Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 102150

Area Forecast Discussion
National Weather Service Missoula MT
250 PM MST Sun Dec 10 2017

.DISCUSSION...High pressure remains firmly in place across much
of the western United States of America which has resulted in the
formation of strong valley inversions. Within these inversions we
are seeing persistent low clouds (stratus), freezing fog and
stagnant air. Some light snow flurries are even being seen under
the stratus as higher level clouds aloft `seed` moisture into the
stratus and much colder valley inversion layer. This
seeder/feeder process is particularly challenging to forecast and
thus a fairly broad bush of occasional flurries are in the
forecast each time we believe sufficient moisture exists. However
even the most productive periods of flurries will produce no more
than a skiff of snow and minimal, if any impact. Otherwise expect
temperatures each day to follow a very similar trend, with
mountains seeing pleasant, warm days and cold clear nights while
the valleys experience continued cold conditions with very narrow
diurnal differences between daytime high and morning low

Confidence remains high that the Northern Rockies will see a
pattern change from this stagnant high pressure pattern we`ve
been stuck in by the end of the day Friday. Details remain murky
at this time, though snow will return to the mountains first on
Friday. Depending on how long it takes for the valleys to break
out of the inversions, will determine how long precipitation
remains snow for the valleys. Freezing rain could be a concern at
some point on Friday and into the weekend as well. Long range
forecast models are in fairly good agreement with a cold system
moving into the Northern Rockies next week that would likely give
snow to all elevations.


.AVIATION...Strong valley inversions continue to be the main story
across the valleys of western Montana and north-central Idaho. Low
stratus and/or fog will continue to impact aviation sites KGPI,
KMSO, and KSMN over the next 24 hours. Light snow flurries will
also be possible, especially around KMSO that could help lower
visibility even further. However, nothing more than a trace is



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