Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 201053

Area Forecast Discussion
National Weather Service Missoula MT
353 AM MST Sun Jan 20 2019

.DISCUSSION...The low pressure system that is expected to bring
winter impacts to the Northern Rockies today and overnight is
beginning to take shape over southeastern Washington this morning.
Light snow is forecast to start falling in north central Idaho
just before sunrise and then track east into western Montana later
this morning. The snow level remains high enough to where most of
the impacts in Idaho will be above 3500 feet. However, a quick
inch of wet snow is possible as low as the Grangeville area early
this morning.

For western-central Montana, snow will begin in the mountains and
on mountain passes along Interstate 90 around 8 AM. For the
valleys, in particular the central and southern Bitterroot Valley,
precipitation will start off as a rain/snow mix until cooler air
wraps around the low pressure system later this afternoon and
evening. This warm layer could even extend up to Lolo. The
location and thickness of the warm layer will dictate how much
snow accumulates in the valleys, including Missoula. As the low
migrates east to the Continental Divide wrap around moisture and
cooler air will aide in an enhanced band of precipitation and is
expected to extend from Missoula, north into the Mission and
Flathead Valleys. Areas north of Missoula now have the best chance
for the higher snow accumulations for the duration of the storm.

A moist northerly flow will set up behind the low pressure this
evening and overnight that will keep light to at times moderate
snow in the forecast. By Monday morning a storm total of 2 to 4
inches of snow, with isolated higher amounts, are expected for
the valleys of west-central Montana. For northwestern Montana,
higher amounts of up to 6 inches is possible. The mountains and
mountain pass of western Montana and north-central Idaho could see
up to 8 inches.

Models are still showing a brief period of stability which should
lead to some overnight clearing Monday night into Tuesday morning.
This could result in a period of excellent radiational cooling and
have thus gone a bit colder than guidance in some of the more
favorable cold spots (Potomac, Polebridge, Georgetown Lake, etc.).
But the clearing is short lived with clouds expected to steadily
increase throughout the day and light precipitation develop during
the afternoon and overnight, especially across Clearwater and
Idaho County.

A more robust weather system will pass over the Northern Rockies
on Wednesday. Although this feature moves through the flow rather
expediently, the dynamics aloft remain more than sufficient for a
period of heavier snowfall along the Idaho/Montana border and
northern Continental Divide area (Bob Marshall Wilderness &
Glacier NP). Meanwhile valley locations will still see some snow,
especially early Wednesday before warmer air raises the snow level
to between 3000 and 4000 feet during the afternoon. Overall
mountain snow accumulations of 6 to 12 inches seem likely with the
valleys experiencing 1 to 4 inches of snow by Wednesday evening.

Thereafter the Northern Rockies is forecast to remain under
northwest flow aloft in between strengthening high pressure
along the Pacific Coast and a deepening, extremely cold low
pressure trough becoming established across much of the eastern
USA. This type of flow pattern is notoriously challenging to
forecast since it tends to possess just enough moisture to produce
some light snowfall, but still not enough to amount to much
regional impact. Temperatures will remain near normal with a
tendency towards slightly colder than normal given the chance for
valley cold pool development.


.AVIATION...Our next winter storm is taking shape in southeastern
Washington this morning and will be moving over the Northern
Rockies through the day today and overnight. Light to moderate
snowfall is expected to impact KMSO by 20/1500Z this morning.
There could be periods this afternoon around 20/2000Z when rain
mixes with the snow, before turning back to all snow this evening.
KGPI will start seeing the snow after 20/1800Z, and is expected
to be all snow for the duration of the event. Light snow will
continue for both terminals until early Monday morning, where KMSO
could see up to 4 inches, and up to 6 inches for KGPI. KBTM and
KSMN will likely miss the higher accumulations, though will see
light snow from this morning through tomorrow morning. Periods of
low visibility and ceilings will still occur.


MT...Winter Weather Advisory until 8 AM MST Monday Butte/Blackfoot

     Winter Weather Advisory from 8 AM this morning to 8 AM MST
     Monday Bitterroot/Sapphire Mountains...Lower Clark Fork
     Region...Potomac/Seeley Lake Region.

     Winter Weather Advisory from 8 AM this morning to 8 AM MST
     Monday Missoula/Bitterroot Valleys.

     Winter Weather Advisory from 11 AM this morning to 8 AM MST
     Monday Flathead/Mission Valleys...Kootenai/Cabinet Region...
     West Glacier Region.

ID...Winter Weather Advisory until 8 AM MST Monday Eastern Lemhi
     County...Western Lemhi County.

     Winter Weather Advisory until 7 AM PST Monday Northern
     Clearwater Mountains...Southern Clearwater Mountains.

     Winter Weather Advisory until 8 PM PST this evening ABOVE 3500
     FEET for the Lower Hells Canyon/Salmon River Region...
     Orofino/Grangeville Region.


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