Winter Storm in Idaho, February 4 to 7, 2020

Area Forecast Discussion
National Weather Service Missoula MT
204 PM MST Tue Feb 4 2020

.DISCUSSION...Tonight kicks off a very active period of weather
for central Idaho and western Montana. An atmospheric river will
bring a lot of moisture, followed by warm air and even more
moisture during a period extending from tonight through Friday.
This is a very atypical atmospheric river for the region. Normally
they approach from the southwest or the west, but this one is
approaching from the northwest. As a result, we don't have any
good historical systems to compare to it.

Snow will start tomorrow across the region with rapidly rising
snow levels in central Idaho tomorrow afternoon. Most of the lower
elevations of central Idaho will start as snow early tomorrow
morning and transition to rain by the evening. Higher elevation
locations like Elk City, Dixie, Pierce, Weippe, and Lolo Pass will
stay snow into the evening with the potential for 8 to 12 inches
of snow depending on location.

Snow levels will be slower to rise in western Montana, where it
will be late Thursday morning or Thursday afternoon before the
transition from snow to rain starts for valleys. During this
transition, some valleys may receive some freezing rain, with
locations in northwest Montana (like Kalispell to Eureka) being
the most likely to be affected by freezing rain.

The heaviest precipitation will be in the Bitterroot and
Clearwater mountains of central Idaho and western Montana, where
two day storm totals will be over 2 inches of liquid equivalent.
The highest elevations will see most of that, with around 2 feet
of snow likely. The Sapphire mountains, southern Mission, and
southern Swan mountains will also have similar amounts of
precipitation. With the warm air coming in with the atmospheric
river, expect dense, wet snow to create a slushy mess in a lot of
locations. Winter Storm Warnings have been issued the mountains
of central Idaho, with many other locations having Winter Weather
Advisories issued due to the impacts of the slushy, wet snow that
is expected.

Friday, the amount of moisture starts to decline with some cooler
air moving into the region. However, unstable conditions and
plenty of moisture left over will promote snow bands Friday
morning. These can cause very rapidly changing conditions.

The active weather pattern will continue through the weekend as
an upper level low moves into the Northern Rockies. Model guidance
has come into better agreement with the evolution of a cold
frontal boundary moving across eastern Washington into western
Montana Saturday morning and afternoon. Widespread precipitation
will develop along this boundary, with snow accumulating down to
2,000 feet. Mountain locations can expect to see another round of
moderate snowfall Saturday and Sunday. Although valley
accumulations remain uncertain, at least minor snowfall
accumulations (1-2 inches) are likely for most areas.

The weather pattern next week looks to remain active as a large
block of high pressure remains centered near 40N, 150W. This will
keep the Northern Rockies in a cold northwesterly flow pattern
with weather systems periodically expected to impact the area.

&&

.AVIATION...Regional radar and satellite imagery this afternoon
shows the leading edge of the atmospheric river has pushed onshore
in the Pacific Northwest. Low to mid level cloud decks will
continue to stream into the Northern Rockies this evening with
terrain obscurations becoming common. Light snowfall will spread
across north central Idaho and western Montana tonight. KMSO,
KGPI, and KSMN will see periods of MVFR snowfall tonight through
Wednesday. Precipitation rates will increase late Wednesday
morning through the afternoon. Periods of IFR and potentially
LIFR visibility from snowfall are possible at KMSO and KSMN after
05/1500z.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory from 1 AM Wednesday to 5 AM MST Thursday
     Bitterroot/Sapphire Mountains.

     Winter Weather Advisory from 5 AM Wednesday to 5 AM MST Thursday
     Potomac/Seeley Lake Region.

     Winter Storm Watch from late Wednesday night through late
     Thursday night Potomac/Seeley Lake Region.

     Winter Storm Watch from late Wednesday night through late
     Thursday night Butte/Blackfoot Region.

     Winter Weather Advisory from 1 AM to 3 PM MST Wednesday Lower
     Clark Fork Region.

     Winter Weather Advisory from 4 AM to 3 PM MST Wednesday
     Missoula/Bitterroot Valleys.

ID...Winter Weather Advisory from 9 PM this evening to 4 PM PST
     Wednesday Lower Hells Canyon/Salmon River Region.

     Winter Storm Warning from midnight tonight to 4 AM PST Thursday
     Northern Clearwater Mountains.

     Winter Storm Warning from midnight tonight to 4 AM PST Thursday
     Southern Clearwater Mountains.

     Winter Weather Advisory from 4 AM to 3 PM MST Wednesday Western
     Lemhi County.

     Winter Weather Advisory from 9 PM this evening to 4 PM PST
     Wednesday Orofino/Grangeville Region.

&&