Winter Storm and Winter Weather in Idaho, November 4 to 5, 2022

Area Forecast Discussion
National Weather Service Missoula MT
332 AM MDT Fri Nov 4 2022

...HEAVY MOUNTAIN SNOW AND STRONG WINDS THROUGH SATURDAY...
...ARCTIC AIR TO BRING COLD IMPACTS AND SNOW NEXT WEEK...

.DISCUSSION...The leading edge of a multi-day Atmospheric River
event has begun to move over the Northern Rockies this morning.
This wave of moisture will bring light to moderate snowfall to
the mountains and valleys along the MT/ID border through mid
morning today. Winter Weather Advisories remain in place for
north-central Idaho, the Lower Clark Fork region, as well as the
Kootenai/Cabinet regions, where the valleys could see around 1 to
3 inches of snow. The lowest valleys in north-central Idaho, such
as Orofino, will only see trace amounts to 1 inch in the grassy
areas. The grades and benches above Orofino and vicinity will see
slightly higher amounts of around 4 inches. While 1 to 3 inches
of snow doesn't seem that much, the heavy and wet nature of the
snow will increase the risk of tree limb and power line damage
across these areas. Breezy southerly winds this morning will also
impact travel for US-95 from Whitebird Hill to Grangeville and
vicinity, with low visibility due to blowing snow.

The bulk of the moisture associated with the Atmospheric River
will arrive this afternoon, and continue to stream overhead
through Saturday. This storm system will bring a host of impacts
that will effect travel throughout the region. First, Snow levels
will briefly rise to around 6000 feet this afternoon and evening
as warm air advection accompanies the surge of moisture. Snow
levels will quickly lower back down to valley floors Saturday
morning as the cold front pushes through. Heavy snow and strong
winds will create very difficult to impassable conditions in the
backcountry during this time, as remote trails and roads could
become blocked due to downed trees and/or snow drifts. Winter
Storm Warnings remain in place for the Clearwater Mountains in
Idaho, and the Bitterroot/Sapphires in western Montana from 11AM
PDT/Noon MDT today through tomorrow morning.

Another impact from this storm system will be strong damaging
winds that will develop ahead and behind the Pacific cold front
that will begin to track across the region tonight. The front
will first impact northwest Montana after midnight tonight, then
central Idaho and western Montana soon after sunrise Saturday
morning. High Wind Watches remain in place for the Flathead,
Mission, Missoula and Bitterroot valleys and for the
Butte/Blackfoot region, where wind gusts of 50-60 mph are
possible. High resolution models suggest convection along the
front could help mix down even stronger winds gusts for a brief
period of time, especially in the Bitterroot Valley. Highly
traveled corridors along Hwy-2 near MacDonald Pass and I-90 near
Butte, could see travel further impacted due to the potential for
snow squalls.  A burst of heavy snow, strong winds, and a rapid drop in
temperature will create an environment favorable for roadways to
quickly freeze. Atmospheric conditions are also looking quite
favorable for snowbands developing across the rest of western
Montana by late Saturday evening and overnight. If these bands do
develop, isolated areas of moderate to heavy snowfall will be
possible for both the valleys and mountains through early Sunday
morning.

Ensemble model guidance remain in good agreement with a Pacific
trough deepening southward across the Pacific Northwest Sunday.
This will aide in the southerly progression of an Arctic airmass
moving out of Alberta Canada and over the Continental Divide into
western Montana. The arctic front is expected to move into
northwest Montana by Sunday morning, then the rest of western
Montana late Sunday night. A dramatic decrease in temperatures,
along with gusty east-northeast winds and brief periods of
moderate to heavy snow can be expected along the front. High
temperatures Monday are not expected to eclipse 20F across
northwest Montana, and not get out of the 20s for west-central
Montana. This will be quite the shock to the system, as well as to
anything not winterized for the season.

We continue to watch for the potential of a surge of moisture
moving over the Northern Rockies Monday and Tuesday, as the
Pacific low pressure system moves on shore. This moisture will be
overrunning the arctic airmass already in place, that will
promote widespread snowfall for all elevations. Stay tuned as we
refine the forecast in the next few days.

&&

.AVIATION...South winds have developed across the Northern Rockies
this morning and are expected to increase in magnitude throughout
the day. Additionally, radar is demonstrating an area of moisture
pushing into the region, at KMSO, KGPI, and KSMN ceilings are
expected to lower in response. Also, a period of light and
potentially at times, moderate snow is expected at KGPI and KSMN
before changing to light rain. After 05/0000z winds will increase
gusting 25 to 30 kts at area terminals, it will not be until the cold
front passage on Saturday when gusts will peak between 38 to 50
kts. Additionally, post frontal snow levels will quickly decrease
to valley levels and snow squalls will be possible during
Saturday evening at KGPI, KMSO, and KBTM greatly reducing
visibility and ceilings at times.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...     Winter Storm Warning from noon today to noon MDT Saturday ABOVE
     5000 FEET for the Bitterroot/Sapphire Mountains.

     Winter Weather Advisory until noon MDT today for
     Kootenai/Cabinet Region.

     Winter Weather Advisory from noon today to noon MDT Saturday
     ABOVE 5000 FEET for the Potomac/Seeley Lake Region...West
     Glacier Region.

     High Wind Watch from late tonight through Saturday morning for
     Flathead/Mission Valleys.

     Winter Weather Advisory until noon MDT today for Lower Clark
     Fork Region.

     High Wind Watch from Saturday morning through Saturday afternoon
     for Missoula/Bitterroot Valleys.

     High Wind Watch from Saturday morning through Saturday afternoon
     for Butte/Blackfoot Region.

ID...Winter Weather Advisory until 11 AM PDT this morning for
     Northern Clearwater Mountains...Southern Clearwater
     Mountains.

     Winter Storm Warning from 11 AM this morning to 11 AM PDT
     Saturday ABOVE 5000 FEET for the Northern Clearwater
     Mountains...Southern Clearwater Mountains.

     Winter Weather Advisory until 11 AM PDT this morning for
     Orofino/Grangeville Region.

     Winter Weather Advisory until 11 AM PDT this morning for Lower
     Hells Canyon/Salmon River Region.

     Winter Weather Advisory from midnight tonight to 3 PM MDT
     Saturday for Eastern Lemhi County...Western Lemhi County.

&&