Heavy Snow in Idaho, December 1 to 3, 2013

Area Forecast Discussion...UPDATED
National Weather Service Missoula MT
1150 PM MST Sat Nov 30 2013

.UPDATE...Upgrading the winter storm watch to a winter weather
advisory for the Flathead and Mission valleys. Freezing rain is
currently being reported across the Flathead valley and will
create slick travel conditions. Snow is expected in these
locations by Monday, making for a prolonged period of travel
difficulties.

&&

.AVIATION...Periodic snow/rain showers with pockets of freezing
rain will impact aviation across the Northern Rockies during the
next 24 hours. Valley fog and freezing fog will persist within
some valleys, while others will experience low stratus. Terrain
obscurations will be commonplace during this time. The snow/rain
showers will become much more numerous after 01/1800Z, marking the
beginning of a prolonged winter storm scenario over the region.

&&

.PREV DISCUSSION... /ISSUED 1118 PM MST Sat Nov 30 2013/

UPDATE...Upgraded the winter storm watch to a winter storm
warning for the Kootenai/Cabinet region in northwest Montana.
Freezing rain is currently being reported with ice forming on
roadways making for slick travel conditions. Snow will also come
into the picture tomorrow, especially for locations above 2500
feet. Travel conditions will continue to deteriorate through Monday.

.PREV DISCUSSION... /ISSUED 937 PM MST Sat Nov 30 2013/

UPDATE...Adding to the complexity of the upcoming weather
forecast, we have issued a Wind Advisory for the Grangeville area
and the Salmon River Drainage along Highway 95 starting Sunday
morning and continuing into Monday morning.

DISCUSSION...Conditions favor increasing south winds across much
of north-central Idaho and western Montana Sunday. However, the
Grangeville area and south along Highway 95 through the Salmon
River Drainage should some of the strongest winds with potential
gusts up to 55 mph later Sunday morning through early Monday
morning. Once the cold front passes through the area Monday
morning, winds will become more westerly and not quite as strong.

PREV DISCUSSION... /ISSUED 353 PM MST Sat Nov 30 2013/

HEAVY MOUNTAIN SNOW FOLLOWED BY ARCTIC AIR MONDAY NIGHT...

DISCUSSION...

Tonight a complex weather system really starts to impact the
region. Because this is such a complex storm system, we want to
highlight the biggest impacts that are expected:

1) HEAVY MOUNTAIN PASS AND BACK COUNTRY SNOWFALL: An atmospheric
river of tropical origins is currently making its way towards the
west coast and the Cascades. After causing heavy rain and flooding
there, the moisture plume will head eastward and impact the
northern Rockies. Areas of upslope flow in central Idaho and
western Montana will be hardest hit. This river will bring strong
winds and 200 percent of the normal amount of water to the western
slopes of the northern Rockies.

Mountain snow will be measured in feet with this system by the
time it is done, and Lolo, Lost Trail, and Lookout passes will be
have heavy snow tomorrow afternoon and evening. Those on
back country roads should be prepared for heavy snow so as to not
become trapped.

2) VALLEY RAIN CHANGING TO SNOW/CHANCE FOR MIXED PRECIP: While the
heavy snow is impacting the mountains, many of the valleys will be
warm enough for rain. Some isolated pockets of cold air may
persist in northwest Montana and cause very localized icy
conditions. Freezing rain Sunday morning is expected to be much
less than this morning. Throughout the night precipitation may
turn from rain to snow, and then possibly back to rain again
Monday morning.

Expect variable and rapidly changing conditions in the valleys of
northwest Montana, especially in the foothills surrounding the
valleys. Travel conditions will be difficult with the potential
for snow, slush, and ice through Monday morning.

3) ARCTIC FRONT: Timing is difficult as models still cannot agree
upon when the arctic front will arrive. The current consensus is
for northwest Montana to be impacted Monday late afternoon or late
evening and the front moves across western Montana and central
Idaho by late Tuesday morning. Confidence is high that the cold
air is coming, however timing remains uncertain.

There are several impacts that will make being outdoors or
traveling next week difficult and dangerous at times.
- Flash freezing of wet roads with the arrival of the cold air
- Low visibility and difficulty driving with blowing snow
- North winds 20-30 mph, especially through Hellgate and Bad Rock
canyons.
- Wind chills below -20 across northwest Montana Wednesday morning
- Coldest temperatures in 2 1/2 years from central ID through
western MT

The pattern will remain very cold through next weekend, with only
marginal chances of additional light snowfall. Overnight
temperatures will consistently be in the sub-zero and single digit
range for western Montana valleys, while central Idaho valleys
will be only slightly warmer. Daytime temperatures will remain
well below freezing. There is little sign of a pattern change
even after next weekend.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...WINTER STORM WATCH from Sunday evening through late Tuesday
     night Butte/Blackfoot Region...Missoula/Bitterroot Valleys.

     WINTER STORM WARNING until 1 AM MST Wednesday
     Bitterroot/Sapphire Mountains...Lower Clark Fork Region.

     WINTER WEATHER ADVISORY until 1 AM MST Wednesday
     Flathead/Mission Valleys.

     WINTER STORM WARNING until 1 AM MST Wednesday Potomac/Seeley
     Lake Region...West Glacier Region.

     WINTER STORM WARNING until 1 AM MST Wednesday Kootenai/Cabinet
     Region.

ID...WINTER STORM WATCH from Sunday evening through late Tuesday
     night Eastern Lemhi County...Western Lemhi County.

     WINTER STORM WARNING until Midnight PST Tuesday Night Northern
     Clearwater Mountains...Southern Clearwater Mountains.

     WIND ADVISORY from 10 AM Sunday to 7 AM PST Monday for the Camas
     Prairie...Grangeville and Highway 95 IN THE Lower Hells
     Canyon/Salmon River Region...Orofino/Grangeville Region.

&&

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