Winter Storm in Montana, December 11 to 12, 2018

Area Forecast Discussion
National Weather Service Missoula MT
402 AM MST Tue Dec 11 2018

.DISCUSSION...Scattered snow showers continue to move through the
Northern Rockies this morning and are capable of producing a
period of moderate snowfall rates with accumulations up to an
inch. The next front is currently over central Washington and
will be moving through the Northern Rockies later today into
Wednesday morning. Again, this front will bring a considerable
amount of snow and wind related impacts to the region over the
next 24 to 36 hours.

Snow: The mountains along the Montana/Idaho border and around
Glacier NP will see accumulations of 8 to 15 inches by mid day
Wednesday, creating difficult to hazardous driving conditions
over area mountain passes, including Lookout, Lolo, Lost Trail and
Marias passes. The narrower valleys of northwest Montana should
see 2 to 8 inches of snow through this evening. Downsloping
effects will limit snow accumulations in the broader valleys of
western Montana and southwest Montana. These locations should
receive 1 to 4 inches of snow.

Wind: Strong westerly winds will be associated with the system.
The higher terrain will see wind gusts up to 50 mph creating
areas of blowing snow. Southerly winds will be steadily increasing
throughout the day across the Camas Prairie of Idaho with Highway
95 from Whitebird Hill to Grangeville potentially receiving gusts
up to 50 mph. These winds will linger during the onset of snow
Tuesday evening potentially creating difficult driving conditions
south of Grangeville. Another area of concern for winds will be
across southwest Montana and Lemhi County on Wednesday afternoon.
Strong westerly winds gusting to near 40 mph will impact the
MT-200, US-12 and I-90 corridors west of Missoula.

The front exits the region on Wednesday leaving the Northern
Rockies under northwesterly flow aloft. This flow pattern is
conducive to scattered snow showers lingering across the region,
especially the higher terrain of north central Idaho and west
central Montana. A transitory ridge slides over the region on
Thursday, but the models are indicating a stream of moisture will
make its way into northwest Montana potentially increasing the
coverage and intensity of precipitation.

Forecast models have continued their trend in weakening the cold
frontal passage on Friday and even delaying it until Saturday
morning. All this seems to be in response to the main branch of
the polar jet stream being forced northeast across Canada towards
the Northwest Territories. As a positive aside, the much delayed
and weaker front should yield at least breezy southerly winds and
thus a rather warm afternoon on Friday. Temperatures in many
places could run some 10 to 15 degrees above normal and quite
possibly produce our warmest day since before Thanksgiving! As
mentioned earlier. the frontal passage will now likely occur
sometime between late Friday night and early Saturday morning and
could be less than ceremonious with only light snow possible,
mainly in the mountains. This could even prove optimistic if the
weakening trend in the models continues.

For the weekend the models have followed a trend we suspected
last night and the result doesn't appear too bad if you like
drier, relatively warmer weather for the weekend. All the long
range models are now directing substantially more jet stream
energy southward with the next approaching upper level trough.
This is producing a deeper off shore trough and could be more
likely to cut off energy over the southwestern states. Farther
north the leading arch of the jet stream will become north/south
orientated on Sunday leading to a much slower inland progression
of any moisture. Thus chances for precipitation this weekend were
significantly decreased, though variable amounts of lingering
cloud cover should keep temperatures from rising much above
normal.

The next chance for precipitation doesn't enter the forecast realm
until Tuesday when another acceleration in the jet stream pushes
into the west coast. But remaining energy across the southern
subtropical jet stream has us placing lower confidence in these
precipitation chances with the good possibility that the polar jet
is once again forced north into Canada.

&&

.AVIATION...Scattered showers continue to move through the
Northern Rockies this morning. These showers are mainly confined
to the higher terrain of north central Idaho and west central
Montana. A period of clearing last night allowed for the
development of low stratus and fog over portions of Lemhi county
and southwest Montana. KSMN is currently the only terminal being
impacted by these low clouds and fog. The fog should dissipate as
the next system approaches. A strong cold front is still expected
to impact the Northern Rockies later today into Wednesday morning.
This system will cause widespread snow and strong winds. All
terminals will be impacted by this system at some point over the
next 24 to 36 hours.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Warning from 2 PM this afternoon to 11 AM MST
     Wednesday Lower Clark Fork Region.

     Winter Storm Warning from 2 PM this afternoon to 11 AM MST
     Wednesday West Glacier Region.

     Winter Weather Advisory from 2 PM this afternoon to 9 AM MST
     Wednesday Kootenai/Cabinet Region.

     Winter Storm Warning from 2 PM this afternoon to 11 AM MST
     Wednesday Bitterroot/Sapphire Mountains.

     Winter Weather Advisory from 5 PM this afternoon to 11 AM MST
     Wednesday Potomac/Seeley Lake Region.

ID...Winter Weather Advisory from 1 PM this afternoon to 4 AM PST
     Wednesday Orofino/Grangeville Region.

     Winter Storm Warning from 1 PM this afternoon to 10 AM PST
     Wednesday Northern Clearwater Mountains...Southern
     Clearwater Mountains.

     Winter Weather Advisory from 8 PM this evening to 11 AM MST
     Wednesday Eastern Lemhi County...Western Lemhi County.

&&