Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 151106
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
406 AM MST Fri Dec 15 2017

.DISCUSSION...

Pockets of freezing fog and low stratus impacting visibility have
once again developed across the valleys overnight, although
passing high clouds have kept the fog from becoming widespread. A
cold front is on track to head into northwest Montana late this
morning/early afternoon, which may initially bring light freezing
rain to some of the northwest MT valleys before turning to all
snow. As this front heads into the rest of northwest Montana
Friday afternoon, gusty west winds can cause areas of reduced
visibility from blowing snow for the US-2 corridor Columbia Falls
east to Marias Pass along the Continental Divide.

The precipitation will continue to push southeast through north-
central Idaho and the rest of western Montana late today, bringing
periods of widespread snowfall throughout the evening commute.
Accumulating snow over I-90 and US-93 will likely result in slower
travel times this evening and through the overnight period.
Mountain pass snowfall amounts are forecast to be around 2-4
inches, while valleys will likely see around 1-3 inches. Winter
Weather Advisories remain in effect associated with the winter
driving hazards across the area.

Lingering snow showers are anticipated through Saturday. In
addition, there is potential for Flathead Lake enhanced snow
showers along Highway 35 due to the instability and local
convergence in place. Showers will then begin tapering off by
late afternoon as drier air moves in.

A strong Pacific jet stream continues to be forecast for the
Northern Rockies during the Sunday-Monday period, bringing
another round of snow to the region. The most significant impacts
from this system will be slick roadways and low visibility at
times from snow and gusty wind over area mountain passes. The
wind, snow combination will be most pronounced along the
Continental Divide (think Marias, MacDonald, Homestake passes).
Snow levels are anticipated to be mostly above the valleys during
this event, however Monday morning could pose a problem with
isolated low elevation slick spots hidden by predominantly wet
pavement. A low probability hazard will be freezing rain as the
airmass over the region slowly warms at the ridgetops.

Unlike yesterday model runs, there doesn`t appear to be a distinct
break in precipitation between the Sunday-Monday system and the
relatively cold system Wednesday-Thursday. In other words, high-
elevation snow will likely continue between systems and keep
pass roadways slick at times, even on Tuesday.

Wednesday-Thursday period continues to appear as a moderate winter
weather impact for most elevations across the Northern Rockies. A
cold core low pressure circulation is expected to drop over the
Great Basin from the north, drawing very cold air from east of
the Continental Divide. Concurrent with the cold air intrusion
west of the Divide, the low-level pattern will be favorable for
snow accumulations at most elevations on Wednesday morning.
Widespread winter driving conditions with slick roadways and low
visibility due to a combination of snow and wind appear plausible.
Valley snowfall will be interrupted Wednesday night by gusty
easterly winds, particularly near Hellgate (I-90) and Badrock (Hwy
2) canyons. Road surfaces near these canyons could be dangerously
slick near the onset of gusty east wind.

Relatively cold air will settle into Northern Rockies valleys
Thursday and Friday, with mainly mountain-based light snow
continuing intermittently. The cold airmass will cool
temperatures to around zero degrees for high valleys near the
Continental Divide by Friday morning.

&&

.AVIATION...
Low stratus and freezing fog will bring reduced visibility and
mountain obscurations to KGPI, KMSO, and KSMN through mid
morning. Strong west winds are increasing over the Continental
Divide and terrain of northwest Montana this morning ahead of the
cold front. As the cold front tracks southeastward today,
light accumulating snowfall will head into northwest Montana
reaching KGPI by approximately 15/2000Z, and then KMSO 16/0000Z.
KSMN and KBTM is forecast to see the front around midnight.
Showers will linger into Saturday.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory from 5 PM this afternoon to 11 AM MST
     Saturday Bitterroot/Sapphire Mountains.

     Winter Weather Advisory from 5 PM this afternoon to 11 AM MST
     Saturday Missoula/Bitterroot Valleys...Potomac/Seeley Lake
     Region.

     Winter Weather Advisory from 11 AM this morning to 11 AM MST
     Saturday Flathead/Mission Valleys...Lower Clark Fork
     Region...West Glacier Region.

     Winter Weather Advisory from 8 AM this morning to 11 AM MST
     Saturday Kootenai/Cabinet Region.

ID...Winter Weather Advisory from 4 PM this afternoon to 10 AM PST
     Saturday Northern Clearwater Mountains...Southern
     Clearwater Mountains.

     Winter Weather Advisory from 4 PM this afternoon to 10 AM PST
     Saturday ABOVE 2000 FEET for the Lower Hells Canyon/Salmon
     River Region...Orofino/Grangeville Region.

&&

$$


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