Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 172231
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
331 PM MST Sun Dec 17 2017

...VERY HEAVY SNOW EXPECTED THIS WEEK...

...DANGEROUSLY COLD BY CHRISTMAS DAY...

.DISCUSSION...Freezing rain has been reported in northwest
Montana this afternoon and will remain possible until there is
sufficient moisture for a return of snow, which we expect this
evening. However the threat for freezing rain will transition
southward this evening and overnight to portions of north central
Idaho and west central Montana. Additionally, gusty winds is
expected through Grangeville this evening and with the recent
snow, blowing snow will be possible this evening into Monday.

Moisture will continue to increase tonight, especially in the
Clearwater Mountains of NC Idaho and in NW Montana where the high
terrain are expected to begin a prolonged snow event. Periods of
intense snowfall are likely to develop overnight and through
Monday morning over Lookout Pass and Marias Pass. Northwest
Montana valleys will also see some snow through Monday, but snow
levels remain a challenging aspect of the forecast. The main snow
event arrives Monday night into Tuesday across NW Montana when a
period intense snowfall is expected north of and including Highway
2, the Glacier Region, Marias Pass, and I-90 over Lookout Pass.
The higher terrain, Lookout Pass and Marias Pass have the
potential for many feet of snow, while the low elevations should
see anywhere from 9-15 inches by early Wednesday. Gusty winds in
the high terrain and over mountain passes will cause blowing snow
as well.

The Tuesday into Wednesday snow event across northwest Montana
will come to an end Wednesday once the next trough and cold
fronts arrive. This will shift the snow southward and bring
increased west winds Wednesday along with colder temperatures. A
Canadian cold front is also likely to pass Wednesday afternoon and
evening, which will bring gusty north and east winds. Periods of
intense snowfall, low visibilities, and very slick roadways are
possible with each of the cold fronts on Wednesday. Snow will
quickly decrease late Wednesday with drier air moving into the
region.

But the active weather doesn`t just end there! This morning`s
long range models have all continued to show a significant shift
towards a much deeper upper level trough digging out of British
Columbia and across the the Northern Rockies/Pacific Northwest on
Friday. The parent low responsible for this energy appears to have
origins from not just the Gulf of Alaska, but also associated
with an extremely cold Polar Low over NE Siberia. This developing
trough would certainly be cold on its own. However, as it digs
south Thursday through British Columbia, the energy will also draw
frigid arctic air in place across central Canada westward over
the Canadian Rockies. By Friday morning the trough will be
entering the NW USA and all operational models
(GFS/ECMWF/Canadian) along with their ensembles agree that a
significant push of frigid arctic air will enter from both the
north and east, spilling into the Continental Divide with
impressive force. Winds gusting to near 50 mph will be possible
through Bad Rock and Hellgate Canyon as well as a top Marias and
MacDonald Passes. Moisture will be limited with this system, but
the preexistence of cold air should aid in efficient snow
production. Thus blowing and drifting, along with the additional
falling snow could become quite problematic if your Christmas
travels begins on Friday.

Light snow may continue Friday night into Saturday morning. But
these impacts will quickly pale in comparison to the extrodinarily
cold air mass that models show taking up residence next weekend
and into the following week. Breezy conditions will likely remain
in place overnight into Saturday, resulting in a dangerous wind
chill threat. Temperatures will be slow to warm Saturday as winds
gradually calm. And Christmas Day could be one of the coldest in
many years with low temperatures that morning below zero and high
temperatures struggling to rise into the single digits above zero.
But the cold is likely to not end there and quite possibly become
even more impressive as all models show a reinforcing shot of
arctic air spilling into Montana by Monday morning with the
coldest, most stable conditions quite possible during the
following week.

The fact that all long range models show this pattern with a
striking amount of consistency lends towards an unusually high
amount of confidence in the extended forecast. Please keep
abreast of the latest updates and forecast developments if you
are planning holiday travel this week, next weekend and beyond.

&&

.AVIATION...A messy mix of snow, freezing rain and rain is to be
expected at KGPI and KMSO this evening and overnight before
transitioning to all rain Monday morning. Rain is likely to remain
in the forecast through Tuesday before transitioning back to snow
by Wednesday morning. Meanwhile, a few snow showers will be
possible at KBTM and KSMN this evening, but for the most part this
airfield will remain downsloped from precipitation. But terrain
areawide is expected to be frequently obscured by showers and low
clouds.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...     Winter Storm Warning until 11 AM MST Wednesday Lower Clark Fork
     Region.

     Winter Weather Advisory until 11 AM MST Monday Flathead/Mission
     Valleys.

     Winter Storm Watch from late Monday night through Wednesday
     morning Flathead/Mission Valleys.

     Winter Storm Warning until 11 AM MST Wednesday West Glacier
     Region.

     Winter Weather Advisory until 11 AM MST Monday Kootenai/Cabinet
     Region.

     Winter Storm Warning from midnight Monday night to 11 AM MST
     Wednesday Kootenai/Cabinet Region.

     Winter Weather Advisory until 11 AM MST Monday Potomac/Seeley
     Lake Region.

     Winter Storm Watch from late Monday night through Wednesday
     morning Potomac/Seeley Lake Region.

ID...Winter Weather Advisory until 10 AM PST Monday Northern
     Clearwater Mountains.

     Winter Weather Advisory until 10 AM PST Monday Southern
     Clearwater Mountains.

&&

$$



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