Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 211118
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
418 AM MST SUN DEC 21 2014

...DIFFICULT TRAVEL OVER MOUNTAIN PASSES EXPECTED TODAY...
...POSSIBLE WINTER STORM CHRISTMAS EVE DAY INTO CHRISTMAS...


.DISCUSSION...The moist weather system continues to bring a
variety of impacts today. So far several locations have kept
enough cold air in place to keep the precipitation type as all
snow...Highway 83 from Seeley Lake south to Highway 200 including
Greenough Hill all the way to Bonner have experienced at least 4
inches early this morning. Snow has also been observed near Bull
Lake, along US-2 from Libby to Marion and also 2 to 4" has fallen
near Olney north of Whitefish. It`s difficult to tell, but
Lookout Pass has probably seen at least 6" of snow so far with
snow- covered roads.

The Camas Prairie and Grangeville area has been experiencing
strong gusty winds to 50 mph this morning and winds there are
expected to weaken somewhat towards daybreak.

Expect the steady precipitation to transition to showers today.
Strong mid- level winds near 50 mph may be able to mix down
across portions of southwest Montana especially along MT-141 north
of Avon, MacDonald Pass on US-12 this afternoon. These dangerous
cross-winds could impact high profile vehicles in these areas.

High pressure will build over the region Monday and Tuesday.
A few snow showers are expected each day. The biggest impact from
this ridge will be the potential for fog Monday morning and more
likely Tuesday morning.

The ridge will be short lived as a long advertised low pressure
system will move through the region Christmas Eve and much of
Christmas day. For several days there has been considerable
uncertainty for timing and strength of this system. It appears
that models are coming into some agreement regarding timing of
this storm. This system will first impact Northwest Montana
Wednesday afternoon then slide southward impacting central Idaho,
west central and southwest Montana Wednesday evening through
Christmas afternoon. Timing for the best potential for heaviest
precipitation will be Wednesday evening through midday Christmas.

How much snow will fall over the area?? Much of this question
depends on snow levels and where the heaviest boundary of precip
forms. Snow levels will start off fairly low impacting all
mountains and passes with travel concerns. The big question is
when will snow levels fall to the valley floors. Models are coming
into agreement that snow levels will crash to the valley floors
late Wednesday evening and overnight. Most valley locations should
wake up to snowfall on Christmas morning.

Conservative snowfall amounts show anywhere from 4-8 inches in
the mountains and 2-4 inches in the valleys. This could change
considerably depending on where the precip boundary sets up, when
snow levels lower (for valleys) and how long it lingers over an
area. It would not be surprising if amounts are higher increasing
travel impacts for Christmas eve and day. STAY TUNED TO SUBSEQUENT
FORECAST FOR BETTER TIMING AND LOCATION FOR THIS SYSTEM.

Late this week and next weekend high pressure develops off the
west coast leaving the region in a northerly flow aloft. Expect
scattered snow showers each day with slightly below normal
temperatures.

&&

.AVIATION...As the steady precipitation turns to showers by late
this morning there will be some improvement with ceilings and
visibility. Gusty winds will develop as the atmosphere becomes
unstable today. Strong gusts near 40 knots are possible near the
Continental Divide especially from between Helmville and Avon
south to KBTM this afternoon. Numerous snow showers will be
expected through Monday across the Clearwater, Bitterroot, Flint
to Anaconda Ranges and from the Mission Mtns. to Glacier National
Park...so expect lower ceilings and visibility reductions.




&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...WINTER WEATHER ADVISORY until 5 AM MST Monday ABOVE 4000 FEET
     for the Lower Clark Fork Region.

     WINTER WEATHER ADVISORY until 5 AM MST Monday
     Bitterroot/Sapphire Mountains...Butte/Blackfoot Region...
     Potomac/Seeley Lake Region...West Glacier Region.

ID...WINTER WEATHER ADVISORY until 5 AM MST /4 AM PST/ Monday ABOVE
     4000 FEET for the Eastern Lemhi County...Northern
     Clearwater Mountains...Southern Clearwater Mountains...
     Western Lemhi County.

     WIND ADVISORY until 7 AM PST this morning Orofino/Grangeville
     Region.

&&

$$




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