Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
328 AM EST Wed Mar 03 2021
Valid 12Z Wed Mar 03 2021 - 12Z Sat Mar 06 2021
A mid-level closed low emerging from Southern California this
evening will move eastward across the Four Corners Thursday before
ejecting into the Southern Plains on Friday. Enhanced mid-level
divergence, height falls, a LFQ jet streak divergence maxima, and
intensifying southerly WAA will combine to produce moderate to
heavy snow in the terrain of the Four Corners mountains this
evening and Thursday. Snow levels are expected to climb as high as
6000-8000 ft during the heaviest precip, confining the significant
snow to these elevations. The heaviest snow through Thursday is
likely to be in the San Juans which will benefit from upslope
enhancement due to the orthogonal southerly flow, and WPC
probabilities are high for 6 inches each of the two days.
Otherwise, WPC probabilities are moderate for 6 inches on D1 in
the southern Wasatch, extending into most of the CO Rockies on D2.
Lighter snows accumulating to a few inches are likely D2 across
the Palmer Divide.
A closed mid-level low is likely to move slowly west of the coast
Thursday and Friday, with pronounced southerly 700-500mb flow
drawing tropically sourced moisture northward along the coast of
northern CA and into WA/OR. As the low slowly migrates eastward, a
surface cold front will move onshore, but not until late D3
/Friday evening/, which will finally bring a slow wane to the
precipitation. However, a prolonged period of warm/moist advection
ahead of this front will drive significant precipitation across
the Coastal Ranges and into the WA/OR Cascades. Snow levels will
rise on this warm flow, and are expected to generally be above
4000 ft in WA/OR, before falling to 2500-3000 ft on D3. WPC
probabilities D2 are high for 6 inches in the Olympics, where
isolated totals approaching 2 ft are possible. By D3, there will
be enough of an eastward push in the flow to drive heavy 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 to high for 6 inches in these ranges,
with local maxima above 12" likely.
For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.