Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 231101
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
401 AM MST Fri Feb 23 2018

.DISCUSSION...

Snow will linger across portions of north central Idaho and
southwest Montana through this morning. Minor travel impacts, due
to snow on the roadway and low visibility, will be present during
this time. Aside from near zero and sub zero temperatures this
morning across the Northern Rockies, the remainder of today will
be fairly benign.

The next Pacific weather system will move over the Northern
Rockies this evening through Saturday, creating travel impacts due
to snow accumulation on roadways. North central Idaho and
far northwest Montana will be the main areas of concern,
particularly the mountain passes along the Idaho-Montana border
(e.g., Lolo, Lookout, Lost Trail passes). This upcoming snow
impact will be modest in comparison to recent arctic outbreaks,
with the Pacific airmass moderating temperatures throughout the
region. The northern Clearwater Mountains of Idaho are likely to
receive the most snow (6-10 inches), with lower elevations of
north central Idaho in the 2-4 inch snow accumulation range. The
snow potential is anticipated to diminish by late Saturday
afternoon.

The second weather system of the weekend will enter the Northern
Rockies early Sunday morning and quickly spread snowy conditions
across much of the area. This feature continues to have far more
moisture, instability and lift as compared with the
Friday/Saturday storm. Something that will be especially evident
by a swift moving cold front that will bring strong mountain top
winds gusting up to 50 mph throughout the day. These winds through
a moist mid level flow will also promote heavy snow at times with
rates possibly eclipsing 1 inch per hour, especially during the
afternoon.

The best chance for Northwest Montana accumulations will occur
early Sunday with 1 to 4 inches expected in the valleys and 3 to 6
inches across the higher terrain. Models all indicating the cold
front will be gaining strength as it moves quickly south
throughout the day. And while heavy snow is to be expected
along the higher terrain of the Bitterroot and Clearwater
Mountains, valley amounts could become quite interesting should
convective snow features develop along and immediately proceeding
the cold front as the GFS and NAM suggest. At present time both
these models would support a good amount of snow (possibly 4 to 8
inches) falling with the southern Bitterroot Valley (south of
Stevensville to Lost Trail Pass). This unique potential for
heavier valley accumulations could elevate what is easily a system
with impacts worthy of a Winter Weather Advisory into a Winter
Storm Warning event and thus we encourage you to follow the
forecast closely these next 36 hours.

Monday will see residual post frontal snow showers becoming more
frequent at peak afternoon heating and then decreasing
significantly as stability takes over late in the day. Increasing
subsidence (sinking motion in the atmosphere) will briefly take
hold overnight, producing rapid clearing and excellent potential
for radiational cooling. Thus Tuesday morning could end up quite
cold and even below zero in many places. Especially in locations
with exceptionally deep snowpack and/or those who received a
uniform layer of new snow that is several inches deep. But the
brief presence of morning sun will be quickly replaced by
increasing clouds as NW flow aloft draws yet another snowy period
across the Northern Rockies. Models are unsure as to how long this
moist flow will persist. But the latest data would support
enhanced chances for snow through the rest of next week.

&&

.AVIATION...Light snow will exist across north central Idaho and
southwest Montana air space through roughly 23/1600Z, lessening
visibility and ceilings at times for KBTM and KSMN. After a
reprieve from snow thereafter, snow will redevelop and intensify
across the Northern Rockies after 24/0000Z and continue to be
present for the rest of the next 24 hours. Most of snow will be
occurring along the Montana-Idaho border areas, where terrain
obscurations will be commonplace. KGPI, KMSO, and KSMN have a low
chance of diminished flight categories during this time.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...ID...Winter Weather Advisory until 8 AM PST this morning Northern
     Clearwater Mountains.

     Winter Weather Advisory until 9 AM MST this morning Western
     Lemhi County.

     Winter Weather Advisory until 8 AM PST this morning Lower Hells
     Canyon/Salmon River Region...Orofino/Grangeville Region.

     Winter Weather Advisory until 8 AM PST this morning Southern
     Clearwater Mountains.

&&

$$



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