Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
440 PM EDT Sun Mar 28 2021

Valid 00Z Mon Mar 29 2021 - 00Z Thu Apr 01 2021

...Pacific Northwest and Northern Rockies...
...Day 1...

A deep and anomalously cold trough for late March pushes
east-southeast across WA this evening with the main energy
shearing east along the Canadian border through Monday night and a
reinforcing trough amplifying the trough, digging it to CO through
Tuesday. 0.75 inch PWs (one standard deviation above normal) is
pushing into WA this afternoon with snow levels crashing from
3000ft to 1500ft by this evening as moisture and precip rates
taper off. The surge in moisture advection and lift reaches the
northern ID/MT Rockies this evening and NW Wyoming late tonight.
Day 1 snow probabilities are high for 6 or more inches for the
WA/OR Cascades, Bitterroots and ridges south of Glacier NP as well
as moderate for the Wallowa in northeast OR and the Tetons.


...Colorado Rockies...
...Day 2...

The amplifying wave from the Pacific Northwest tonight shifts down
the northern Rockies Monday into Tuesday, settling over CO.
Easterly upslope flow turns colder with rain changing to snow
along the Front Range after a cold frontal passage Monday evening
with the upper trough axis only slowly shifting east through
Tuesday, extending the snow in central/southern CO, particularly
from the Palmer Divide to the Pikes Peak area and down the Sangre
de Christos. The 12Z ECMWF, UKMET, and Canadian GDPS continue to
produce more QPF and resultant snow than the 12Z NAM and GFS with
preference remaining with the non-NCEP grouping. Several inches of
snow are possible in the foothills and mountains of central to
southern CO with Day 2 snow probabilities of 10 to 20 percent for
6 or more inches on central CO ridges.


...Far Northern New York/New England...
Day 1...

Low pressure now over southeastern Ontario will track across far
northern Maine tonight. Warm sector snow will change to rain or
sleet by this evening with pockets of light freezing rain in far
interior Maine. The upper trough will swing east across Maine
early Monday with wrap around snow on a westerly component of low
level flow makes for upslope snow in the windward terrain of the
Tug Hill, northwestern Adirondacks and northern Greens where there
are 10 to 40 percent Day 1 snow probabilities for 4 or more inches.

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

Jackson