Winter Storm Watch 6, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
319 AM MST THU DEC 17 2015

...Valley snow transition to freezing rain late Thursday into
Friday...

.DISCUSSION...

Scattered snow showers continued through the early morning hours
across western Montana, this combined with yesterdays snowfall
has left area roads quite slippery, use caution during the morning
commute. Otherwise, the first half of today will be cold but
relatively uneventful weather-wise with attention turning to a
weather system and associated warm front approaching from the
southwest.

The warm front will begin to impact north central Idaho by early
afternoon with a gradual increase in light snowfall. Snow will
transition to rain late this evening for the valleys as warmer air
aloft moves overhead. Pockets of light freezing rain will be
possible late tonight for the lowest valleys, which may lead to
localized slippery road conditions.

Across western Montana, this same warm front will lead to areas of
snow developing this evening. Warm air aloft will transition the
snow to rain for lower elevations on Friday morning. Cold air will
remain trapped in many valleys, setting up a potential for areas
of light freezing rain with the most likely areas being Hamilton
to Missoula northward into the Mission Valley. Also valleys west
of Missoula on I-90 and Hwy 200 will have a chance of seeing freezing
rain. Areas east of Hwy 93 in northwest Montana and area mountain
passes, should see precipitation remain primarily as snow. Travel
could become particularly hazardous on Friday morning, especially
as road conditions become extremely slick and changeable over
short distances.

A cold front will drop southward across the region Friday evening
through Saturday morning ending the freezing rain threat but
resulting in light snow accumulations in the valleys with several
additional inches of snow for mountain passes. Travel over
mountain passes will continue to be hazardous, especially for
Lolo, Lost Trail, Homestake, and MacDonald Passes.

A cool and unsettle pattern will persist through next week.
Periods of light snow will be a near daily occurrence in the
mountains, with frequent light snow or flurries in the valleys
too.

The one really stand-out period next week will be Tuesday into
Wednesday. Models agree well on a cold front during this period
that has strong jet support and a lot of wind with it. How much
wind exactly varies from model to model, but there if this
forecast continues central Idaho could see winds 30 mph with
periods of moderate snow Tuesday night. Back country locations and
the mountains could have several inches of snow and winds 40 mph
causing whiteout conditions. Portions of western Montana would
likely see similar impacts with a little less snow, but similar
winds.

&&

.AVIATION...Patchy freezing fog will continue at KGPI through
about 17/1600Z. High/mid level cloud cover will overspread the
region from west to east today with light to moderate snow
developing around 18/0100Z at KSMN, 18/0300Z at KMSO and KBTM, and
by 18/0900Z at KGPI, then continue into Friday morning. As a warm
front passing through early on Friday snow will transition over to
rain/freezing rain at KMSO by 18/1200-1400Z. Also strong 700-750
mb winds of 50-55 kt Friday morning may lead to some ridgetop
level turbulence.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory until 9 AM MST this morning
     Bitterroot/Sapphire Mountains...Butte/Blackfoot Region...
     Missoula/Bitterroot Valleys.

     Winter Weather Advisory until 9 AM MST this morning
     Flathead/Mission Valleys...West Glacier Region.

     Winter Weather Advisory until 9 AM MST this morning
     Potomac/Seeley Lake Region.

ID...None.
&&