Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 201007

Area Forecast Discussion
National Weather Service Missoula MT
307 AM MST Fri Jan 20 2017

.DISCUSSION...A deep, broad low pressure system off the Pacific
Northwest Coast is causing southwest flow into the Northern
Rockies this morning. A shortwave ridge moving through this
morning is providing enough stability for fog this morning. The
surface layer is loaded with moisture due to rain yesterday and
warm temperatures causing snow melt, so it does not take much to
cause fog this morning.

Also overnight temperatures have dropped below freezing, so recent
rain and snow melt is refreezing on the ground. The ground
temperatures remain very cold, helping the refreezing to occur
overnight. Watch for icy conditions on roads and sidewalks this

Today, a shortwave trough will move through the southwest flow and
cause some snow showers, mainly in the mountains. It will also
help to mix out the valley fog.

A large low pressure that develops along the west coast over the
weekend will provide periods of light snowfall across the northern
Rockies. The first wave will produce very light snowfall Saturday
through Saturday night. The next push of moisture will be
stronger, and produce a few inches of snowfall across the area
Sunday night into Monday. Roadways can become slick and snow
covered, especially mountain passes.

As the energy of the low tracks south and east, the northern
Rockies will remain under a northerly flow will persistent light
snow showers possible over the area Tuesday through Wednesday. A
strong high pressure will start to build along the western U.S. by
late week that will support the return of cool valley inversions
and air stagnation issues.


.AVIATION...Fog is affecting many area terminals this morning,
with the greatest impact at KGPI and KMSO. Rain and snow melt
yesterday are making a lot of moisture available for fog in
combination with increasing stability. However, the surface
pressure pattern is not ideal for fog and some weak disturbances
moving through the building shortwave ridge are causing the fog
conditions to vary considerably. Expect this to continue through
the morning hours until the sun comes up and has a few hours to
mix up the boundary layer and break up the fog. Also, a shortwave
trough will arrive today causing stability to decrease and help
the fog mix out today.


MT...Dense Fog Advisory until 8 AM MST this morning Lower Clark Fork
     Region...Missoula/Bitterroot Valleys...Potomac/Seeley Lake


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