110 
FXUS65 KMSO 132203
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
303 PM MST SUN DEC 13 2015

...Second round of snow tonight will heavily impact travel across
portions of central Idaho and southwest Montana...

.DISCUSSION...

While today's winter storm has been mostly underwhelming for
western Montana valleys from the Bitterroot to the Flathead,
higher elevations and mountain passes have seen plenty of snow
over the last 24 hours, with just shy of a foot reported at
several locations. The valleys of MT which have missed out thus
far have another shot at accumulating snow this afternoon and
evening as a cold front sweeps through the area, ending the south
winds which have dried out the lower atmosphere thus far today and
prevented precipitation from reaching the ground in those areas.

The focus of the storm now shifts to an area stretching from
central Idaho to southwest Montana, where nearly a foot of snow is
possible in some areas overnight. Valleys south of I-90 will pick
up several inches as well, especially the upper reaches of the
Clark Fork. In addition, the front pushing through tonight will
cause winds to increase across southern Lemhi County ID and along
the I-90 corridor from Deer Lodge eastward, leading to reduced
visibilities. A winter storm warning continues for those areas.
Another winter storm warning lingers for the mountains of west-
central MT and central ID into tonight, as several more rounds of
snow may impact travel in those areas. Elsewhere, advisories
cover much of our area, with the exception of northwest MT which
has likely seen most of its impactful snow already; only a few
showers are expected overnight in that area.

Colder air moving in tonight will lead to a colder day Monday,
with most locations below freezing through the day. Wrap-around
snow will continue to impact southwest Montana, especially areas
near the Continental Divide. In addition, northerly flow and
lingering moisture could lead to continued snow showers for the
Camas Prairie of Idaho. Elsewhere, occasional snow showers are
possible, but widespread impacts to travel are not expected.

A northerly flow behind the exiting low pressure will keep light
snow showers in the forecast for the day Tuesday, especially for
areas east of Missoula and south of I-90. Only light additional
snow accumulations are expected.

Models continue to show a shortwave trough sliding through a
northwesterly flow on Wednesday. Some models are trending the wave
stronger each run, with a mid level low pressure moving over
north- central Idaho. This type of weather pattern is historically
very good for snow production in all valley locations of western
Montana and central Idaho. How much snow is still in question, as
models still differ on the placement of the heaviest
precipitation. There is still an outside chance that this system
continues to strengthen which could push the majority of the
energy south of the area. Stay tuned to the forecast as we
continue to iron out the details.

Much cooler air will spill south out of Canada for the rest of
the week. Low temperatures for western Montana will be in the low
teens, with the highs struggling to get out of the 20s. North-
central Idaho will also see below normal temperatures, with highs
around the freezing mark.

As we get to next weekend, another North Pacific system will
impact the Northern Rockies. A warmer airmass will be associated
with this system, that could setup an overrunning scenario. Long
range forecast models differ with how much of a warm airmass will
move in, and the location of where the boundary sets up. More
details on this in the coming days.


&&

.AVIATION...
Snow will continue for north-central Idaho and southwest Montana
this afternoon through Monday with persistent terrain
obscurations. Northwest Montana could see isolated light showers
linger through Monday, but minimal impact is expected. Snow
showers in west-central Montana will decrease as well, as a cold
front moves from northwest to southeast this evening. The cold
front is expected to reach KMSO around 14/0000Z; light scattered
snow showers with lower ceilings are then possible through around
14/04000Z. Looking at current model and satellite trends, the next
round of moderate snowfall, low visibility, and low ceilings for
KBTM and KSMN are forecast to be after 14/0400Z.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Warning until 11 PM MST Monday Butte/Blackfoot 
     Region.

     Winter Storm Warning until 11 AM MST Monday Bitterroot/Sapphire 
     Mountains...Lower Clark Fork Region.

     Winter Weather Advisory until 11 AM MST Monday 
     Missoula/Bitterroot Valleys.

     Winter Weather Advisory until 11 AM MST Monday Flathead/Mission 
     Valleys.

     Winter Weather Advisory until 11 AM MST Monday Potomac/Seeley 
     Lake Region...West Glacier Region.

ID...Winter Storm Warning until 11 PM MST Monday Eastern Lemhi County.

     Winter Storm Warning until 11 AM MST /10 AM PST/ Monday Lower 
     Hells Canyon/Salmon River Region...Northern Clearwater 
     Mountains...Southern Clearwater Mountains...Western Lemhi 
     County.

     Winter Weather Advisory until 10 AM PST Monday ABOVE 3000 FEET 
     for the Orofino/Grangeville Region.

&&

$$