Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 082144

Area Forecast Discussion
National Weather Service Missoula MT
244 PM MST Thu Dec 8 2016

...Periods of snow into early next week...

.DISCUSSION...A weakening weather disturbance will push a surge of
moisture northward over Idaho and western Montana this evening
into early Friday. A moderate amount of snow will cause travel
impacts from the Camas Prairie and Orofino eastward across the
southern Bitterroots and Lemhi County. Further north and east
across western Montana, the snow intensities will become lighter
with relatively little or no snow making it to northwest Montana.
The light snow should reach the Camas Prairie and Salmon early
this evening and Missoula to Drummond around midnight.

Friday...during the morning and through about early afternoon, the
northern Rockies will generally be between weather systems with
only very light scattered precipitation. Temperatures will
gradually moderate into Friday.

The second of a series of weather disturbances will move through
the Northern Rockies Friday evening and Saturday morning. This
particular system will have more moisture associated with it
compared to the event Thursday night, so more widespread light to
moderate impacts can be expected for the mountains and valleys of
western Montana and north-central Idaho. One potential impact
that needs to be watched, is the possibility for snow bands to
develop over west-central Montana and the Clearwater Mountains of
Idaho. If these bands do form, much higher impacts for the valleys
in particular would be possible. Otherwise, several inches of
snow will be possible in north-central Idaho and generally 2 to 5
inches for the valleys of western Montana.

Light snow showers could persist throughout the region during the
day on Saturday, before a 3rd weather system moves in Saturday
evening. This one too, will spread a wide swath of light snow
through western Montana and north-central Idaho. Snow band
potential will also be a concern for west-central Montana in the
overnight hours.

Monday the models both bring an arctic cold front into northwest
Montana and thus support another round of decent snow across much
of the Northern Rockies. Strong winds crossing the Continental
Divide with the surge of cold air could produce periods of blowing
and drifting snow along US-2 through Bad Rock Canyon, Columbia
Falls and into Kalispell. But the southward extent of these winds
and even the cold air are still something the models don`t really
agree upon right now. The key difference between the GFS and ECMWF
appears to be in their depiction of a potential interaction
between the polar jet stream and sub- tropical jet stream. The GFS
has a much weaker sub-tropical jet stream and subsequently allows
a full arctic intrusion into Montana and Idaho. However the ECMWF
develops a stronger sub-tropical jet stream, supporting an
atmospheric river event through the Great Basin states and thus
prevents arctic air from fully engulfing the Northern Rockies
until later next week.

By mid-week models do agree that a potential period of clearing
should lead to some very cold conditions area wide. It could be
even colder should the GFS prove correct. Regardless of model
solution, a very active and potentially significant weather
pattern is shaping up for next week which supports both the
potential for more snow and then extreme cold.


.AVIATION...A pleasant day Thursday will shift to more wintry
precipitation areas as the first of a few weather disturbance
moves in overnight. KSMN can expect to see impacts due to light
snow by 09/0300Z this evening, with KMSO and KBTM seeing the
impacts by 09/0800Z. Impacts for these sites will be reduced
visibility, ceilings, and light snow accumulations. Snow should
end by the late morning hours for these aviation sites. Most of
the impacting snow is expected to stay south of KGPI, though light
snow is still possible. There will be a break in the snow through
the afternoon hours Friday, before a more significant disturbance
moves in Friday evening.


MT...Winter Weather Advisory from midnight tonight to 9 AM MST Friday
     Lower Clark Fork Region.

     Winter Weather Advisory from 8 PM this evening to 8 AM MST
     Friday Bitterroot/Sapphire Mountains.

     Winter Weather Advisory from 11 PM this evening to 9 AM MST
     Friday Missoula/Bitterroot Valleys.

ID...Winter Weather Advisory from 7 PM this evening to 9 AM PST
     Friday Orofino/Grangeville Region.

     Winter Weather Advisory from 6 PM this evening to 9 AM MST
     Friday Eastern Lemhi County...Western Lemhi County.

     Winter Weather Advisory from 7 PM this evening to 7 AM PST
     Friday Northern Clearwater Mountains...Southern Clearwater


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