Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 300045
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
645 PM MDT Wed Mar 29 2017

.UPDATE...The Winter Weather Advisory for Lemhi County is now in
effect.

&&

.DISCUSSION...Snow has already begun to accumulate over Lost
Trail Pass so the advisory has been updated.

&&

.AVIATION...Rain will persist at KGPI and  KMSO through the
evening hours then taper off to scattered showers after midnight.
KBTM and KSMN will see a development of scattered showers early
this evening. These showers are anticipated to persist through
Thursday. Lowering of ceilings and visibility will occur at all
terminals in and around showers.


&&

.PREV DISCUSSION... /ISSUED 231 PM MDT Wed Mar 29 2017/

DISCUSSION...Westerly flow will continue to bring abundant
moisture to the region the rest of today into tonight, with
central Idaho seeing the heaviest precipitation. Favored areas of
Clearwater and Idaho counties will see from 0.50 to 1.50 inches of
moisture, leading to an increased potential for rock/mudslides
along steep highway corridors. Snow levels will generally be above
6000 feet, keeping snow impacts minimal except for the highest
passes, such as Lost Trail.

By early Thursday, a mid level circulation is expected to develop
over southern Idaho. While most of the region will experience
periods of light precipitation, this circulation will lead to an
area of enhanced precipitation first over southern Idaho county
early in the day dropping into Lemhi county by afternoon. A brief
period of snow accumulation may occur for White Bird Grade early
in the morning with Lost Trail Pass and Gilmore Summit
occasionally experiencing snow covered roadways during the day,
although warm road surface temperatures will counter this.

Colder air will settle into the area Thursday evening while
showery activity remains across Southwest Montana, so roadways
down to 5000 feet including Butte and Georgetown Lake may see a
period of wintry driving conditions. Drier air over takes the
region Friday with any leftover showers ending.

A shortwave ridge of high pressure builds over the Northern
Rockies on Saturday morning. Many valleys could see morning fog,
but given the increasing sun angle and less-than-ideal ridge axis
position, any fog that forms should be short-lived and only patchy
in nature. During the day Saturday into Sunday, westerly flow
will bring scattered showers with seasonal temperatures.

The next low pressure system will drop out of Canada on Sunday
night into Monday. This system looks very similar to the system
currently impacting the area, with Lemhi county and southwest
Montana seeing the best moisture and precipitation. Backcountry
and mountain passes could again see snow accumulations, with the
best accumulation periods being overnight. Snow that falls during
the day will likely have a hard time adding up due to relatively
mild temperatures and radiational heating.

By Tuesday morning, the system migrates east of the region and
another dry period is forecast for mid next week. A larger and
more widespread wet system may move in by the end of the work
week, but models disagree on timing and intensity of this
feature.

AVIATION...Rain will persist at KGPI and  KMSO through the
evening hours then taper off to scattered showers after midnight.
KBTM and KSMN will see a development of scattered showers early
this evening. These showers are anticipated to persist through
Thursday. Lowering of ceilings and visibility will occur at all
terminals in and around showers.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...None.
ID...Winter Weather Advisory until 9 AM MDT Friday ABOVE 6500 FEET
     for the Eastern Lemhi County...Western Lemhi County.

&&

$$


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