Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 161057
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
357 AM MST Sat Dec 16 2017

.DISCUSSION...A secondary wave of energy moving through northwest
flow aloft has re-invigorated snow showers this morning,
generally along and south of the Interstate 90 corridor. These
showers will likely continue into mid morning, generally weakening
and decreasing in coverage by this afternoon. Roads will remain
slick across much of western Montana and central Idaho, so travel
with care!

A large subtropical ridge over the eastern Pacific Ocean will extend
north and east towards the Northern Rockies from Sunday into early
next week. This will result in a steady atmospheric stream of
moisture with some rise in snow levels during this period. Light
snow will begin by Sunday morning across central Idaho to
northwest Montana and intensify in the mountains later in the
day.

Another surge of moisture is still expected Sunday night into
Monday. Moderate snow will fall at times at mountain passes along
the Montana and Idaho border and at Marias Pass. For valleys on
Monday morning, snow will change to rain or freezing rain which
will impact the morning commute. Also with increasing westerly
winds aloft causing down-sloping some western Montana valleys
will see a break in the precipitation Monday morning. Expect snow
to continue in the mountains during the day Monday. High
temperatures will range between 6 to 8 degrees above normal.

With an increasing southerly wind gradient over central Idaho and
with recently snowfall, blowing and drifting snow could be an
issue from Whitebird Hill to Grangeville and the southern Camas
Prairie by Sunday evening and continuing towards daybreak Monday.
This would lower visibility and could make travel along portions
of Highway 95 difficult.

A feed of relatively warm and moist air will continue Monday night
into Tuesday morning, particularly for the Clearwater Mountains in
Idaho and northwest Montana. This will continue decent snowfall
for the higher terrain. Valleys will likely turn back to snow
Monday night, especially in northwest Montana, then transition
back to rain or a rain snow mix during the day Tuesday as a warm
front moves through the region.

The forecast still looks interesting starting Tuesday night into
Wednesday as a Canadian cold front meets with moist westerly flow
aloft. As cold air surges southward at the surface late Tuesday
night into Wednesday, a rapid transition in precipitation type
from rain to snow will occur for most all elevations. A period of
moderate to heavy snow will be possible, particularly across
northwest Montana near the Continental Divide Wednesday morning.
This coupled with breezy winds could lead to a period of very
difficult travel during the Wednesday morning commute. Cold air
will be fully entrained over the region by Wednesday evening,
however drier air should bring an end to snow from north to south.

Northwesterly flow looks to develop on Thursday and remain into
the weekend. This flow pattern appears to be relatively cold and
fairly dry, but weak disturbances have the potential to bring
light snow.

&&

.AVIATION...Snow showers with lower ceilings and visibility will
linger this morning as an upper level trough moves through. KMSO
and KBTM will have the best chance for lingering snow showers.
High pressure builds in by tonight and Sunday morning which will
allow for some valley fog or stratus to form from central Idaho
into western Montana. Moisture moving around the upper ridge will
bring light snow to northwest Montana by 17/1200Z Sunday morning.
Also another concern will be the increased riming potential thanks
to a very moist layer between 6,000 and 15,000 feet MSL Sunday
morning.


&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory until 11 AM MST this morning
     Butte/Blackfoot Region.

     Winter Weather Advisory until 11 AM MST this morning
     Bitterroot/Sapphire Mountains.

     Winter Weather Advisory until 11 AM MST this morning
     Missoula/Bitterroot Valleys...Potomac/Seeley Lake Region.

     Winter Weather Advisory until 11 AM MST this morning Lower Clark
     Fork Region.

ID...Winter Weather Advisory until 10 AM PST this morning
     Orofino/Grangeville Region.

     Winter Weather Advisory until 11 AM MST this morning Eastern
     Lemhi County...Western Lemhi County.

     Winter Weather Advisory until 10 AM PST this morning ABOVE 2000
     FEET for the Lower Hells Canyon/Salmon River Region.

     Winter Weather Advisory until 10 AM PST this morning Northern
     Clearwater Mountains...Southern Clearwater Mountains.

&&

$$


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