Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
432 PM EDT Fri Sep 27 2019

Valid 00Z Sat Sep 28 2019 - 00Z Tue Oct 01 2019

...Western U.S....

...A prolonged heavy snow event focused at higher elevations
across the Pacific Northwest and in particular on eastern slopes
of the northern Rockies in north-central Montana continues into
Sunday night...

Deep low pressure currently over southwest BC stalls over interior
WA/OR late tonight through Sunday. Cold air from northern Canada
is drawn south by this low/trough and surface high pressure
stalling near the northern BC/Alberta border tonight into Sunday
and reaches the northern MT border tonight. This high pressure to
the north which includes a surface ridge to northern MN combined
with a lee trough/low over WY/eastern MT by Saturday provides a
broad area of northeasterly flow and cold air wedge into
north-central MT. This flow is upslope for the eastern slopes of
the Rockies in north-central MT. Moisture streaming up the Great
Plains with origins over the Gulf of Mexico aids heavy precip
rates particularly for eastern slopes of the Rockies until
shortwave activity rounding the low/trough pushes across the
northern Plains Sunday night into Monday.

Multiple feet of snow are possible above about 4000ft in the TFX
CWA for areas north of Helena. Also of particular concern is a
potential band of snow extending along the cold air wedge front
across northern and northeastern MT Saturday night into Sunday
night. Dynamic cooling in enhanced precip along this front could
encourage snow accumulation in this area, particularly if it is
strong nocturnally.


The probability of significant icing is less than 10 percent.

The WPC WWD is expected to remain active on day shifts only for
this significant snow event through Saturday. The WPC WWD is
scheduled to resume normal operations on Oct 3.

Jackson