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fxus65 kmso 071524 

Area forecast discussion
National Weather Service Missoula Montana
824 am MST Sat Dec 7 2019

Update...a warm layer of air above the valley floors is producing
conditions for light freezing rain this morning. Of particular
concern is the Interstate 90 corridor between Missoula and Lookout
Pass and Highway 200 from Ravalli to Heron. Although freezing
rain will be light, slick roads are possible due to a light glaze
of ice. We have issued a Winter Weather Advisory to reflect these
conditions. Roads will improve by late morning as the sun warms up
surface conditions. However, snow will intensify over Lookout
Pass late this afternoon and evening, producing another period of
potential travel impacts late tonight into Sunday morning.


Aviation...fog has developed this morning across much of western
Montana, despite increasing clouds aloft, and is affecting
visibility at kmso and kgpi. Like yesterday, these aviation
impacts are likely to linger throughout the morning but gradually
show improvement around 07/1700 at both airfields. Light rain is
likely to begin at kgpi later this morning, but not at kmso until
later tonight. But any significant obscuration is not expected
until this evening at both airfields. Kbtm and ksmn will remain
free of significant weather issues until late Sunday morning at
the earliest.


Previous discussion... /issued 328 am MST Sat Dec 7 2019/

Discussion...despite the steady increase in clouds and mainly
mountain precipitation, most of the broader valleys will
experience yet another unseasonably warm early December day. In
fact, the snow level today will likely rise to around 5000 feet
over much of north central Idaho and western Montana, in response
to warm air advecting into the region from our southwest. This
shift in the forecast towards a warmer solution, is somewhat
significant with regards to the expected transition to snow later
tonight and overall forecast snowfall totals. But certainly not
enough to warrant adjustments to any of our presently issued
winter weather advisories. They all seem to handle the potential
societal impacts quite well.

What has changed is the overall impression that the back door or
southwesterly moving cold front will cross the Continental Divide
and brush across northwest Montana Sunday morning. The most reliable
model data seems to suggest that the winds and cold air will be a
bit delayed and likely not all that strong. As of right now it
seems the strongest winds are likely to occur in Kalispell by
mid/late Sunday morning and be generally back to light, northerly
by early evening. While the Flathead Valley is likely to see
little snow with this system, the same cannot be said for the West
Glacier and Seeley/Swan regions where 12 to 18 inches are likely
throughout the higher terrain and generally 2 to 5 inches within
the valleys.

The delayed, shallower push of cold air will make it across
Flathead Lake during the afternoon, but likely not be sufficient
to crest Evaro hill into Missoula. Thus the Missoula valley along
with points generally south of I-90 will remain in the warm sector
(think light rain or rain/snow mix) until late Sunday afternoon.
This is when a weakening area of pressure, that's shifting south
out of Canada all Sunday, will pass by Missoula and bring colder
air to the rest of the area. It will bring about a transition of
any remaining precipitation over to snowfall. But by this time it
seems that only scattered light snow would be possible in the
Missoula/Bitterroot valley. In fact the delayed cold air has
resulted in a general halving of forecast snowfall amounts for
most of SW Montana. However, the surrounding higher terrain
should still get some much needed snowfall with advisories still
applicable for the higher terrain and Mountain Pass travel.
Showers will gradually reduce in coverage and intensity Sunday
night, but likely still linger about the higher terrain into at
least Monday.

As previously noted in yesterday's discussion, showers are
possible on Monday. Forecast soundings suggest that very light
freezing rain could develop, mainly from the Flathead and mission
valleys, to the Glacier National Park region Monday morning. With
northwest flow continuing into Monday night, showers will persist.
Other areas may see patchy freezing fog develop both Monday and
Tuesday mornings.

Temperatures will moderate by mid to late week back to normal and
slightly above across the region. A wave will bring snow showers
Wednesday morning, mainly over northwest Montana and portions of
north central Idaho.

Moist westerly flow begins by Thursday, bringing the chance for
precipitation area-wide. There is concern Thursday morning that
light freezing rain could be a possibility in the valleys of
central Idaho into western Montana, but at this time we opted not
to add it to the forecast package due to low confidence. On the
other side of the Coin, Mountain locations will finally experience
an uptick in snow through at least Friday and maybe Saturday.


Mso watches/warnings/advisories...
Montana...Winter Weather Advisory from 3 PM this afternoon to 11 am MST
Sunday above 4000 feet for the Bitterroot/Sapphire

Winter Weather Advisory until 11 am MST Sunday Lower Clark Fork

Winter Weather Advisory from 3 PM this afternoon to 6 PM MST
Sunday West Glacier region.

Winter Weather Advisory from 2 am to 6 PM MST Sunday
Potomac/Seeley Lake region.

Winter Weather Advisory from 2 am to 6 PM MST Sunday
Butte/Blackfoot region.

Idaho...Winter Weather Advisory from 2 PM this afternoon to 10 am PST
Sunday above 4000 feet for the northern Clearwater
Mountains...southern Clearwater Mountains.

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