Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 191054

Area Forecast Discussion
National Weather Service Missoula MT
354 AM MST Sun Nov 19 2017

...Slick Mountain Pass Travel Tonight through Monday...

.DISCUSSION...Another dry and mild day is on tap today after a
cold start. Temperatures have been fluctuating up and down early
this morning thanks to shallow temperature inversions, and a
strengthening pressure gradient with increasing winds at times.
At 3 AM, the Bert Mooney Airport was sitting at 10 degrees while
northern Butte was near 30 degrees! Another example includes the
Bitterroot Valley where temperatures near the river are near 20
degrees while up on the side benches temperatures are close to
freezing. Expect high temperatures to rise into 40s in the
valleys today amidst scattered high clouds that will increase and
lower by this evening.

Glancing at satellite imagery this morning reveals an anomalous
plume of moisture, from subtropical origins, aimed at Vancouver,
Canada. This feature will eventually shift eastwards and bring
moderate to heavy precipitation to the mountains by tonight. There
will be periods of heavy snow after midnight through Monday
morning that will make traveling across Lookout Pass on
Interstate 90 a little tricky. Also Marias Pass, Lolo Pass and
Lost Trail Pass will see accumulating snow with some gusty winds.
In fact, winds gusting near 40 mph will create blowing and
drifting snow at times at Marias Pass. Due to the fact that the
moisture is moving in during the night and some valley locations
may be hovering near freezing, there will be a chance for freezing
rain. Snow levels are expected to rise above 5,000 feet during the
day Monday then lower some by Monday night. Expect gusty winds
over 40 mph in the Camas Prairie and Grangeville areas tonight
into Monday. There could be a few southerly gusts over 50 mph
especially between Whitebird Grade and Grangeville which would be
a dangerous crosswind for high profile vehicles on Highway 95.

The Pacific Northwest will be under the influence of a wet and
mild flow pattern through Thanksgiving. This means that, while
high temperatures should be in the 30s this time of year, high
temperatures are likely to be in the 40s and 50s throughout the
week, and there is a chance for a few spots to approach 60 on
Wednesday and Thursday which could be near record high
temperatures for this time of year.

Precipitation is expected to be plentiful throughout much of the
region during this week. Precipitation arrives as snow in the
mountains, rain in valleys, and possibly freezing rain in the
valleys of northwest Montana, on Tuesday. Rising snow levels
through the middle of the week will limit snow to the highest
terrain, so even most of the mountain passes are likely to see
rain. The northern half of Idaho and into northwest Montana can
plan on seeing the bulk of the moisture this week.

Thanksgiving continues to be forecast as the warmest day of the
week. Breezy conditions are expected due to a cold front moving
through the region, which will also bring increasing rain chances
and lowering snow levels late in the day. Mountain passes may
start to see snow by Friday with a passage of a cold front
through Idaho and western Montana.


.AVIATION...Patchy freezing fog is possible for some northwest
Montana valleys 19/1000Z through 19/1800Z, however no fog or
freezing fog is anticipated at any major area terminal. Expect
mainly VFR conditions today but mountain wave activity near the
Continental Divide especially near Marias Pass will contribute to
strong or perhaps severe turbulence through 20/0800Z. A plume of
moisture will bring lowering ceilings and precipitation starting
by 20/0600Z across northwest Montana and then affecting mountain
passes by 20/1100Z with moderate to heavy snowfall. Also due to
the wetter than normal nature of this system, moderate to heavy
icing is possible up to around 12,000 feet MSL on Monday,
especially along and west of the Montana/Idaho border.



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