Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
426 AM EDT Thu Mar 18 2021

Valid 12Z Thu Mar 18 2021 - 12Z Sun Mar 21 2021


...Northeast...
Day 1...

Models show waves of low pressure moving across the central
Appalachians and mid Atlantic before moving offshore south of Long
Island and southern New England.  Moisture is forecast to be
lifted by the mid level front north of the low pressure axis
across the southern tier of NY, northern PA, the Catskills,
Berkshires, and then to southeast MA.

There remains considerable spread in the accumulation forecasts
from the models, likely due to different handling of the intensity
of the mid level front.  Since the antecedent conditions are
unfavorable for snow (warm with rain), the majority of solutions
have the event as mostly rain, and then ending as a 2-3 hour
period of snow before drying aloft leads to the event ending.

The heaviest snow is likely in the Catskills, Berkshires, and
southeast MA, where WPC probabilities are as high as 20-30% for 4
inches due to 1"/hr snowfall rates. The uncertainty involves both
duration of snow as noted, but also intensity.  The WRF ARW/NSSL
WRF, 00z High Res Rapid Refresh and NAM CONUS Nest each show
potential for several inches in southeast MA tonight.
The low pressure and associated front move east on Friday,
bringing the event to a close.

...Western U.S....
Days 1-3...

A closed mid-level low will move east on Thursday and Friday
towards the Pacific northwest, shearing into an upper trough as it
moves inland across WA/OR/northern California Saturday morning.
Prolonged 700-500mb southwest flow will spread moisture inland Thu
and Fri, but with relatively high freezing levels. The combined
day 1 plus day 2 snow should lead to an area of 1-2 feet of snow
in the mountains of northern and central CA.  Height falls with
the upper trough bring cooler temps on Day 2 and spreads heavy
snows across the WA Olympics. Total snowfall may exceed 2 ft in
the highest terrain of the Olympics.

On Day 2, several inches of snow are likely in the forward flank
jet maxima crossing southern Idaho to northwest WY, where 300 mb
divergence maxima promote ascent as they crosses the terrain. As
the upper trough moves east on Sat-Sun., coupled upper
divergence/lower convergence maxima cross the UT Wasatch,the WY
Tetons, continuing east across the WY Bighorn Mountains and the
ranges of western CO.  Several inches of snow are expected in
favored terrain.

High WPC probabilities for 6 inches are confined to the
Siskiyous/Shasta area of northern CA on Day 1, in the Sierra
Nevada, Olympics, and Boise Mountains Day 2, and the Tetons/UT
Uinta Mountains Day 3.


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

Petersen