Winter Storm Watch 5, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
213 PM MST Wed Feb 26 2014

...Strong Arctic Push with Blizzard Conditions Possible in parts
of Western Montana Thursday night and Friday...

.DISCUSSION...Weak high pressure will remain over the region
tonight with cool, but dry air persisting in the valleys of
north-central Idaho and western Montana.

Thursday through Sunday Morning...

The increasing southwest flow and associated moisture on Thursday
is a bit delayed in the latest data, not spreading across central
Idaho and western Montana until Thursday afternoon. This will
shift the potential for mixed precipitation and freezing rain
towards Thursday afternoon into Thursday evening, which could be
both a positive and a negative. The good: Freezing rain and ice
have a harder time accumulating on surfaces during the day, even
when the sky is expected to be cloudy. The bad: This will shift a
slight chance for freezing rain into the evening hours where
surfaces will have a chance to quickly loose their heat and
potentially become icy. The area with a slight chance for freezing
rain will be narrow, but possibly quite significant. The greatest
areas of concern for freezing rain are along I-90 from St. Regis
to Butte, US-93 from Missoula to Hamilton, MT-200 from Ravalli to
Trout Creek and in NW Montana MT-56 (Bull Lake Road) and US-2 from
Libby to Troy.

An arctic air mass poised over central Canada will be invading
western Montana beginning Thursday night into Friday, eventually
moving across north-central Idaho Friday night. This is an
exceptionally cold air mass which will be ushered into western
Montana by very strong northeast winds. The greatest wind and cold
impacts will be targeting western Montana all the way through the
weekend, while north-central Idaho impacts will mostly be centered
around steady light to occasionally moderate snow.

WIND AND SNOW: Northeast winds will be developing through the gaps of
the Continental Divide late Thursday afternoon, eventually surging
into the northern Flathead valley Thursday evening. Cold easterly
winds will punch through the Blackfoot region Thursday night,
spilling into the Missoula valley by early Friday morning. Wind
speeds associated with this influx of arctic air will be 25 to 35 mph
sustained with potential gusts in excess of 50 mph locally. The
combination of these winds, lots of light fluffy snow already on the
ground and several inches of new snow will create considerable
blowing and drifting. In fact, local blizzard conditions
(visibilities less an 1/4 mile and roads blocked by snow drifts)
are quite possible through the gaps of the Divide, the northern
Flathead and Missoula valleys.

Speaking of new snow: Models are generating a focused area with
the potential for heavy snow early Friday morning generally 50
miles north and south of the I-90 corridor where an additional 3
to 5 inches, locally up to 9 inches are expected. As was the case
last weekend, the heaviest amounts will occur in a narrower region
which we hope to further define over the next 24 hours.

WIND AND COLD: The combination of strong winds and a flood of colder
and colder air into the western Montana will create dangerously low
wind chill values. Initially, nasty wind chills will be confined
along the Continental Divide Thursday night and Friday. However, as
arctic air advances through the rest of western Montana, wind chills
will become a real hazard Friday night through Saturday night. Wind
chill values of 20 below to 40 below zero are possible. Newborn
livestock would be highly vulnerable under these conditions.

NORTH-CENTRAL IDAHO IMPACT: Mountain snow and valley rain can be
expected Thursday through Friday with mountain passes becoming snow-
covered and slick. However, as colder air moves into Idaho, snow
levels will lower to the valley floor Friday night/Saturday. A
westerly flow at the mid/high levels of the atmosphere will maintain
a steady influx of Pacific moisture falling mostly as light to
moderate snow. Accumulating snow is expected at all elevations making
travel difficult. There will also be a threat for freezing rain
during this same time in some of the narrow valleys of north
central Idaho, including portions of US-12 generally from Powell
to Lolo Pass, ID-11 from Weippe to Pierce, Grangemont and along
ID-14 to Elk City. The chance for freezing rain will gradually end
Thursday night into Friday morning as colder air begins its surge
across the region.

For late Sunday through mid next week...

A more westerly flow is expected to develop early next week. This
should direct surges of moisture across north central Idaho and
western Montana. This milder westerly flow should cause
temperatures to gradually moderate. However, there is a lot of
uncertainty in timing of systems/surges of moisture and how quick the
valleys should moderate. Typically (and after every arctic air mass
we have seen this past winter) the forecast model guidance warms
up the valleys much too fast and see no reason it will not happen
again.

&&

.AVIATION...Areas of low stratus in the Bitterroot Valley, Mission
Valley and Camas Prairie will slowly erode and dissipate tonight.
Otherwise, high clouds will gradually increase over the northern
Rockies overnight.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...BLIZZARD WATCH from late Thursday night through Friday afternoon
     Missoula/Bitterroot Valleys...Potomac/Seeley Lake Region.

     BLIZZARD WATCH from Thursday evening through Friday morning
     Flathead/Mission Valleys...West Glacier Region.

ID...None.
&&

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