Lake Wind Advisory 6, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
309 PM MST Thu Nov 14 2013

...Changes to the upcoming weather system this weekend, high-
impact snow still likely...

.DISCUSSION...There have been some changes to the forecast
regarding the cold intrusion of air and snow potential beginning
Friday and lasting through Sunday across the Northern Rockies.

Before we highlight those changes, we want to stress that our
confidence remains high in this system occurring. We still expect
measurable snowfall over mountain passes all the way down to valley
levels.

Here are the impacts expected from this system and forecast changes
that have been made:

1) ARCTIC AIR AND EASTERLY WINDS: Recent forecast models have come
in with a weaker, but still very present, push of arctic air
Friday into Saturday. This has changed from previous forecasts in
the sense that Missoula and locations south may not receive as
strong of an east wind as previous forecast, which could make for
a snowier situation the entire weekend. In addition,
Kalispell, Flathead Lake, and along the Continental Divide should
still see gusty winds with well below normal temperatures, thanks
to the arctic air. However, winds are not expected to be as strong
as previously forecast. Flathead Lake will still see very choppy
conditions Friday, in addition to snowy and cold conditions on
Saturday. Those participating in Mack Days should be prepared for
inclement conditions all weekend.

2) ACCUMULATING SNOWFALL: Confidence remains quite high that
Lincoln/Mineral/Sanders counties in Montana, and Clearwater/Idaho
counties in Idaho will see the best focus of snowfall. Moderate to
heavy amounts are expected over mountain passes and higher
elevation roadways Friday through Saturday. Lower elevations will
begin receiving accumulating snow beginning Friday evening.
Hazardous driving conditions anticipated throughout this region.

In addition, snowfall could become a bigger impact to the
Missoula/Bitterroot valleys, especially Sunday. Our confidence is
increasing that persistent bands of snow will develop Sunday.
The exact location of the bands will dictate who could pick up a
fair amount of snow during the day. Radically reduced visibility
and quickly deteriorating roadways will be an impact under bands
of snow. It is not uncommon to receive upwards of 6 inches from an
especially persistent band of snow.

The coldest air of the arctic outbreak is anticipated to arrive
Monday morning. Temperatures will easily cool to the single digits
for high valleys near the Continental Divide.

Model guidance depicts a unclear weather picture across the
Northern Rockies for Monday onward, however there are some broad
characteristics certain in the long-term forecast: First, strong
west to east flow aloft will be present, likely maximizing cloud
cover. Second, areas that receive snow during the weekend storm
will see an increased probability of inversions, cold pools, and
persistent valley fog. Third, snowless valleys will likely see
temperatures slowly warm to near seasonal averages. Forth,
precipitation chances will remain relatively close to climatology
(somewhat high) through next weekend. Fifth, there appears very
few opportunities for very frigid air to be reintroduced to the
region.

&&

.AVIATION...Broken cloudiness tonight over the Northern Rockies will
become overcast with lowering ceilings by midday Friday. Some high
mountain snow showers will exist during this time, mainly across
northwest Montana and along the Continental Divide. Terrain in
these areas will be obscured at times. A significant cold and
snowy storm system will reach the Northern Rockies on Friday
afternoon. Flying conditions will deteriorate very quickly from
west to east over time, with KGPI and KMSO directly impacted by
late Friday afternoon. Terrain obscurations will be common during
this time, in snow, erratic gusty winds, and low
visibility/ceilings. KBTM will likely be impacted later Friday
evening.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...     WINTER STORM WATCH from late Friday night through Sunday
     afternoon Missoula/Bitterroot Valleys.

     WINTER STORM WARNING from 11 AM Friday to 5 PM MST Sunday Lower
     Clark Fork Region.

     WINTER STORM WATCH from Friday morning through Sunday morning
     Flathead/Mission Valleys...Potomac/Seeley Lake Region...
     West Glacier Region.

     WINTER STORM WARNING from 11 AM Friday to 5 PM MST Sunday
     Kootenai/Cabinet Region.

     WINTER STORM WATCH from late Friday night through Sunday
     afternoon Bitterroot/Sapphire Mountains...Butte/Blackfoot
     Region.

ID...WINTER WEATHER ADVISORY from 10 AM Friday to 10 AM PST Sunday
     Northern Clearwater Mountains...Southern Clearwater
     Mountains.

&&

Kitsmiller/Allegretto/Zumpfe
Aviation...Zumpfe

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