Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 122134

Area Forecast Discussion
National Weather Service Missoula MT
234 PM MST Tue Dec 12 2017

.DISCUSSION...The valley inversions are still the main story for
a bit longer. Patchy to areas of freezing fog and low stratus
continues to be the biggest impact for the next 12-18 hours. A
weak disturbance moves over the top of the ridge Wednesday morning
bringing some light precipitation to portions of west central
Montana into southwest Montana. The aforementioned area could
receive 0.5" to an 1.00" of new snow. A concern exists that this
weak feature could become a seeder/feeder into the trapped valley
inversions, so a few locations could receive even higher amounts.
Confidence is low on this seeder/feeder scenario.

The ridge rebounds Wednesday night into Thursday, keeping the
valley inversions entrenched. This inversion pattern will finally
come to an end on Friday. A cold front will move out of British
Columbia and bring light to moderate snowfall back to the
mountains of north central Idaho and western Montana. Impacts will
be mostly confined to mountain pass roadways crossing Lost Trail,
Lolo, Lookout, and Marias passes. Valleys may see up to an inch
of snow through Saturday, but the better chance for valley
snowfall will come late Sunday through Tuesday. During this time,
the entire western United States will undergo a longwave pattern
change to a much more active, moist weather pattern. Valley
snowfall, cold temperatures, and abundant mountain snow will all
be more prevalent as a result throughout the week next week.

In fact, long term forecast models have shown good agreement this
far out for a potentially significant winter storm to develop on
or around December 20th. Again, this is quite a ways out in the
forecast, but one thing is certain: inversions are leaving us, and
snowfall is returning!


.AVIATION...The strong valley inversions persist tonight, with
low stratus and freezing fog being the biggest impact to the
region`s terminals. A weak disturbance will be moving through the
ridge Wednesday morning, providing an opportunity for light snow
showers (especially to west central and southwest Montana). This
feature moves out by the afternoon and the strong ridge rebounds
with continued strong valley inversions.



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