Winter Storm in Montana, November 10, 2015

Area Forecast Discussion
National Weather Service Missoula MT
259 PM MST MON NOV 9 2015

...SNOW IMPACTING ELEVATIONS ABOVE 2500 FEET THROUGH THURSDAY...
...MORE SNOW IMPACTS ACROSS NORTHERN ROCKIES WEDNESDAY/SATURDAY...

.DISCUSSION...Satellite loop reveals a very broad trough centered
over the western US this afternoon. Embedded within this giant
circulation aloft is found a mid-level low pressure system,
centered over the Idaho and Montana border, and a second wave
found over the Oregon and Idaho border.

The first feature has aided in bringing much of the rain and snow
so far today and is expected to track northeast towards north-
central Montana by later tonight. This first low has already
enhanced the pressure (wind) gradient over the Continental Divide
and therefore cooler air has been slowly been filtering westward
with gusts to 30 mph being felt in the Flathead Valley and also
from Helmville to MacDonald Pass. As this wave tracks eastward by
this evening, the atmosphere will begin to cool and allow for a
transition from rain to snow to occur across western Montana. We
issued a winter weather advisory for northwest Montana, Flathead
and Mission Valleys to allow for slushy accumulation of snow then
refreezing tonight when temperatures fall into the 20s. Expect
periods of light to moderate snow and even perhaps brief periods
of heavy snow in some of the higher locations across southwest
Montana this evening as the cold front connected with this wave
traverses east. The timing of this heavier period of snow may be
closer to between 5 and 9 pm especially affecting areas along I-90
east of Missoula to Garrison Junction, Georgetown Lake and
MacDonald Pass.

In central Idaho, snow levels will drop this evening as colder air
moves in from the north on the backside of the trough. Whitebird
Grade, Lolo Pass, and Lookout Pass will all see snow intensify
in the evening. Road surface models show the pavement reaching
freezing temperatures, so some travel impacts are likely. As the
night goes on, snow levels are expected to reach the Camas
Prairie, the benches around the Clearwater River, and on down to
around 3,000 feet, potentially bringing the first widespread snow
of the season. Northerly flow looks to bring continued snow
showers into the early night. Travel impacts should be minimal as
road surfaces might not get cold enough for accumulations in those
areas. However, given the potential impact of the first snow of
the season, we have a winter weather advisory in place.

The secondary wave is expected to track ever so slowly eastward
towards Dillon by early Tuesday afternoon. This system will keep
snow continuing from the Bitterroots of Idaho through southwest
Montana through Tuesday morning. This system could help keep snow
falling as far north as the Mission Valley Tuesday morning.

Following a relatively dry Tuesday overnight, a fast-moving
Pacific storm system will move across the Northern Rockies between
Wednesday afternoon and Thursday midday. This system will be
accompanied by a strong jet stream, enhancing snow amounts over
mountain pass elevations while keeping most broad valleys
downsloped and relatively dry for most of Wednesday evening. By
pre-dawn Thursday, northwest flow behind the main system will
allow banded snow features to intensify over both mountains and
valleys. There will be a remarkable difference between very snowy
and fairly dry conditions within only a matter of several miles. In
the event that these bands of snow park over a valley location for
several hours, up to 6 inches snowfall and sudden treacherous
driving conditions will develop.

Models suggest that another strong Pacific jet stream will move
over the Northern Rockies on Saturday, with very similar impacts
to the Wednesday/Thursday system. Forecast confidence in this
system coming to pass is currently low due to inconsistencies
among recent model iterations. Even beyond Saturday, there is
little evidence that suggests the constant barrage of storm
system off the Pacific will slow down at all. Colder than average
temperatures appear likely to continue through the next week as
well.

&&

.AVIATION...Lowered ceilings and visibility along with widespread
terrain obscuration will continue through this evening and
overnight as a weather system rolls through the Northern Rockies.
Snow levels are expected to lower to around 2500 feet tonight which
will likely bring snow down to many valley floors, including all
terminal sites. Impacts due to low visibility and ceilings should
be anticipated, especially between 10/0000z and 10/1800z. In
addition, gusty northeast winds across northwest Montana will
affect KGPI through the evening.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
     Tuesday Butte/Blackfoot Region...Potomac/Seeley Lake Region.

     Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
     Tuesday Bitterroot/Sapphire Mountains.

     Winter Weather Advisory from 11 PM this evening to noon MST
     Tuesday Missoula/Bitterroot Valleys.

     Winter Weather Advisory until noon MST Tuesday Lower Clark Fork
     Region.

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

     Winter Weather Advisory until 1 PM MST Tuesday West Glacier
     Region.

     Winter Weather Advisory from 5 PM this afternoon to 11 AM MST
     Tuesday ABOVE 2500 FEET for the Kootenai/Cabinet Region.

ID...Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
     Tuesday Eastern Lemhi County...Western Lemhi County.

     Winter Weather Advisory from 4 PM this afternoon to 4 PM PST
     Tuesday Southern Clearwater Mountains.

     Winter Weather Advisory from 4 PM this afternoon to 11 AM PST
     Tuesday ABOVE 3000 FEET for the Lower Hells Canyon/Salmon
     River Region...Northern Clearwater Mountains...
     Orofino/Grangeville Region.

&&