Special Weather Statement, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
212 PM MDT Tue May 16 2017

...SNOW DOWN TO 3000 FEET TONIGHT ACROSS NORTHERN ROCKIES...

.DISCUSSION...A significant and complex storm system is still
expected to swing through the Northern Rockies tonight through
Thursday morning. The first aspect of the system will be a cold
front finishing its push through the Northern Rockies this
afternoon and evening bringing widespread precipitation. Snow
levels will remain relatively high during this frontal passage,
so any snow should be limited to the higher terrain. As the center
of the low pressure system moves into southwest Idaho, the second
feature to develop will be a trough of warm air loft (TROWAL),
which is associated with an area of enhance precipitation. This
trowal feature appears to develop over west central Montana later
this evening. The overall model consensus has the the heaviest
precipitation from the southern end of the Bob Marshalls over
Seeley Lake and into the Mission valley. It will be under this
feature that will give the valleys their best chance of impactful
snow, such as heavy snow breaking tree branches causing localize
power outages. Confidence is only moderate on the placement of
this trowal feature, but it will have one of the biggest impacts
to the valleys of western Montana. The low moves into southeast
Idaho and slows down allowing the third and final feature of wrap
around precipitation that will be enhanced by upslope along the
Continental Divide down into this southwest Montana. This will be
a prolong precipitation event that will cause southwest Montana to
receive the heaviest snowfall of all areas. The Butte/Anaconda
area could receive 6 to 8 inches with Georgetown Lake, MacDonald
and Homestake passes getting at least a foot of new snow.

Rain and high-elevation snow showers across the Northern Rockies
will continue through Thursday. Mountain passes along the
Continental Divide could see another dusting of snow turning to
light slush on the pavement Thursday morning, creating a brief
winter driving hazard. Otherwise, a slow warming trend will be
present across the region through Friday with variable cloudiness.

Saturday through Monday, models suggest that mild surge of cool
air from the Canada Prairie will run along the east of the
Continental Divide, increasing precipitation chances and
decreasing snow levels for a time. Snow chances appear greatest
for mountain passes along the Divide Sunday evening through Monday
morning. Most of the snow that falls at mountain pass elevations
will likely turn to slush on the pavement, however there may be a
few hours around sunrise Monday when the snow will stick and
create slick driving conditions on the passes.

Dry and warm high pressure is expected to return to the Northern
Rockies Tuesday and Wednesday next week.

&&

.AVIATION...Winter storm conditions will spread across the
Northern Rockies air space during the next 24 hours. Rain will be
present at times near all area terminals through 17/0200Z, with
cloud bases slowly lowering as well. KBTM will experience a mix of
rain and snow after 17/0200Z, becoming all snow after 17/0600Z
through Wednesday morning. KMSO will experience a mix of rain and
snow after 17/0900Z and continue through Wednesday morning. KSMN
will experience a snow and rain after 17/0500Z, all snow 17/0900Z
through 17/1500Z, and returning to a mix of snow and rain
thereafter. KGPI is anticipated to remain all rain, with the most
significant impact 17/0500Z through 17/1000Z. All snow and
snow/rain mix periods will be accompanied by substantially lowered
ceilings and visibility. Terrain will be obscured for much of the
Northern Rockies, especially after 17/0200Z.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Warning from 2 AM Wednesday to 9 AM MDT Thursday
     ABOVE 5000 FEET for the Butte/Blackfoot Region.

     Winter Storm Warning from 9 PM this evening to 2 PM MDT
     Wednesday ABOVE 4000 FEET for the Bitterroot/Sapphire
     Mountains.

     Winter Weather Advisory from midnight tonight to 10 AM MDT
     Wednesday Flathead/Mission Valleys...Missoula/Bitterroot
     Valleys.

     Winter Weather Advisory from 9 PM this evening to noon MDT
     Wednesday Lower Clark Fork Region.

     Winter Storm Warning from 2 AM Wednesday to 9 AM MDT Thursday
     ABOVE 4000 FEET for the West Glacier Region.

     Winter Storm Warning from midnight tonight to 9 AM MDT Thursday
     ABOVE 3500 FEET for the Potomac/Seeley Lake Region.

ID...Winter Storm Warning from 9 PM this evening to 9 PM MDT
     Wednesday ABOVE 4000 FEET for the Eastern Lemhi County...
     Western Lemhi County.

     Winter Storm Warning from 8 PM this evening to 1 PM PDT
     Wednesday ABOVE 4000 FEET for the Southern Clearwater
     Mountains.

     Winter Weather Advisory from 8 PM this evening to 11 AM PDT
     Wednesday Northern Clearwater Mountains.

     Winter Weather Advisory from 8 PM this evening to 1 PM PDT
     Wednesday ABOVE 4000 FEET for the Lower Hells Canyon/Salmon
     River Region.

&&

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