Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 181101
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
400 AM MST THU DEC 18 2014

...SNOW TURNING TO HEAVY RAIN THIS WEEKEND IN CENTRAL IDAHO...

.DISCUSSION...

A very light snow and rain mixture will be present across the
Northern Rockies, particularly northwest Montana and portions of
north central Idaho through today. Precipitation is not
anticipated to be very long-lasting, arriving in waves and lasting
less than an hour per occurrence, however isolated travel impacts
due to recent thin cover of snowfall are possible across much of
northwest Montana. As temperatures warm a bit during the daytime,
the impact of precipitation to the public will lessen
dramatically, with the exception of mountain passes such as
Lookout Pass.

A slightly more potent weather system will move over the Northern
Rockies from the Pacific Ocean Friday evening through Saturday,
bringing several inches snow to mountain pass elevations and at
least a dusting of snow to middle and high elevation valleys.
Snowfall on area roadways, particularly over mountain passes, may
accumulate and create hazardous driving conditions overnight
Friday through midday Saturday. Otherwise, this first weekend
system appears to go through a similar fate of recent systems over
the Northern Rockies, slowly dissipating on a eastward trajectory.
Precipitation and cloud cover will inhibit much warming on
Saturday, however the airmass of this weather system will not be
strikingly different from the airmass currently in-place.

The models are still consistent with the atmospheric river moving
into the Northern Rockies Saturday night through Monday. The
question still remains on how quickly will snow levels rise during
this event. At the onset of precipitation snow levels will be low
enough to cause significant snow on area mountain passes,
especially Lookout pass and Lolo pass by Sunday morning. Then the
precipitation will change over to rain for even the mountain
passes on Sunday with the exception being Lost Trail. Lost Trail
has the potential to see up to a foot of wet and heavy snow during
through out this system. Idaho and Clearwater counties could
receive widespread amounts 1.5 to 2 inches of rain Saturday night
into Monday. This rain could cause a risk of rock and mud slides
particular for Highway 12 and 14.

The upper level flow will transition to a northwesterly trajectory
which will steadily lower snow levels across the region. A few
weak disturbances will move through the area causing period of
widespread snow showers. Again the models are showing a cold
trough of low pressure digging over the region by Christmas.
Certainty is still fairly low on the extent of the impacts from
this system as models diverge on the strength and timing of this
trough.

&&

.AVIATION...Intermittent light snow and rain mix will be present
across portions of northwest Montana and north central Idaho
throughout today. At KGPI, ceiling and visibility will vary a
great deal between bouts of very light precipitation. Terrain will
be obscured for much of the next 24 hours across northwest
Montana, while occasional obscurations will exist elsewhere.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...None.
ID...None.
&&

$$



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