Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 012147

Area Forecast Discussion
National Weather Service Missoula MT
247 PM MST TUE DEC 1 2015

.DISCUSSION...Cold pools and valley inversions remain entrenched
in the valleys of western Montana and to a lesser extent central
Idaho. The extended cold period from these has caused the ground
to become very cold with sub-surface ground temperatures dropping
in the upper 20s, much of the ground is frozen hard. The frozen
ground and entrenched inversions has set up the potential for
freezing rain with the next approaching system.

A weak disturbance remains on track to impact portions of the
Northern Rockies beginning Wednesday morning, and continuing
through the night. Precipitation is expected to be light with this
system, however there is the possibility of warmer air overrunning
the cold pools creating a light freezing rain/drizzle. The amount
of warm air and resultant liquid precipitation remains uncertain,
though right now, northwestern Montana has the highest likelihood
of seeing this outcome. The most probable scenario will be
widespread light snow, with isolated pockets of freezing rain. The
disturbances causing the rain are very small features embedded in a
larger pattern of developing southwest flow aloft. As a result,
timing remains uncertain for the timing of the precipitation. Just
yesterday models suggested precipitation throughout the day
Wednesday, while today they indicate the precipitation in the
morning and overnight with a break during the day. So confidence
in the timing is low.

Most valleys will remain in the cold pools through the day on
Wednesday and some locations through Thursday. By Thursday the
southern end of most valleys like Eureka and Hamilton should be
out of the inversion and cold pools. The higher elevation valleys
like Anaconda, Butte, and Georgetown will also mix out by
Thursday. But Locations such as Missoula, Libby, and Salmon will
likely remain cold and inverted through most of the day Thursday.

By Thursday afternoon, a weather system will be approaching the
Idaho/Montana border. Warm, southwest flow ahead of the cold front
will bring rain to the mountain passes with snow levels above
6000 feet. A cold front will pass through Thursday night into
Friday which will bring lowering snow levels and snow
accumulations to mountain passes. Currently we are looking at
about 2 to 5 inches of snow for Lookout, Lolo, and Lost Trail
passes Friday into Friday evening. Valleys could see some light
snow as the cold front moves through Friday, but no real
accumulation is expected.

Behind the cold front, a brief ridge of high pressure will build
on Saturday. A cool, dry day is expected for Saturday with the
exception of northwest Montana, where we could see a few light
showers linger. Starting Sunday and into next week, the weather
pattern will become active with Pacific storms moving through the
region every other day. Above normal precipitation especially for
mountains, and near normal temperatures are expected through the
first half of next week.


.AVIATION...Low stratus is expected to continue for the valleys of
northwest Montana, with KGPI often seeing ceilings below 1500
feet. A weak disturbance may also lead to some very light snow or
freezing rain on Wednesday at KGPI. KMSO will likely see freezing
fog in the vicinity of the airport early on Wednesday, that will
drift around and occasionally reduce visibility but incoming mid-
level cloud cover may limit the intensity and coverage of fog
compared to recent days. Otherwise, KBTM and KSMN may see patchy
freezing fog in the vicinity of the airport on Wednesday morning
but it is not expected to limit operations.



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