Winter Weather Advisory 30, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
332 AM MDT Sun Mar 28 2021

...Damaging Winds Expected Across North-Central Idaho and Western
Montana Late Sunday Afternoon into Monday Morning...

.DISCUSSION...A strong pacific cold front remains on track to
impact the Northern Rockies later this afternoon through Monday
morning. High Wind Warnings remain in effect area-wide, with the
exception of the Lower Salmon River/Hells Canyon/Riggins area.
NOW is the time to prepare by securing loose items, moving
vehicles which are vulnerable to fallen trees, charging backup
batteries, etc. Southwesterly winds will begin to ramp up late
this morning as the gradient tightens with the approaching
shortwave trough. High temperatures will reach into the 50s and
60s this afternoon under a pre-frontal southwesterly flow.
Damaging winds will arrive across northwest Montana by mid-
afternoon and will slowly shift southward across west central
Montana and north central Idaho Sunday evening, before reaching
Lemhi County, Idaho after midnight.

* Wind Impacts: The latest model trends suggest the threat for
  wind gusts up to 70 mph has increased across northwest Montana
  from 5 to 9 pm MDT. Specific areas to watch for favored to reach
  more widespread damage include: The US Highway 2 corridor from
  Happy’s Inn to Kila, Montana Highway 56 from Bull Lake to
  Montana Highway 200 junction, the Flathead Valley, and northern
  Swan Valley, and areas along US Highway 2 from Columbia Falls to
  West Glacier. Widespread wind gusts between 50 and 60 mph are
  expected with the cold front as it pushes across west central
  Montana and north central Idaho between 6 to 10 pm MDT, before
  reaching Lehmi County and southwest Montana between 9 pm and 1
  am MDT.

* Winter Impacts: High confidence exists for a period of very
  difficult travel conditions across area mountain passes Sunday
  night into Monday morning, particularly for Lolo and Lost Trail
  passes. A brief period (an hour of two) of heavy snow and
  blowing snow will accompany the cold front across higher
  elevation mountain valleys (Butte, Anaconda, Seeley-Swan
  valleys, etc.) and mountain pass areas. Given the convective
  nature of these snow bands along and behind the front, snowfall
  rates between 1 to 2 inches per hour are expected. With
  temperatures falling rapidly behind the front, any wet or slushy
  roadways will likely freeze overnight into Monday morning.

Snow showers will continue Monday afternoon as the Northern
Rockies remains in a cool northwesterly flow. A few showers may
align into bands. The Bitterroot valley and Butte/Blackfoot region
(Philipsburg, Georgetown Lake, Anaconda) will needed to be
closely monitored for any of these bands which linger into the
early evening hours.

Temperatures will remain below average Tuesday and snow and
graupel showers will redevelop during the afternoon. Relatively
warm surface temperatures will limit any accumulations.

A warming and drying trend will occur Wednesday into Thursday as
a broad ridge of high pressure builds across the western US.
Forecast model confidence decreases Friday into the weekend.
Model ensemble clusters analysis suggests a few scenarios are
possible. One scenario suggests a progressive pacific trough
moving across the Pacific Northwest into the Northern Rockies
Saturday into Sunday. The other scenario keeps the Northern
Rockies under a mild southwesterly flow with a broad upper level
ridge centered over the central Rockies into central Montana.
/Lukinbeal

&&

.AVIATION...A powerful pacific cold front remains on track to
impact the Northern Rockies late Sunday afternoon into Monday
morning. Pre-frontal southwesterly winds will begin to increase by
28/1600-28/1800. Damaging winds will reach northwest Montana by
28/2100-29/0000z and will shift across west-central and north-
central Idaho soon thereafter. Damaging winds will reach into
southwest Montana after 29/0300Z and Lemhi County, ID by 29/0600Z.
The strongest peak wind gusts areawide will likely occur across
northwest and southwest Montana, including KGPI and KBTM, where
isolated wind gusts in excess of 50 knots are possible. In
addition to strong winds, a short burst of precipitation will
accompany the cold front, with visibility reductions expected. A
burst of heavy snow is possible at KBTM from 29/0600-29/0900Z.
/Lukinbeal

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...High Wind Warning from 8 PM this evening to 9 AM MDT Monday
     Butte/Blackfoot Region.

     Red Flag Warning from 1 PM this afternoon to 7 PM MDT this
     evening East Beaverhead.

     High Wind Warning from 8 PM this evening to 9 AM MDT Monday
     Bitterroot/Sapphire Mountains.

     High Wind Warning from 9 PM this evening to 6 AM MDT Monday
     Missoula/Bitterroot Valleys.

     High Wind Warning from 3 PM this afternoon to 6 AM MDT Monday
     Lower Clark Fork Region.

     High Wind Warning from 3 PM this afternoon to 6 AM MDT Monday
     Flathead/Mission Valleys.

     High Wind Warning from 3 PM this afternoon to 6 AM MDT Monday
     West Glacier Region.

     High Wind Warning from 3 PM this afternoon to 6 AM MDT Monday
     Kootenai/Cabinet Region.

     High Wind Warning from 9 PM this evening to 6 AM MDT Monday
     Potomac/Seeley Lake Region.

ID...High Wind Warning from midnight tonight to 9 AM MDT Monday
     Eastern Lemhi County.

     High Wind Warning from 8 PM this evening to 5 AM PDT Monday
     Orofino/Grangeville Region.

     High Wind Warning from 8 PM this evening to 5 AM PDT Monday
     Southern Clearwater Mountains.

     High Wind Warning from midnight tonight to 9 AM MDT Monday
     Western Lemhi County.

     High Wind Warning from 8 PM this evening to 5 AM PDT Monday
     Northern Clearwater Mountains.

&&