Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 211200
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
500 AM MST Tue Nov 21 2017

.UPDATE...Issued a dense fog advisory for much of west-central
Montana.

&&

.DISCUSSION...Dense fog has been observed in the Missoula Valley
and other locations are showing a downtrend in visibility. Also
with temperatures near freezing, black ice has been reported by
the Montana Transportation Department. We decided to issue a dense
fog advisory to advise these two impacts until 11 am this morning
for the I-90 corridor between Lookout Pass and Bearmouth, for the
US-93 corridor between Hamilton and Arlee, and Highway 200
between Heron and Clearwater Junction.

&&

.AVIATION...Fog and low clouds will plague valley locations from
northwest Montana into central Idaho through 21/1800Z today. KGPI
and KMSO are the main terminals that will be affected by fog.
Another system will move into the region later today to bring
mainly rain through Wednesday. Expect lowering ceilings and
precipitation from Clearwater County north and east over Lookout
Pass and the Troy areas by 21/2100Z and then to the remainder of
the region after that. Heavy rain and low ceilings are anticipated
across the St. Joe National Forest and Clearwater National Forest
from 22/0000Z through 22/2000Z Wednesday. Also with persistent
rain, the main terminals especially KGPI could see low ceilings
tonight through Wednesday. Freezing levels will rise to 8,000
feet by 22/0000Z, peak around 9,000 feet on Wednesday before
lowering quickly with a strong cold front on Thanksgiving day.




&&

.PREV DISCUSSION... /ISSUED 406 AM MST Tue Nov 21 2017/

DISCUSSION...The rain that fell across the area yesterday has
contributed to fog and low clouds this morning. Expect a slight
delay in your morning commute due to fog. Also be very careful
around curves as there could be black ice lurking which could
cause a loss of control to your vehicle.

An atmospheric river with anomalous moisture lurking west of
California this morning is expected to shift north and eastward
over the Northern Rockies today through Wednesday. This will
cause snow levels to rise to around 8,000 feet by tonight and
bring rain to the region. Heavy rain is expected over the
mountains of central Idaho and into northwest Montana where total
amounts could range from one to 3 inches. For the most part, the
existing snowcover should soak much of the rain, but ponding of
water issues are possible especially near mountain passes
including Lookout and Lolo and areas that have snow berms from
recent snowfall tonight into Wednesday.

With the influx of much warmer air aloft tonight and clouds, low
temperatures tonight will be above freezing in the mountains and
may not dip lower than the 40s in valley locations. With wet
roads, hydroplaning will be a possible risk, so be careful if you
need to be traveling. There could also more chances for fog come
Wednesday morning. High temperatures on Wednesday will be 12 to 15
degrees warmer than normal as many locations will rise into the
50s. While some valleys will see some respite from the rain
Wednesday, north-central Idaho and western Montana mountains will
see an above normal chance for rainfall.

The cold front for Thanksgiving has been trending faster over the
last few model runs so it may pass during the afternoon and
evening rather than the overnight. Strong winds are likely in the
terrain ahead of the front, but the earlier timing leads to the
potential for strong winds even at lower elevations. Snow levels
will also quickly lower with the front so accumulations are
expected to return to mountain passes. Precipitation amounts and
type are less certain at lower elevations due to the strong winds
and drier behind the front. There are indications that enough
atmospheric ingredients may come together to produce snow bands
Thursday night so low elevations may end up seeing some snow
accumulations into Friday morning.

Conditions dry out during Friday with decreasing precipitation
chances. Another warm and wet weather system arrives for the
weekend. A cold front is possible for Sunday night with a return
to snow during Monday.

AVIATION...Fog and low clouds will plague valley locations from
northwest Montana into central Idaho through 21/1800Z today. KGPI
and KMSO are the main terminals that will be affected by fog.
Another system will move into the region later today to bring
mainly rain through Wednesday. Expect lowering ceilings and
precipitation from Clearwater County north and east over Lookout
Pass and the Troy areas by 21/2100Z and then to the remainder of
the region after that. Heavy rain and low ceilings are anticipated
across the St. Joe National Forest and Clearwater National Forest
from 22/0000Z through 22/2000Z Wednesday. Also with persistent
rain, the main terminals especially KGPI could see low ceilings
tonight through Wednesday. Freezing levels will rise to 8,000
feet by 22/0000Z, peak around 9,000 feet on Wednesday before
lowering quickly with a strong cold front on Thanksgiving day.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Dense Fog Advisory until 11 AM MST this morning Lower Clark Fork
     Region...Missoula/Bitterroot Valleys...Potomac/Seeley Lake
     Region.

ID...None.
&&

$$



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