Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 202105
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
305 PM MDT Fri Oct 20 2017

...Wet and windy conditions impacting the Northern Rockies
Saturday night and Sunday...

.DISCUSSION...The cold front that has brought valley rain and
mountain snow has pushed through much of western Montana, and
currently making its way across the Continental Divide this
afternoon. Locations above 5000 feet east of the Highway 93
corridor will continue to experience light to moderate snowfall
reducing visibility for the next couple hours. Areas of slush will
also produce slick driving conditions through the evening. Areas
west of 93 will continue to see scattered valley rain and mountain
snow showers until around sunset.

The stronger and wetter storm system still looks on track to begin
impacting the Northern Rockies Saturday night. 1 to 3 inches of
rain, with isolated pockets of up to 4 inches, still looks likely
for the mountains of the Clearwater NF in north-central Idaho.
Valleys will be on the lower end of this range. Areas of the
Kootenai NF and Glacier NP will also see rainfall totals exceeding
1 to 2 inches of rain. Snow levels will mostly remain around 6000
feet, but could be slightly lower in the heavy downpours. So
several inches of snow can be expected for the higher terrain,
above area passes. Along with the rain and high mountain snow,
strong westerly winds will also occur. Most mountain ranges of
western Montana and north-central Idaho could see gusts exceeding
50 mph at times, and even higher gusts possible along the
Continental Divide. Valleys of western Montana can also expect a
windy day as well, with gusts approaching 40 mph, especially after
the noon hour. Those expecting to venture out in the backcountry
should prepare for raw conditions, with a threat for treefalls,
particularly near recent fire activity from this summer.

Another impact due to the gusty winds, will be choppy conditions
on area lakes, with Flathead Lake being one of the most impacted.
Wave heights of 2 to 3 feet, possibly slightly higher, are likely
on the lake. So use caution if planning to spend any time on the
water.

Next week will start off quite nice with highs and lows on Monday
and Tuesday running near normal as high pressure builds along the
coast. However the extended period of high pressure over the
western USA present in the models just a few days ago now appears
to retrograde (shift west) off the coast, forced west by a
developing upper level storm system coming on shore over central
British Columbia. Long range models have shown steadily growing
confidence in a stronger weather system, capable of tapping into a
much colder air mass present across the Northwest Territories of
Canada.

So what does this mean for Western Montana? A significant change
will be possible late Wednesday as a strong cold front drags much
colder air across the Continental Divide, much like an advancing
arctic weather system. Gusty winds, falling temperatures and rain
transitioning to snow are presently in the forecast for Wednesday
night into Thursday morning. Fortunately this system appears to
move through the area rather quickly, so the potential for greater
public impacts will occur overnight and should be largely done by
Thursday morning. However any changes to the timing of this front
could present challenges to the Wednesday evening commute or more
likely, the Thursday morning commute. So please stay tuned!

Temperatures behind this weather system will become unseasonably
cold in most locations with lows well into the lower teens in some
places. However things will gradually moderate towards normal by
next weekend as northwest flow aloft holds steady over the
Northern Rockies.

&&

.AVIATION...An area of more widespread showers exists this
afternoon generally east of the Highway 93 corridor. This
precipitation has been driving snow levels down at low as 5000
feet, which will lead to locally low visibility and ceilings through
roughly 21/0100z. Airfield KBTM will be the terminal most affected.
Precipitation will become more scattered late this afternoon and
evening, though periods of low visibility and ceiling will be
possible at all terminal locations. Expect widespread
obscurations of the higher terrain through tonight. In addition,
breezy southwest to west winds will be possible in most valleys,
with gusts up to 30 kts possible through roughly 21/0200z. Winds
will decrease there after, though most locations will keep a light
breeze well into the night. Briefly improved ceilings are
expected tomorrow morning before the next wet and windy weather
system enters the region.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...None.
ID...None.
&&

$$



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