Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 202215
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
315 PM MST SAT DEC 20 2014

...DIFFICULT TRAVEL OVER MOUNTAIN PASSES EXPECTED TONIGHT AND
SUNDAY...
...POSSIBLE WINTER STORM CHRISTMAS EVE DAY INTO CHRISTMAS...

.DISCUSSION...A deep moisture plume will bring heavy precipitation
to the region tonight lasting into Sunday. Areas of light
precipitation will continue the rest of this afternoon and early
this evening, ahead of the main surge of moisture set to arrive
mid evening lasting well into Sunday. Snow levels will gradually
rise this evening through Sunday with a burst of heavy mountain
snow initially overnight followed by a change over to a rain/snow
mix or all rain for many mountain passes below 5500 feet on
Sunday. Many lower valleys will see a rain/snow mix change over to
all rain during the early morning hours on Sunday, but a few
pockets of light freezing rain are possible across the lower
valleys of northwest Montana this evening...certainly something to
watch out for. Here are a few specific impacts:

* Lookout Pass on Interstate 90 will experience heavy and
  relatively wet snow by late evening changing to either a
  mixture of rain/snow or just plain rain by daybreak Sunday. Lolo
  Pass will see a similar change to a mixture with rain but the
  timing for that may be a little later Sunday morning and then
  change back to snow by the afternoon.

* Another area of concern could be the Glacier Park area, but with
  strong westerlies aloft, downsloping affects could limit the
  intensity of snowfall and total snow amounts. Nonetheless, road
  surfaces will be slick at times with snowfall.

* Models continue to paint a bull-eyes of over an inch of
  precipitation along MT Highway 83 between Swan Lake and Seeley
  Lake...elevations above 4000 feet look to receive 3 to 5 inches
  of total snowfall through Sunday evening.

* As the plume of moisture shifts south, Lost Trail Pass will see
  heavy snow for much of the day Sunday and could see a brief
  period of rain/snow.

The bulk of the system exits the region by Sunday afternoon, but
showers will continue through early Monday with unstable northwest
flow over the region. Models are hinting at the possibility of
snow bands setting up Sunday Night and early Monday, and colder
air should bring snow levels back to valley floors. This could
mean a couple quick inches for the morning commute Monday. One
factor working against snow bands is drying out of the airmass.
Still, it remains a possibility, and is something we will keep an
eye on.

Most of the region will dry out Monday afternoon into early
Tuesday as weak ridging moves in, giving a window with a good
chance of improved travel conditions.

For Christmas Eve Day into Christmas, we`re still looking at a
possible winter storm which could bring accumulating snow to much
of western Montana and central Idaho. The problem is large
discrepancies continue in model forecasts of the center of low
pressure, and as a result the possible outcomes are anything from
pass-level only snow event with valley rain to widespread snow at
nearly all elevations.

Stay tuned to subsequent forecasts as this system may have
significant travel impacts for the holidays. Confidence should
increase in the near future for the actual placement of potential
snow.

For Friday and into next weekend, colder and generally drier
conditions can be expected for the Northern Rockies, although a
few waves of energy moving through may be able to create snow
showers at times, especially over the mountains.

&&

.AVIATION...A very wet system is expected to move in this evening
and will bring a mixture of rain and snow to area terminals. Most
terminals should start off as snow or rain/snow mixture but is
expected to become all rain by Sunday morning. There is also a
slight chance of freezing rain early Sunday morning at KMSO and
KGPI so temperatures should be monitored closely as it transitions
to rain. The heaviest bands of precipitation is expected to be
between midnight tonight and noon on Sunday. During this time
ceilings will lower and visibilities should be reduced. Rain
showers are expected to linger through Sunday afternoon.



&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...WINTER WEATHER ADVISORY until 5 AM MST Monday ABOVE 4000 FEET
     for the Lower Clark Fork Region.

     WINTER WEATHER ADVISORY from 7 PM this evening to 5 AM MST
     Monday Bitterroot/Sapphire Mountains...Butte/Blackfoot
     Region...Potomac/Seeley Lake Region...West Glacier Region.

ID...WINTER WEATHER ADVISORY until 5 AM MST /4 AM PST/ Monday ABOVE
     4000 FEET for the Eastern Lemhi County...Northern
     Clearwater Mountains...Southern Clearwater Mountains...
     Western Lemhi County.

&&

$$



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