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

Valid 12Z Sat Apr 13 2019 - 12Z Tue Apr 16 2019


...Central and Southern Rockies to the High Plains...

A deep upper trough over the Southwest and southern Rockies will
continue to shift east, with a well-defined surface low developing
over Texas later today.  Upslope flow will continue to support
periods of snow along the Sangre de Cristo Mountains, producing
additional locally heavy amounts on Saturday.

Meanwhile as low deepens, moderate to heavy precipitation rates
supported by a favorable upper jet couplet and low to mid level
frontogenesis may result in rain changing to snow in some areas
along the western edge of the comma-head developing over the Texas
Panhandle.  While the NAM has backed off some, it remains the cold
outlier - showing snow accumulations that are unlikely given the
high April sun angle.  WPC probabilities have decreased here as
well, now showing a less than 10 percent chance for accumulations
of 4-inches or more.

...Great Lakes...

The upper trough moving across the southern Plains on Saturday is
forecast to lift to the northeast across the Mississippi and into
the lower Ohio valley on Sunday.  Favorable upper jet forcing
along with low to mid level frontogenesis will continue to support
a solid area of precipitation northwest of the low center.  Models
remain in better agreement, with rain changing to snow from
northern Illinois and Indiana into lower Michigan on Sunday. 
Overnight models have trended a little colder and probabilities
for significant accumulations have increased, with the latest WPC
guidance showing a Moderate Risk for snow amounts of 4-inches or
more centered across central Lower Michigan on Day 2.

...Pacific Northwest and Northern California to the Northern
Rockies...

A shortwave trough and associated surface low will continue to
move east across western Canada, with a trailing front slipping
southeast across the Pacific Northwest and northern Rockies. 
Falling snow levels and continued onshore flow behind the front
will support some locally heavy totals across the northern
Cascades, where a Moderate Risk for snow accumulations of 8-inches
or more is shown 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 into
Monday from the Idaho-Wyoming border into northwestern Wyoming,
where WPC probabilities show a Moderate Risk for accumulations of
8-inches or more on Day 2.

Models show a more defined shortwave moving onshore late Monday
into early Tuesday, bringing snows back into the southern Cascades
and northern Sierra on Monday and then into central Idaho and
western Wyoming once again by Tuesday morning.

Pereira