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000
FXUS65 KMSO 242027
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
127 PM MST Wed Feb 24 2021

...Heavy snow Event for Central Idaho, Very Breezy Conditions
Elsewhere...

.DISCUSSION...Skies are clearing temporarily and snow is melting
on roads. Overnight, clouds will begin to increase again as our
next impactful system pushes onshore. A fetch of sub-tropical
moisture originating north of Hawaii will focus in on central
Idaho. Although, there will also be periods of snow showers and
snow bands in Western Montana.

Beginning Thursday morning, light snow will develop in the Idaho
panhandle and spill into the western parts of Lincoln and Sanders
Counties. Expect snow rates to rapidly increase as the first
short wave in this system destabilizes the atmosphere and snow
showers or snow bands develop. In general for Western Montana,
snow totals will be relatively low with the highest amounts
likely along I-90 and to the south. We will stay on the northern
side of the Jetstream, which will keep us in an unstable
atmosphere. There will be areas, especially in the mountains,
that will see moderate to heavy snow showers at times. This will
be the greatest impact as road conditions will vary greatly
depending on where and when showers set up. These showers will be
capable of gusty winds, blowing snow around, and creating low
visibilities.

Snow will begin late morning Thursday and continue through
Saturday. There will be multiple periods of moderate to heavy snow
and high probability of snow rates exceeding 1" per hour at
times. The area from Pierce south to Dixie and eastward towards
the Bitterroot Crest will be the most affected. Snow totals will
once again be quite high with the highest peaks receiving 2 to 3
feet, most elevations of the Clearwater Mountains are expected to
receive 11 to 20 inches. The lower valleys around Riggins can
expect 1 to 3 inches of snow. The Camas Prairie and Whitebird
Grade will see 3 to 5 inches of snow and although the timing is
not perfect a southerly gradient will force wind gusts to 40 MPH
Thursday afternoon. Any available transportable snow may blow
around causing reduced visibility and drifting on roadways.

The models diverge by Sunday but the possibility of continuing light
snow exists, with amounts of up to 1 inch of snow possible. The
probability of light snow continuing is around 30%, especially in
the mountains.

Starting Monday there is considerable uncertainty in the forecast.
Usually this means that there is a lot of spread in the various
model solutions, but in this case it means that the models are
almost evenly split between 2 scenarios. Both scenarios have a
trough moving out of the northeast Pacific Ocean and into the
western US, where they differ is in the track of the trough
afterwards. One scenario has the flow splitting with the trough
digging south quickly leaving the northern Rockies under a ridge
of high pressure with warm temperatures and mostly clear skies.
The other scenario keeps the trough on a more northerly track and
it brings cooler temperatures and more snow showers to the
northern Rockies. We expect the models to begin favoring one
scenario over the other the soon, so then we will be able to
provide a more detailed forecast. In the mean time, the forecast
will remain with near average temperatures and moderate chances of
precipitation.

&&

.AVIATION...A very dry, stable air mass is settling in over the
region causing clear skies and light winds in many locations.
However, it is also trapping some low level moisture in some
valleys causing stratus. With snow cover and temperatures coming
near or above freezing, we expect some snow melt to keep the
lowest levels moist. A ridge building over the area with low level
moisture is normally a recipe for some fog and mist. However, a
deepening trough off the British Columbia coast will keep enough
of a pressure gradient over the region to drive light southerly
winds overnight. So there is a chance of some patchy fog and
stratus, but it is not yet expected to affect any of the area`s
major terminals.


&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory from 8 AM Thursday to 8 AM MST Friday
     West Glacier Region.

     Winter Weather Advisory from 8 AM Thursday to 8 AM MST Friday
     Kootenai/Cabinet Region.

     Winter Weather Advisory from 8 AM Thursday to 5 PM MST Friday
     Lower Clark Fork Region.

ID...Winter Weather Advisory from 4 PM Thursday to 7 AM PST Friday
     Lower Hells Canyon/Salmon River Region.

     Winter Storm Watch from Thursday morning through Friday
     afternoon Northern Clearwater Mountains...Southern
     Clearwater Mountains.

     Winter Weather Advisory from 4 PM Thursday to 7 AM PST Friday
     Orofino/Grangeville Region.

&&

$$