Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
456 PM EDT Sat Apr 13 2019

Valid 00Z Sun Apr 14 2019 - 00Z Wed Apr 17 2019

...Great Lakes region to northern New England...

An amplified southern stream shortwave trough and associated
surface low will continue to develop and track towards the
northeast over the next two days.  A rather pronounced deformation
zone on the northwest side of the surface low will support a swath
of steady stratiform precipitation from Oklahoma to Michigan.  A
coupled jet structure and low-mid-level frontogenesis, combined
with positive vorticity advection, will support deep layer ascent.

Although the majority of the precipitation from this event is
expected as rain, the northern boundary of the comma head region
will likely be marginally cold enough to support wet snow.  Near
surface temperatures should be at or just above freezing, and snow
accumulations will be dependent on snowfall rates and also time of
day given higher sun angles in mid-April.  Models remain in
agreement with rain changing to snow from northern Illinois and
southern Wisconsin to central Michigan on Sunday.  The NAM was
colder than the remaining deterministic guidance, so a GFS/ECMWF
blend for thermal profiles worked well.  WPC probabilities are
moderate for 4+ inches of snow from near Chicago to central
Michigan for the Day 1 period, assuming the snow readily
accumulates.


...West Coast to the Northern Rockies...

There will be two separate weather features of interest to affect
the western U.S. through the next three days.  The first will be a
shortwave trough that will be accompanied by a cold front that
will track southeast across the northern Intermountain West and
the northern Rockies.  Snow levels will fall with the passage of
the front, and continued post-frontal onshore flow will support
some moderate snowfall amounts across the northern Cascades, where
a High Risk for snow accumulations of 4 inches or more is
indicated by the WPC probabilities on Day 1.  This front is
expected to linger across the northern Rockies through the
remainder of the weekend.  This along with a low amplitude
shortwave may support a period of heavier snow Sunday night and
into Monday from the Idaho-Wyoming border into northern Utah, with
amounts on the order of 4 to 8 inches likely with locally higher
amounts possible.

The second system will be a well defined shortwave moving onshore
late Monday into early Tuesday, bringing accumulating snow back
across the southern Cascades and northern Sierra on Monday and
then across the elevated terrain of the northern Intermountain
West and northern Rockies by Tuesday.

Hamrick