Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
403 AM EDT Wed Oct 09 2019

Valid 12Z Wed Oct 09 2019 - 12Z Sat Oct 12 2019

Days 1-3...

...Major early season snow storm for the mountains of southern
Montana and across the Northern Plains...

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

A powerful, early-season winter storm will continue to bring heavy
snows to the northern Rockies into Wednesday.  Snows will move
slowly east across the northern High Plains today before
continuing farther east across the Dakotas, and also moving south
into the central Rockies on Thursday.  Snows will continue across
the Dakotas through Friday into early Saturday, where heavy
accumulations are becoming more likely.  There is potential for a
couple of feet of snow in portions of northern North Dakota, which
would constitute record breaking snow amounts.

The 00z NAM/UKMET/ECMWF Models and 12z ECMWF Ensemble Mean remain
in good agreement, showing a 700 mb front and embedded circulation
crossing from near the MT border with ID and WY this morning to
eastern MT/WY later today and across the western Dakotas tonight.
Heavy snow occurs along the circulation path in the southern MT
ranges, including the Beartooth and Bighorn Mountains, including
adjacent WY.
WPC probabilities for the 24-hr period ending 12Z Thursday (Day 1)
continue to show a greater than 70 percent chance for
accumulations of 12 inches or more across some of the higher,
orographically favored terrain. 

Meanwhile,  strong low to mid-level frontogenesis will support a
band of moderate snows dropping southeast across eastern Montana
and the western Dakotas.  WPC Day 1 probabilities show a 70
percent chance or greater for snow accumulations of 4 inches or
more extending across most of southern and central MT to 50
percent in northeastern Montana.

On Thu and Thu night, the front and embedded circulation drift
east across the eastern Dakotas.  West of the front, a period of
heavy snow is expected across the Dakotas.
Most of the eastern Dakotas near the MN border are expected to
have a mixture of precip types, cutting down on sleet/snow
amounts.

Increasing divergence along the right-entrance region of a
strengthening upper jet is expected to support increasing snowfall
rates along the well-defined low to mid level front moving east
across the Dakotas.  With the models showing better agreement,
probabilities for heavy snow accumulations for the Day 2 period
(ending 12Z Friday) show a 50 percent chance or greater for
amounts of 8 inches or more from northern South Dakota into east
central North Dakota.  Meanwhile, locally heavy amounts are
forecast to move south from the northern into the central Rockies,
including the northern Colorado ranges.

As the 700 mb circulation drifts slowly northeast Fri out of the
Dakotas and across northwest MN to the Canadian border early
Saturday, snows continue in northern North Dakota until winding
down as the circulation reaches the Canadian border.
Several additional inches are possible, up to another foot in
portions of eastern ND.
The precip type transition zone drifts east across MN, with
uncertainty regarding the duration of snow and reduced amounts
where duration is limited.

WPC Day 3 probabilities (12z Fri to 12Z Saturday) show a 50
percent chance or greater for accumulations of 8 inches or more
across a good portion of central into eastern North Dakota.

The probability of significant icing is less than 10 percent Days
1-3.

Petersen