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

Valid 12Z Sun Mar 28 2021 - 12Z Wed Mar 31 2021

...Pacific Northwest and Northern Rockies...
...Days 1-2...
A 700 mb trough moves east southeast from the northeast Pacific
and onshore across British Columbia, driving a 50-60 kt 700 mb jet
across the WA Olympics and Cascades, and then northern ID to
northwest MT.  A surge in tropically sourced moisture precedes the
associated cold front bringing 0.75 inch PW air (one standard
deviation above normal) into western WA this morning.
Heavy snow is expected with high Day 1 snow probabilities of a
foot or more for the WA Cascades. Locally as much as 18-24 inches
is forecast in favored windward locations in the northern
Cascades.

The surge in moisture advection and lift that crosses the Pacific
Northwest today moves inland across the northern Rockies tonight
and early Monday. Although the northern stream trough moves
quickly east across the northern Plains Monday, residual moisture
remains in place in the ranges of western MT Monday where Day 2
snow probabilities are moderate to high for 4 inches and low for 8
or more inches along the Rocky Mountain front.

On Day 3/Tue., the slow approach of a 700 mb ridge from the
eastern Pacific advects drier air aloft across the northwest,
limiting snow potential.

...Colorado Rockies...
...Day 3...

A 700 mb wave with an associated pool of  enhanced moisture moves
across CO in association with a cold front. After the frontal
passage, return flow results in boundary layer moisture
convergence in the mountains west of the CO Springs, CO area,
including locations around Pikes Peak. The ECMWF, UKMET, and
Canadian regional GEM show more QPF and resultant snow than the
00z NAM and GFE, and the former were given more weighting to
retain continuity. Several inches of snow are possible in the
foothills and mountains of central to southeast CO .  The event
should wind down as the 700 mb wave/boundary layer moisture
convergence maxima move south of out CO.

Day 3 snow probabilities ending 12Z Tuesday have 30 to 50 percent
probabilities for 4 inches of snow in the foothills tot he front
range of central to southeast CO.


...Northern New York/New England...
Days 1-2...

The forecast closed low moving east from the northern Great Lakes
across southern Quebec advects both warmer air aloft and moisture
up across Maine today into this. Cold enough antecedent air allows
some snow to fall across higher elevations of interior Maine and
well as northern Aroostook Co where Day 1 snow probabilities are
moderate for 4 or more inches.

After the cyclone departs, cold advection develops across western
to northern NY, with snows occurring in the Tug Hill and western
Adirondacks, where a westerly component of low level flow turns
upslope in the windward terrain. Low probabilities exist for 4
inches of snow.  The probabilities are limited by the duration of
snow in each area, given initial temps are too warm for snow.
On Mon, snow coverage wanes as a low level ridge crosses from the
Great Lakes, with drying aloft causing snow to taper.

The warm advection results in a change over to rain with some
pockets of freezing rain anticipated early today, particularly for
the mountains of western to northern Maine where there are Day 1
probabilities up to 30 percent for measurable ice accretion.

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

Petersen