Area Forecast Discussion


451 
FXUS65 KMSO 180221
AFDMSO
Area Forecast Discussion
National Weather Service Missoula MT
721 PM MST Sat Feb 17 2018
.UPDATE...Latest radar imagery is showing another round of heavy
snow headed towards the Flathead Valley tonight. Currently the 
winds are sustained at 28 mph at Glacier Park International 
Airport and is expected to increase further tonight with wind 
gusts of 40 to 45 mph expected by around 10 pm. With conditions 
expected to worsen tonight, a Blizzard Warning was issued for the 
Flathead and Mission valleys. 
&&
.AVIATION...Prolonged heavy snow will continue at KGPI today
through Sunday. Gusty east-northeast winds will gust to 35-40 KTS
at times, creating blowing snow as early as 18/0100Z and lasting 
through the day Sunday. This will create near-blizzard conditions
at the terminal that could last as long as 18/1500Z or 
thereafter. Even when snow ends, blowing snow will create poor 
visibility. 
For KMSO: snow will be moderate to heavy tonight through Sunday,
with gusty east winds occurring 18/1100Z onward. Snow may be
transportable, creating a blowing snow issue at the terminal and 
low visibility as a result. Snow intensities tonight between
17/2300Z and 18/0900Z could reach 1 inch per hour at times. 
For KBTM and KSMN: periodically low vis/ceiling will be common
after 18/0200Z through 18/1500Z from snow and blustery east-
northeast winds. Snow could be moderate to heavy at times tonight
before easing up by morning. 
&&
.PREV DISCUSSION... /ISSUED 226 PM MST Sat Feb 17 2018/ 
..WINTER STORM NOW THROUGH SUNDAY...
DISCUSSION...The winter storm is upon us, and impacts are already
beginning to surface across western Montana and north central
Idaho. Since this is a difficult system with many components to
consider, let's hit them one by one:
1) PROLONGED MODERATE TO HEAVY SNOW: Snow has been observed across
most of the Northern Rockies today, with travel conditions
steadily deteriorating with time. The exceptions to this are central
Idaho valleys (Grangeville, Orofino, and Salmon) and the southern
Bitterroot valley, where rain or a rain/snow mix has been 
occurring. NW MT has been seeing continuous heavy snowfall, and is
expected to continue to see persistent heavy snowfall for the 
duration of this storm. For the Missoula valley, the northern 
Bitteroot valley, and along the I-90 corridor from Lookout Pass to
Butte: snow may briefly mix with rain for a period this afternoon
and evening, but will then change back to snow and stay moderate 
to heavy through the night into Sunday. Snowfall rates could be in
the 1 to 2 inch/hour range, particularly across northwest and west
central Montana. Across the board, mountainous terrain will
continue to get hammered with up to two feet of snow by Sunday, 
with winds creating dangerous backcountry conditions. 
2) BLOWING SNOW/BLIZZARD CONDITIONS: The arctic front is still 
expected to move through the region tonight. Gusty winds and 
blowing and drifting snow are going to produce very low 
visibility and poor travel conditions throughout western Montana 
on Sunday. (Central ID is not expected to see these conditions.) 
Drifts may become large enough Sunday morning in the 
Flathead/Mission valleys, and along Highway 2 east and west of 
Kalispell, creating impassible travel conditions. Blizzard 
conditions could become commonplace through Sunday. Cold air will 
consistently move into the region Sunday and Sunday night so the 
main concern will transition to very cold wind chills. 
3) FLASH FREEZE OF WET ROAD SURFACES: For areas who receive rain 
or a rain/snow mix today into tonight, expect dangerous travel
conditions to occur rapidly as colder air moves in and freezes 
water on roadways, creating a layer of ice beneath snow that falls
on top of it. Rapidly changing conditions, in addition to lowering
visibility, will create very dangerous travel Sunday morning. 
4) BITTERLY COLD TEMPS: The entire region will be entrenched with
cold air by Monday morning and wind chills of negative 15 to 
negative 30 will be common. 
Well below normal temperatures will continue Tuesday and Wednesday 
as the arctic air slowly modifies. A few lingering flurries will 
continue on the terrain but overall it will be dry with winds 
becoming light. Forecast models diverge considerably regarding 
snow potential Wednesday through Friday as they vary on the timing
of disturbances in northwest flow. So although the details are 
lacking, northwest flow typically means active weather, so expect periods
of snow showers at the very least. 
AVIATION...Prolonged heavy snow will continue at KGPI today
through Sunday. Gusty east-northeast winds will gust to 35-40 KTS
at times, creating blowing snow as early as 18/0100Z and lasting 
through the day Sunday. This will create near-blizzard conditions
at the terminal that could last as long as 18/1500Z or 
thereafter. Even when snow ends, blowing snow will create poor 
visibility. 
For KMSO: snow will be moderate to heavy tonight through Sunday,
with gusty east winds occurring 18/1100Z onward. Snow may be
transportable, creating a blowing snow issue at the terminal and 
low visibility as a result. Snow intensities tonight between
17/2300Z and 18/0900Z could reach 1 inch per hour at times. 
For KBTM and KSMN: periodically low vis/ceiling will be common
after 18/0200Z through 18/1500Z from snow and blustery east-
northeast winds. Snow could be moderate to heavy at times tonight
before easing up by morning. 
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Warning until 5 PM MST Sunday Butte/Blackfoot 
     Region...Potomac/Seeley Lake Region.
     Winter Storm Warning until 5 PM MST Sunday Bitterroot/Sapphire 
     Mountains.
     Winter Storm Warning until 5 PM MST Sunday Missoula/Bitterroot 
     Valleys.
     Winter Storm Warning until 5 PM MST Sunday Lower Clark Fork 
     Region.
     Blizzard Warning until 11 AM MST Sunday Flathead/Mission Valleys.
     Winter Storm Warning until 5 PM MST Sunday West Glacier Region.
     Winter Storm Warning until 5 PM MST Sunday Kootenai/Cabinet 
     Region.
ID...Winter Weather Advisory until 5 PM MST Sunday Eastern Lemhi 
     County...Western Lemhi County.
     Winter Storm Warning until 4 AM PST Sunday Northern Clearwater 
     Mountains...Southern Clearwater Mountains.
&&
$$