Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 131100

Area Forecast Discussion
National Weather Service Missoula MT
400 AM MST SAT FEB 13 2016

.DISCUSSION...A Pacific cold front has pushed across Idaho and
western Montana early this morning, producing widespread valley
rain and mountain snowfall. While showers are expected to continue
through the morning hours, a gradual increase in activity is
likely this afternoon as the main upper level low pressure trough
and an associated jet streak slide over the region.

There will be a brief lull in the showers early this evening. But
yet another moist, Pacific weather system will be quick to enter
the region and bring a steady increase in valley rain and
mountain snow. The time between the exiting system and
approaching one overnight is sufficiently brief to prevent much
warm air advection. Thus the snow level is expected to remain
below most passes with anywhere from 3 to 6 inches likely to fall
over Lookout, Lolo and Lost Trail Passes through Sunday. Winter
Weather Advisories may need to be considered should periods of
more intense snow appear likely and this is a decision that could
be made later this afternoon.

This weather system only marks the beginning of what will be a
multi-day period of persistent moist flow across the Northern
Rockies Region as an impressively strong upper level jet stream
(+180kts) screams into southern British Columbia. An `Atmospheric
River` of moisture, with its origin actually extending well into
the South Pacific near American Samoa, will work in tandem with
our mountainous geography to produce up-slope enhanced
precipitation along the Montana/Idaho border region. The
Clearwater & Bitterroot Mountains will be experiencing the
heaviest precipitation, though the snow level will be quite high
during this time (+6000ft) and likely reside well above almost public
and widely accessible recreation areas. Thus our focus for impacts
is beginning to shift towards hydrology. Please see the section
below for a more detailed analysis of our concerns.

By Tuesday a ridge of high pressure appears to influence the
region, shifting the jet stream and moisture feed north into
Canada. This brief shift in the weather pattern appears to last
till late Wednesday before yet another trough of low pressure
moves inland and across the Western United States. Confidence in
the details are low at this time. But it would appear as though
another strong cold front and the return of at least some mountain
snow are at hand by the later part of next week.


.AVIATION...Patchy valley fog will remain present within the
vicinity of KGPI and KMSO with occasional impacts to visibility
being possible through around 13/1600. As of 13/1050z the cold
front is on the doorstep of KBTM and should push through the
terminal by 13/1100z, bringing gusty NW winds and brief moderate
rain. Otherwise expect occasional light rain showers to persist
throughout the day at most all terminals with KSMN seeing the
least chance for showers. Much of the precipitation will come to
an end by 14/02000 but yet another Pacific storm system will begin
to enter the region later tonight, bringing a better chance for
heavier rainfall by Monday to KGPI and KMSO.


.HYDROLOGY...Rock fall/mudslides and/or debris flows will become
increasingly possible in North Central Idaho and across parts of
Western Montana where the heaviest precipitation is expected to
fall Sunday night through Monday evening. Elsewhere across the
lower terrain warm temperatures and persistent rainfall will aid
in quickly melting any remaining valley snow pack. This could lead
to significant ponding of water in areas where significant snow
remains on the ground and might be blocking drainage pathways. The
brief return of high pressure and above normal temperatures by the
middle of this week may further lead to melting of low elevation
snow pack and the attributed drainage complications.



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