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144
FXUS65 KMSO 221125
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
424 AM MST MON DEC 22 2014

.DISCUSSION...Northwest flow will keep the threat for showers
across western Montana this morning into the afternoon. Radar
trends have not shown any decrease in coverage and weather model
guidance suggest the potential for continued moderate snow impacts
for southwest Montana. Therefore will extend the winter weather
advisory for the Butte and Blackfoot Regions until 11 am MST this
morning.

Drier air will begin to move into the region by this evening
allowing for shower coverage to decrease. With some clearing
expected tonight and building surface high pressure, freezing fog
could develop in the valleys across western Montana and central
Idaho. Untreated surfaces could become slick in these areas
possibly impacting travel come Tuesday morning. There should be
enough clearing to allow for locations across southwest Montana
with fresh snowcover to cool to near zero.

The big weather story this week is the much advertised strong
winter storm which will move through and impact much of the Northern
Rockies the middle of this week. Several uncertainties the past
several days have limited confidence for the impact of this storm.
These are:   TIMING, LOCATION, SNOW LEVELS AND SNOW AMOUNTS.

Well, confidence has increased for some of these variables.

Northwest Montana...Low pressure will enter far Northwest Montana
(along the Idaho border) Tuesday night and Wednesday, however
impacts will be very minimal with high snow levels.

CONFIDENCE HAS INCREASED that this storm system will extend
southward late Wednesday afternoon to include central Idaho, west
cenrtal and southwest Montana.

CONFIDENCE HAS INCREASED THAT rain will change to snow during the
evening hours Christmas Eve with SNOW LEVELS lowering to the
valley floors. The bulk of precipitation, or snowfall is expected
Wednesday evening through Thursday morning. It`s likely the region
will wake up to a White Christmas morning.

HOWEVER:

CONFIDENCE IS STILL LOW TO MODERATE FOR HOW MUCH SNOW will fall.
Current thinking is the biggest impacts and most confidence will
be over the mountain passes which would impact travel.
Conservative thinking for snow amounts late Wednesday afternoon
through Christmas morning over mountain pases are:

Lookout, Lolo, McDonald, Homestak Passes 3-6"
Lost Trail Pass 6-9"

In general, there is confidence for a conservative 2-3" inch snow
across all of the valleys in western Montana. There is potential
that the narrower valleys in far Northwest Montana near Noxon and
Troy as well as the Bitterroot/Missoula Vallesy to receive upwards
of 5" if the right conditions set up. Stay tuned.

The valleys in central Idaho will likely see a 2-4 inch snow with
locally higher amounts.

Beyond Christmas, models are trending towards a ridge developing
over the gulf of Alaska by this weekend. This set up could allow
cold air from Canada to move into the region. While the coldest
air looks to stay east of the Continental Divide at this time,
colder temperatures should nonetheless be expected late this
weekend into next week.

&&

.AVIATION...Relatively moist and unstable northwest flow will
support showers especially across western Montana and along the
Continental Divide today. Expect periodic ceiling and visibility
reduction in these areas. Drier air will begin to work its way
into the area especially after 22/2200z which will work to
suppress showers by this evening. Can`t totally rule out isolated
snow showers tonight across the highest elevations. Strong surface high
pressure...1037 millibars...will be centered over Lemhi County,
Idaho extending north into western Montana by Tuesday morning.
This will allow for colder...stable and possible fog or stratus
development.




&&

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

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

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

&&

$$





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