Winter Storm Watch 1, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
229 PM MST Sat Jan 7 2017

.DISCUSSION...Radar and Satellite show an area of precipitation
moving into Central Idaho this afternoon. This is being caused by
a slowly approaching warm front, and will cause light snow
tonight through much of tomorrow. It has been almost a week since
we had any new snow on the roads, so a winter weather advisory has
been issued for some locations in central Idaho and West Central
Montana.

Tomorrow, as more warm air moves into the area, some occasional
freezing rain or sleet will impact portions of Central Idaho as
the warm air overruns an entrenched cold air mass. Camas Prairie
and other low elevation locations in central Idaho will likely
transition to rain fairly quickly with southerly winds and
precipitation. The Salmon River valley in Lemhi County, Idaho will
be the exception. It is unlikely this system will be able to scour
out the very cold air in that valley. Even if it does break out,
it has been so cold for so long down there that it will take
considerable time for exposed ground and roads to warm up, so any
rain that falls would still freeze.

In Western Montana the concern is also the cold air entrenched in
the valleys. It has been very cold for a long time, and the sub-
surface temperatures are below 20F, so if and when precipitation
Sunday night into Monday turns to rain, it will freeze on the
roads. The most likely locations for freezing rain or sleet in
Western Montana are the Bitterroot Valley and Interstate 90 from
Missoula to Haugan. There is also a chance for freezing rain in
portions of the Mission Valley and the Highway 200 corridor from
Ravalli to the Idaho state line. It is also possible that most of
the precipitation falls as snow in Western Montana, however it is
very likely that at least some of it falls as freezing rain on
Monday Morning.

Winter storm watches have been issued for Lemhi County Idaho, the
Bitterroot Valley, and Interstate 90 from Missoula westward. While
confidence on the amount and timing of freezing rain remain low,
these are the locations that have the highest risk for both amount
and duration of freezing rain.

Scattered snow showers linger around the region on Tuesday causing
a couple of inches of new snow in the higher terrain with minimal
impact in valley locations. The models continue to indicate a more
significant system pushing through the Northern Rockies on
Wednesday. This system could bring a significant amount of snow to
region. Current forecast path of the storm appears to be north
central Idaho into southwest Montana. This storm system appears to
pull in arctic air over the Continental Divide. The models are
disagreeing on how cold it will get, but temperatures should fall
to below average readings.

For the far extended, going into next weekend, forecast models
are in good agreement with a short wave ridge building over the
Northern Rockies. However it doesn't look like it will last very
long. Another weather disturbance will move into the region
Saturday and Sunday with more mountain snow, and possibly valley
snow.

&&

.AVIATION...High and mid level clouds have begun to stream over
the Northern Rockies ahead of our next weather disturbance. Light
to moderate snow will spread north through Idaho into western
Montana overnight. Snow will begin impacting KSMN with lowering
visibility and ceilings around 08/0200 this evening. Impacts due
to snow will begin for KMSO and KBTM by 08/03-0400Z, and KGPI
after midnight around 08/0800Z. Snow accumulations for the air
terminals are expected to be light with 1 to 2 inches in a 12
hour period.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory until 5 PM MST Monday
     Bitterroot/Sapphire Mountains.

     Winter Storm Watch from Sunday afternoon through Monday
     afternoon Lower Clark Fork Region...Missoula/Bitterroot
     Valleys.

     Winter Weather Advisory until 2 PM MST Sunday Lower Clark Fork
     Region.

     Winter Weather Advisory until 2 PM MST Sunday
     Missoula/Bitterroot Valleys.

ID...Winter Weather Advisory until 2 PM MST Sunday Eastern Lemhi
     County...Western Lemhi County.

     Winter Weather Advisory until 10 PM PST Sunday Lower Hells
     Canyon/Salmon River Region...Orofino/Grangeville Region.

     Winter Weather Advisory until 4 PM PST Monday Northern
     Clearwater Mountains...Southern Clearwater Mountains.

     Winter Storm Watch from Sunday afternoon through Monday
     afternoon Eastern Lemhi County.

&&