Area Forecast Discussion
Issued by NWS Boise, ID

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FXUS65 KBOI 181707
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
1007 AM MST Mon Dec 18 2017

.DISCUSSION...Moist zonal flow across the Pacific NW will bring
cloudy skies to southwest Idaho and southeast Oregon through
Tuesday with a chance of snow across the west central Idaho and NE
Oregon mountains. The bulk of the moisture associated with this
flow will stay north of a line from Baker City, OR to McCall, ID
through Tuesday evening. Then, a potent low pressure system from
the Gulf of Alaska will move south along the British Columbia
coast and weaken considerably as it moves across our region on
Wednesday afternoon. This system will bring widespread precipitation
on Wednesday with snow levels falling to the valley floors as the
cold front moves through. Rain changing to snow is expected in
the lowest valleys, with snow accumulations less than an inch
expected. Areas above 6000 feet in the central Idaho mountains
can expect 5 to 10 inches by the time precipitation ends Wednesday
night. Current forecast reflects this well with no updates.

&&

.AVIATION...Generally VFR under mid-high ceilings. Local MVFR/IFR
ceilings and isolated snow showers across Baker County and the
central Idaho mountains with mountain obscuration. Surface winds:
variable, 5-15 kts. Winds aloft at 10 KFT MSL: westerly 30-40
kts, becoming SW by 19/12Z.

&&

.PREV DISCUSSION...
SHORT TERM...Today through Tuesday...The ridge will continue to
flatten today, with lingering moisture noted across Baker County
of Oregon, and the West Central and Boise Mountains of Idaho. This
will leave light snowfall over these impacted areas through
Tuesday, though accumulation is not anticipated to be significant.
Patchy fog possible this morning again for the valleys, and any
fog development should lift by the late morning hours. The flow
looks to turn nearly zonal by Tuesday morning, with a shift to
more southwesterly on Tuesday afternoon ahead of the approaching
trough. In response, wind across southeastern Oregon will increase
beginning Tuesday afternoon as the pressure gradient tightens
across the area. Temperatures across the area will continue to
warm, with values expected to be 5 to 10 degrees warmer than
normal by Tuesday.

LONG TERM...Tuesday night through Sunday...Models continue to
forecast colder weather following the passage of a deep upper level
trough from the Gulf of Alaska. As it approaches Tuesday night it
will spread showers across our area, with snow levels 4000 to 5000
feet. Precipitation will continue on Wednesday, with snow levels
lowering to around 3000 feet. A mix of snow and rain is expected in
the lowest valleys, with snow accumulations less than an inch. Areas
above 6000 feet in the central Idaho mountains can expect 5 to 10
inches by the time precipitation ends Wednesday night. The trough
will exit to the southeast on Thursday. Northwesterly flow aloft
will bring a second trough from British Columbia over our area
Friday and Friday night. Precipitation with this system will be in
the form of snow at all elevations, falling mainly over the
mountains of northeast Oregon and central Idaho. The Snake River
Valley could also see some accumulation. After this system passes
the flow will be out of the north, allowing arctic air to push south
into the Intermountain Region. Temperatures Sunday morning will be
as cold as the single digits in the lower valleys, and below zero in
the mountains.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...None.
OR...None.

&&

$$

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DISCUSSION...KA
AVIATION.....AB
PREV SHORT TERM...KB
PREV LONG TERM....JT



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