low slowly migrates eastward, a surface cold front will move
onshore. A prolonged period of low level pre-frontal convergence
combines with the warm/moist advection ahead of the front to
produce prolonged low elevation rain and high elevation snow
across the OR/WA Cascades and WA Olympics.
On Day 2, WPC probabilities are high for 8 inches in the Olympics,
where isolated totals approaching 2 ft are possible.

On Day D3, the cold front moves onshore in California and
progresses inland. The surge in moisture and low level convergence
with the front drives a period of snow across the northern Sierra,
Siskiyous/Shastas/Trinities, and into the OR/WA Cascades, as well
as continuing in the Olympics. WPC probabilities are moderate for
8 inches in these ranges, with local maxima above 12" likely.
There is considerable spread in the QPF and resultant snow totals
in the ranges of WA/OR/northern CA, so probabilities aren't high
for heavy snow exceeding 8 inches yet.

Northern New York/northern Vermont mountains Day 2...
The slow moving 700 mb drifting east from New England allows
moisture to wrap around the circulation and advect across the NY
Adirondack Mountains and then into the northern VT Green
Mountains.  Areas where the overlap of moisture and boundary layer
convergence/ascent where the northwest winds turn upslope results
in snow showers Thu night across these mountains. High mean layer
relative humidity near 90 percent Thu night supports an extended
period of snow showers. On Fri as the low pulls away, drying aloft
develops across the region along with sinking motion, supporting
the snow tapering in coverage/intensity.  Greater weighting was
given to the higher resolution runs and their ability to better
match where upslope flow produce snow showers. Several inches of
snow are possible in the northern Green Mountains of VT.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen