Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 PM EDT Sun Apr 07 2019

Valid 00Z Mon Apr 08 2019 - 00Z Thu Apr 11 2019


...Northeast...

Low to mid level theta-e advection ahead of an approaching low
into an west-east oriented baroclinic zone, with cold high
pressure to the north, will set the stage for mixed precipitation
developing across northern Vermont and New Hampshire Monday
morning and wet snow farther to the east across central and
northern Maine.  Low pressure deepening over the Gulf of Maine on
Monday is expected to lock in the cold air, maintaining mostly
snow across portions central and northern Maine -- where WPC Day 1
probabilities (ending 00Z Tuesday) indicate a Moderate Risk for
snow accumulations of 8-inches or more.  Farther west, WPC
probabilities show a Slight Risk for ice accumulations of 0.25
inch or more across much of Vermont and New Hampshire along the
White Mountains into northern Maine on Day 1.

...Northern and Central Rockies to the Plains...

With a shortwave ridge overhead, snow levels are expected to
remain high across the northern Rockies through Monday, with any
significant accumulations largely limited to the higher peaks of
the central Idaho, southwestern Montana and northwestern Wyoming
ranges.  Snow levels will begin a marked decrease on Tuesday
however as a deepening upper trough moves across the West. 
Favorable upper jet forcing in addition to low to mid level
convergence/frontogenesis are expected to support increasing
probabilities for locally heavy accumulations across the central
Idaho to southwestern Montana and northwestern Wyoming ranges on
Tuesday.

Late Tuesday into Wednesday is expected to mark the beginning of
the next powerful storm to impact the northern and central Rockies
and Plains.  Models show an upper low closing off over the central
High Plains on Wednesday with a powerful surface cyclone
developing as well.  Consensus of the 12Z deterministic guidance
shows the low deepening into the lower 980s/upper 970s as it
tracks from eastern Colorado into Kansas on Wednesday.  Strong
upslope flow along with deep forcing will support moderate to
heavy snow accumulations developing across the northern into the
central Rockies by early Wednesday.  Rain changing to snow is
expected farther east across the Plains on Wednesday, with heavy
accumulations possible across portions of eastern Wyoming and
western South Dakota by late Wednesday.  By the end of the period,
expect accumulating snow to stretch as far east as the upper
Mississippi valley.

Pereira