Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 101438
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
738 AM MST Mon Dec 10 2018

.UPDATE...Issuing a winter weather advisory across Northwest
Montana. An area of snow will continue through mid morning and
produce an inch or two of snow that will cause slippery travel
during the morning commute.


&&



.AVIATION...A cold front will steadily move across the Northern
Rockies today causing ceilings to lower. This front will also
bring widespread light snow which will causing periodic visibility
issues at area terminals, mainly at KGPI through 10/1800z. The
precipitation decreases in intensity and coverage throughout the
night.




&&

.PREV DISCUSSION... /ISSUED 352 AM MST Mon Dec 10 2018/

DISCUSSION...The first cold front is currently knocking at the
door of the Northern Rockies this morning. This front will steadily
move through the region bringing widespread light snow to the
area. The mountain passes along the Montana/Idaho border will have
the biggest impact from this disturbance with Lookout, Lolo and
Marias passes potentially receiving up to 3 inches of new snow
over the next 12 to 18 hours. Valleys inversions will diminish as
the front passes allowing temperatures to potentially spike to
the warmest readings over the past several days. After the passage
of the front the upper level flow becoming northwesterly which
will keep scattered showers lingering across the region. However
some snow could last into the evening hours over the Camas
Prairie in Idaho which may bring an area of concern, particularly
along US-95 from Whitebird Grade to Cottonwood where the
potential for 1 to 3 inches of snow exists by midnight tonight.

A transitory ridge will give the region a respite from
precipitation on Tuesday before the next round of moisture moves
into the area. An even stronger cold front will bring significant
impacts to the Northern Rockies late Tuesday into Wednesday.
But first we see a southerly pressure gradient setting up ahead
of the front up over Hells Canyon to Camas Prairie. Wind gusts of
30 to 40 mph will be possible by Tuesday evening for Riggins up to
Grangeville. The aforementioned front will have substantially
more moisture to work with as it moves the region and thus the
higher terrain along the Montana/Idaho border could receive
upwards of a foot of new snow by Wednesday morning. However strong
westerly winds along and behind the front will have downsloping
effects on the valleys. Most valley locations should receive 1 to
3 inches of new snow with higher amounts possible in the narrower
valleys of northwest Montana and north central Idaho where
downsloping affects will be less felt. The combination of heavy
snow with strong westerly winds has led to the issuance of several
Winter Storm Watches for the higher terrain. Southwest Montana
down into Lemhi County will be impacted by the strongest winds by
Wednesday afternoon, where wind gusts will have the potential of
reaching 40 mph. The snow and winds will steady diminish
throughout Wednesday night.

Much of the Northern Rockies will reside between weather systems
on Thursday and Friday with the main upper level jet near or north
of the Canadian border. Some precipitation may continue across far
NW Montana, mainly to areas north of US-2 with the Purcell and
Whitefish Mountains bearing the brunt of this limited moisture.
Elsewhere breezy winds appear possible which subsequently could
push temperatures to the warmest readings this month in many
places on Friday. Meanwhile in the higher terrain it will be
downright windy, especially along the Continental Divide. We could
even see some more strong winds developing throughout Grangeville
and the Camas Prairie/Hells Canyon area.

Models are struggling to pin down both the path and strength of
the next weather system, advertised to be through Idaho and
Montana sometime Saturday. This wave seems to have a weakening
trend in the models over the past 24 hours and could ultimately
be delayed or at best, significantly weaker as it tries to pass
through a weak upper level ridge. If the weakening trend
continues, temperatures this weekend could end up quite a bit
warmer than forecast.

The pattern really starts to diverge as we look beyond the
weekend. The ECMWF transfers some jet energy south towards Baja of
California, Mexico and thus keeps the NW USA under a very wet,
albeit warm westerly flow for several days. However the GFS is
absent this southward energy transfer and ultimately keeps the
subtropical jet stream 100 to 200 miles farther south over Mexico.
And this would permit only brief moisture over the Northern
Rockies before it would shift north into Canada as ridging is then
forecast to build across the desert southwest.

AVIATION...A cold front will steadily move across the Northern
Rockies today causing ceilings to lower. This front will also
bring widespread light snow which will causing periodic visibility
issues at area terminals. The precipitation decreases in intensity
and coverage throughout the night.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Watch from Tuesday morning through Wednesday
     morning Kootenai/Cabinet Region...Lower Clark Fork Region.

     Winter Weather Advisory until 10 AM MST this morning
     Flathead/Mission Valleys...Kootenai/Cabinet Region...West
     Glacier Region.

     Winter Storm Watch from Tuesday afternoon through Wednesday
     morning West Glacier Region.

     Winter Storm Watch from Tuesday afternoon through Wednesday
     morning Bitterroot/Sapphire Mountains.

ID...Winter Storm Watch from Tuesday morning through Wednesday
     morning Northern Clearwater Mountains.

     Winter Storm Watch from Tuesday afternoon through Wednesday
     morning Southern Clearwater Mountains.

&&

$$


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