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

Valid 00Z Thu Oct 10 2019 - 00Z Sun Oct 13 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 be evolving overnight
into Thursday...with heavy snows in the Rockies to spread out onto
the Northern Plains.  Snowfall over the Dakotas is expected to
linger into Saturday as the system becomes occluded...resulting in
heavy accumulations are becoming more likely.  There is potential
for a couple of feet of snow in portions of eastern North Dakota,
which would constitute record breaking snow amounts.

Overall...the 12Z suite of guidance was in good agreement with the
large-scale features.  Some minor...albeit important...differences
exist between the models with regard to the rain/snow line.  The
12Z NAM became a deeper and colder outlier while the GFS nudged
towards a somewhat colder solution over the Plains in Day 1 and 2.
 The move by the GFS helped align it better with the European
solution which helped provide a bit more confidence in terms of
rain/snow/mixed precipitation types in the system. 

Meanwhile,  strong low to mid-level frontogenesis will support a
band of moderate snows dropping southeast across eastern Montana
and the western Dakotas late tonight and early Thursday morning. 
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...Snowfall rates will be increasing across
the Dakotas...especially over parts of North Dakota as a
well-defined deformation zone develops. That should decrease the
chances of multiple precipitation types.  The models are pointing
to a period of deep-layer frontogenesis co-located in a layer of
negative Saturated Geostrophic Equivalent Potential Vorticity...a
region favored for enhanced snowfall rates in banded
precipitation.  In addition, there will be increasing divergence
along the right-entrance region of a strengthening upper jet to
support broad-scale upper level ascent.

With the models showing better agreement, probabilities for heavy
snow accumulations for the Day 2 period (ending 12Z Friday) have
continued to increase compared with previous runs...especially in
the chances or greater than 8 and 12 inch amounts 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.

By Saturday, the mid/upper level circulation center continues to
drift out of the Dakotas and across northwest Minnesota to the
Canadian border.  This will allow for snows to gradually taper off
across the Dakotas as drier air works in from the west.  Farther
east, the risk of snows will start to be limited by a developing
dry slow as the entire system occludes.

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

Bann