Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
418 PM EDT Thu Sep 26 2019

Valid 00Z Fri Sep 27 2019 - 00Z Mon Sep 30 2019

...Western U.S....

...A prolonged heavy snow event focused on eastern slopes of the
northern Rockies in north-central Montana begins Friday afternoon
and continues into Sunday night...

Deep low pressure currently over southeast Alaska shifts south
down the the AK/BC coast to the Pacific Northwest into Friday
before stalling over interior WA/OR through Saturday night. Cold
air from northern Canada is drawn south by this low/trough and
surface high pressure stalling near the northern BC/Alberta border
and reaches the northern MT border Friday night. 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 which is upslope for
north-central MT. Moisture streaming up the Great Plains with
origins over the Gulf of Mexico arrives in MT Friday afternoon and
aids heavy precip rates particularly for eastern slopes of the
Rockies in MT until shortwave activity rounding the low/trough
pushes across the northern Plains in the Sunday night timeframe.

Multiple feet of snow are possible above about 4000ft in the TFX
CWA north of areas west of Helena. Also of particular concern is a
potential band of snow extending across northern and northeastern
MT on the north side of the precip shield Saturday night into
Sunday night. Dynamic cooling in this swath could enhance 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