Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 201021
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
320 AM MST SAT DEC 20 2014

...DIFFICULT TRAVEL OVER MOUNTAIN PASSES EXPECTED TONIGHT AND
SUNDAY...
...POSSIBLE SNOW IMPACTS OVER CHRISTMAS EVE AND CHRISTMAS DAY...

.DISCUSSION...Widespread dense freezing fog with 1/4 mile
visibility or less in the Missoula and central to northern
Bitterroot valleys will persist through the mid morning hours. The
fog will slowly begin to lift and scour out by mid afternoon.

An anomalous moisture plume is still expected to bring heavy
precipitation to the region by tonight. The first slug of
moderately heavy rain will arrive across central Idaho by this
afternoon. Snow levels will gradually rise this evening into the
overnight.

* Lookout Pass on Interstate 90 will experience heavy and
  relatively wet snow from after midnight tonight 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-eys of over an inch of
  precipitation along MT Highway 83 between Swan Lake and Seeley
  Lake...elevations above 4000 feet could see 4 to 6 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.

Monday and early Tuesday a ridge of high pressure will put a
brief rest to precip over the region. Valley fog is a possibility
Mon and Tue morning under this ridge.

Late Tuesday through Christmas...All models continue to show a
rather strong and wet weather disturbance moving through the
Northern Rockies. The common denominator is that this period will
be wet BUT there continues to be a lot of DISCREPANCY on the
details on where exactly this system moves through the region.
This is a pattern favorable for significant precipitation or snow
in a band just north of the surface low pressure. The problem is
now each model shows a different placement of this low (band of
precip) anywhere from Northwest Montana to central Idaho.
Models should begin to come into some sort of consistency in the
next 3 days for confidence to increase enough for details of
placement and rain/snow amounts potential over the region.

STAY TUNED TO SUBSEQUENT FORECASTS for this may be a significant
system and confidence will increase for exact placement of
potential snow during the critical peak travel over the Christmas
holiday.


&&

.AVIATION...Dense fog will persist at KMSO through 20/1500z. Fog
will gradually lift and clear out at KMSO by 20/2000z. Otherwise
expect cloudy skies with mountain obscurations at times today. The
next storm system will enter the region this evening and persist
through Sunday morning. Valley rain will occur at all terminals,
limiting visibilities and lowering ceilings at times.



&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...WINTER STORM WATCH from this evening through late Sunday night
     Bitterroot/Sapphire Mountains...Butte/Blackfoot Region...
     Lower Clark Fork Region...Potomac/Seeley Lake Region...West
     Glacier Region.

     DENSE FOG ADVISORY until 9 AM MST this morning
     Missoula/Bitterroot Valleys.

ID...WINTER STORM WATCH from this evening through late Sunday night
     Eastern Lemhi County...Northern Clearwater Mountains...
     Southern Clearwater Mountains.

&&

$$



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