Ice Storm in Idaho, December 7 to 8, 2015

Area Forecast Discussion
National Weather Service Missoula MT
434 AM MST MON DEC 7 2015

...Winter driving conditions over Lookout & Lolo Passes this morning...

...Very wet and windy across the entire Northern Rockies through
Thursday...

.DISCUSSION...The first in a series of strong weather disturbances has
started to impact the Northern Rockies this morning with
precipitation entering NW Montana and NC Idaho. Snow levels appear
to be running around 4000-4500 feet. Expect snow levels to remain
fairly steady this morning then gradually rise during the
afternoon and evening. It's snowing over Lolo and Lookout passes
and cameras indicate accumulations over the roads. Some brief
heavy snow could occur over these passes which would limit
visibility. This combined with 2-4 inches of snow expected has
prompted the issuance of a winter weather advisory for Lolo and
Lookout Passes through noon, when snow levels should increase and
accumulating snow impacts will end.

A second surge of precipitation will slam central Idaho by early to
mid afternoon. This strong westerly flow pattern is very
favorable for orographic lift, which will enhance precipitation
over the mountains, including Lost Trail Pass. Moderate to heavy
snow at times is expected between mid afternoon and late evening.
The combination of 4-6 inches of snow, limited visibility, and heavy
snow at times has prompted a winter weather advisory for Lost
Trail Pass.

Outside of mountain pass snow TODAY, impressive rainfall amounts
of 1-2 inches are likely in the central Idaho and Northwest
Montana mountains. Winds will be breezy again this afternoon, especially
for the Camas Prairie in Idaho. Grangeville in particular is
expected to again see gusts of 30 to 45 mph during the afternoon
and evening, as southerly winds become strong. Other valley
locations should see breezy winds, 15 to 25 mph with some higher
gusts. Mountains will have 40 to 60 mph winds as moisture streams
in from the Pacific. On Tuesday, winds will again be breezy.

A couple of more strong surges of moisture and wind will again
slam into the Northern Rockies and provide more abundant rains
with snow levels remaining above all pass levels. The first one
will be TUESDAY AFTERNOON/EVENING, and then another WEDNESDAY
AFTERNOON/EVENING. A cold front wednesday will cause strong winds
to shift from southerly to a more westerly direction. Valleys can
expect windy conditions for a few hours as the front passes, with
gusts to 30+ mph.

Each storm is riding through an impressive westerly 150 to 170
knot jet that extends back into the central pacific. As mentioned
above, these strong westerly wind and moist air provide the
ingredients for high amounts of precipitation due to enhanced
orographic lift. Total rainfall amounts this week will be
widespread 2-5 inches in the mountains of central Idaho, Northwest
Montana, and West Central Montana. Local amounts in far Northwest
Montana may reach 6 inches.

Lesser amounts are likely in the valleys this week.
Mission/Flathead Valleys: 0.75" to 1.25".
Missoula Valley: Less than 0.50".
Bitterroot Valley: Less than 1.00".
Butte: Less than 0.25".

IMPACTS from storms this week.
Excessive runoff.
Ponding of water on roads.
Rock slides and mudslides.
Avalanche concerns in the higher elevations.
Strong gusty winds in the mountains and Camas Prairie at times.

By the end of the week through early next week the pattern shifts
to one of a trough across the Northwest U.S. as the strong jet
stream buckles to the south. Colder air will settle over the
region. Under the trough expect unsettled weather, or daily snow
showers.

&&

.AVIATION...Showers and vicinity showers are forecast for the next 24
hours across the area. KBTM is the one TAF site that should be
mostly shadowed from rain until Monday evening, and may only see
an increase in cloud cover until then. Nevertheless,
mountain obscurations will be widespread. Precipitation type is
expected to be rain, since snow levels rise above valleys before
precipitation moves in. Breezy winds will be present this
afternoon, and by the evening, winds aloft become strong, quickly
increasing with height as a frontal feature moves through the
strong flow aloft.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory until noon MST today for I-90 Lookout
     Pass to Haugan in the Lower Clark Fork Region.

     Winter Weather Advisory until noon MST today for Lolo Pass in
     the Bitterroot/Sapphire Mountains.

ID...Winter Weather Advisory from noon today to 11 PM MST this
     evening for Lost Trail Pass in the Eastern Lemhi County.

&&