Winter Storm Watch 4, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
356 AM MST Fri Feb 3 2017

...Mix of valley snow, rain and freezing rain this weekend will
create slick travel...

...Feet of snow expected in the mountains/mountain passes this
weekend into early next week...

.DISCUSSION...Very cold temperatures have developed across western
Montana and central Idaho this morning, with most locations in the
single digits for central Idaho and well below zero across western
Montana. An approaching weather system will begin to increase
cloud cover this morning, which will keep temperatures from
warming much in the valley locations.

The first wave of moisture in a series of very moist weather
systems will begin to spread into central Idaho and western
Montana this afternoon. Most locations will initially start out as
snow this afternoon. However, as warmer air works into the region,
pockets of freezing rain are expected to develop tonight into
Saturday morning, particularly through central Idaho and west
central Montana. Further north, across northwest Montana, enough
cold air is expected to linger to allow precipitation to fall as
all snow through Saturday. Needless to say, slick roads should be
expected area wide for travelers this weekend, with changeable
conditions over short distances. By Saturday afternoon, an
increase in winds aloft is expected to begin to cause some
downsloping effects, mainly for larger valleys such as the
Missoula/Bitterroot and Mission valleys. Breezy afternoon winds
are expected to allow the atmosphere to mix, which will bring many
valleys up into the mid to upper 30s. As precipitation type
transitions to rain across the lower valleys of Idaho on Saturday
into Sunday, the combination of melting snow and rain could cause
some drainage problems and ponding of water on roadways. (Please
see the HYDROLOGY portion of this discussion for more
information.)

Precipitation on Sunday will continue to be focused over the mountains
along the ID/MT border, as well as the mountains along the
Continental Divide (including and north of the Mission/Swan
ranges). These areas will continue to be pummeled by moisture
Sunday, Monday, and Tuesday as well. Additional one to two feet of
snow is possible during this timeframe down to mountain pass
levels.

For valleys, strong upper level winds will make it tough for much
moisture to reach valley floors due to downsloping effects Sunday
into Monday. The exception to this could be the northern Flathead
valley, the Tobacco valley, and west through Libby/Yaak/Troy.
These locations could overcome the strong winds and keep snow
falling throughout this time period.

Further enhancement of precipitation will then occur Monday
afternoon with decreasing downsloping effects, causing measurable
precip to finally make it down to the majority of valley floors.
Northwest Montana will likely remain as mostly snowfall Sunday
into Monday, however there could be a brief period of rain mixing
with snow Monday before the cold front finally crosses. Locations
who experienced rain or a rain/snow mix up to this point will
transition to all snow by Monday night.

Impacts during this time will continue to be intense, heavy,
dense mountain snowfall and inclement backcountry conditions, in
addition to a sloppy rain/snow mix in valleys. Additional snowmelt
due to elevated temperatures and snow levels will keep low-land
flooding one of our biggest concerns. Standing water on roads and
properties will be likely.
 &&

.AVIATION...The next Pacific weather system will start spreading
moisture over north central Idaho early this morning and into
western Montana by mid morning. Areas of light snow will gradually
spread into the region, mainly affecting KMSO and KGPI between
03/1500z and 03/2100z. Widespread obscurations of the terrain due
to snow and low ceilings should be anticipated. Late this
afternoon and evening, precipitation may begin to transition to a
mix of snow and freezing rain across central Idaho and west
central Montana. Airfield KMSO stands the best chance of seeing
some light freezing rain this evening and overnight tonight.
Meanwhile, moderate to heavy snow is expected to develop across
northwest Montana this evening and overnight, impacting KGPI with
low ceilings and visibility.

&&

.HYDROLOGY...A period of prolonged, intense precipitation will be
likely for north central Idaho and northwest Montana locations
Saturday through Monday. Rising snow levels and moderating valley
temperatures will be conducive to melting snow, and will be
coupled with prolonged, moderate to heavy rainfall. Initially, the
snowpack will absorb precipitation, but given the very long
timeframe for continued rain and melting snow, further impacts
will be possible, if not probable. Of most concern will be runoff
and standing water on roadways and properties, particularly those
with poor drainage.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory from 3 PM this afternoon to 5 PM MST
     Sunday Bitterroot/Sapphire Mountains.

     Winter Weather Advisory from 2 PM this afternoon to 5 PM MST
     Sunday Lower Clark Fork Region.

     Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
     Sunday Flathead/Mission Valleys...Kootenai/Cabinet Region...
     West Glacier Region.

     Winter Weather Advisory from 6 PM this evening to 11 AM MST
     Saturday Potomac/Seeley Lake Region.

     Winter Weather Advisory from 4 PM this afternoon to 11 AM MST
     Saturday Missoula/Bitterroot Valleys.

ID...Winter Weather Advisory from 2 PM this afternoon to 4 PM PST
     Sunday Northern Clearwater Mountains...Southern Clearwater
     Mountains.

     Winter Weather Advisory from 2 PM this afternoon to 11 AM MST
     Saturday Eastern Lemhi County...Western Lemhi County.

     Winter Weather Advisory from 11 AM this morning to 10 AM PST
     Saturday Orofino/Grangeville Region.

&&