Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 192209

Area Forecast Discussion
National Weather Service Missoula MT
309 PM MST FRI DEC 19 2014


.DISCUSSION...Snow and rain have started falling lightly across
the Northern Rockies already today, and will continue
intermittently through the day Saturday.

Saturday evening the precipitation really starts to increase as
an atmospheric river of warm, moist air streams into the region.
Snow levels will start around 3,500 feet and start to rise
Saturday night to around 6,000 feet in some locations by Sunday
afternoon. This is expected to cause very difficult travel
conditions over mountain passes with heavy wet snow followed by
rain at times. The ample moisture from the atmospheric river will
cause high precipitation rates at times, which in turn may cause
heavy snow. There is still considerable uncertainty with the
timing of snow level changes, and this coupled with the high rate
of precipitation could cause very big errors in the snowfall
forecasts near the pass levels. As a result, a winter weather
watch has been issued that includes all the area passes for
Saturday night through Sunday. As the storm approaches and we
become more confident of snow levels, the watches may transition
to warnings for the high impact areas and advisories elsewhere.

Meanwhile, confidence is high that there will be a lot of snow at
the highest elevations. Above 6,000-7000 feet, snow amounts in
the Bitterroot, Clearwater, and portions of the Mission and Swan
Ranges will be measured in feet.

Sunday night into Monday, snow showers will linger behind a weak
cold front associated with the system, with snow levels falling.
Due to the warm nature of the atmospheric river, very little snow
is expected in the valleys.

Looking ahead to the Christmas holiday, forecast models are
showing a potent storm tracking through the Northern Rockies
Christmas Eve and into Christmas Day. This storm will have the
potential to bring accumulating snow impacting travel. There is
still a great deal of uncertainty however as various models are
tracking the surface low pressure center hundreds of miles apart,
and they are also varying from run to run. These different tracks
could result in anything from snow at valley floors to only at the
higher elevations. We will be keeping a close eye on this storm,
and should be able to provide better information as we get closer
and forecast certainty increases. For now, be aware that holiday
travel may be impacted.

Colder air will move into the area behind the Christmas storm,
pulling temperatures across the Northern Rockies below freezing.
Temperatures will be cold, but not exceptionally so (mostly just
colder than what we`ve seen recently).


.AVIATION...Intermittent showers will continue through the evening
and tomorrow. Expect mountain obscurations with variable ceilings
and moderate visibility restrictions. Precipitation will be mixed
this afternoon becoming mostly snow tonight. KGPI will be most
impacted by visibility and low ceilings at times, though all area
terminals will be affected.



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