Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 171118 CCA
AFDMSO

Area Forecast Discussion...CORRECTED
National Weather Service Missoula MT
418 AM MST Fri Nov 17 2017

.DISCUSSION...As expected an upper level disturbance propagating
across the Northern Rockies has initiated some loosely organized
snow showers, the strongest of which passed over Missoula within
the past 30 mins. During this time visibility dropped briefly to
1/2 mile with large powdery snowflakes resulting in a few tenths
accumulation. Additional showers are developing over the terrain
along the Idaho/Montana border and will move across parts of Idaho
and Montana throughout the morning hours. Fortunately all of this
shower activity is occurring within the area presently
highlighted by a Winter Weather Advisory, which will obviously
remain in effect through the rest of this morning. It seems that
individual showers are incapable of producing much more than a
few tenths of an inch of snow at a time. But the cumulative
effects of multiple showers progressing over the same area could
result in upwards of 2 inches accumulation by mid morning.

The main upper level trough will move over the region later this
afternoon and should help initiate another round of shower
activity across a similar area near the Idaho/Montana border.
Atmospheric profiles do still support a period of more organized
or banded snow showers and thus there could be merit in extending
the Winter Weather Advisories into this evening. But have decided
to let the scenario evolve a bit more and give the next shift a
chance to make a final determination.

High pressure will begin to build over the area after the upper
level trough exits to the east, shifting the threat for any
additional light snow showers nearer to the Canadian border by
Saturday morning and then north of the border by Saturday
afternoon. This should produce a rather pleasant, albeit cold
weekend across the Northern Rockies. In fact, the combination of
few, if any clouds, fresh snow and very light winds could produce
a particularly cold morning Sunday across much of SW Montana and
Lemhi County, Idaho. Colder locations like Butte, Potomac and
Ovando along with the surrounding higher terrain could even dip
towards zero for a time Sunday morning.

Thanksgiving week continues to look wet. Sunday night through
Monday a pretty wet weather disturbance will move through the
Northern Rockies. Accumulating snows are anticipated over all of
our mountain passes. A mix of snow and rain is expected across
valley locations. There is some uncertainty regarding
temperatures. A 500-1000 foot drop in snow levels could mean some
accumulating snows in the valleys. This will have to be watched.

A brief ridge will build over the area Monday night into Tuesday
night. This time of year with such a low sun angle and recent
precipitation it won`t take much clearing or stable air to produce
fog with pretty low visibilities down to a mile or so in favored
valleys for fog. These include the Bitterroot/Missoula/Flathead
and Mission valleys and the lower valleys of Northwest Montana and
central Idaho.  This will also be closely monitored as it could
have a huge impact for holiday travel.

Another weather disturbance looks to impact the region later
Wednesday into Friday. Warmer air will keep snow levels above
most of our mountain passes.

Models seem to be pretty consistent building a ridge for the
weekend after Thanksgiving. Fog with low visibilities will once
again be possible in the favored valleys mentioned above. The fog
this time of year can easily persist through much of the day under
this pattern.

&&

.AVIATION...The main takeaway for today is that terminal sites
will likely experience periodic reductions in visibility and
lowered ceiling heights through this afternoon and likely into
this evening. Periods of visibility reductions to 1/2 mile will
quickly be followed by greater than 6 mile visibility through much
of the day. Winds will also be breezy at times with gust of 25-35
kt possible between the snow showers this afternoon.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...     Winter Weather Advisory until 11 AM MST this morning
     Missoula/Bitterroot Valleys.

     Winter Weather Advisory until 11 AM MST this morning
     Bitterroot/Sapphire Mountains...Lower Clark Fork Region.

     Winter Weather Advisory until 3 PM MST this afternoon West
     Glacier Region.

ID...Winter Weather Advisory until 10 AM PST this morning ABOVE 2500
     FEET for the Northern Clearwater Mountains...Southern
     Clearwater Mountains.

&&

$$


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