Area Forecast Discussion
Issued by NWS Boise, ID

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FXUS65 KBOI 231033
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
333 AM MST Mon Jan 23 2017

.SHORT TERM...Today through Tuesday...Large upper level low
spinning off the coast of Washington and Oregon is starting to
move southeast, and will open up and extend eastward by late this
afternoon. Overnight the axis will move through as a narrow high-
amplitude ridge develops just offshore in its wake. By Wed
afternoon, this ridge tilts northeast into western Canada, and a
trough axis extends from near Lake Hudson back west-southwest into
Idaho and Oregon. QPF from this system is very difficult to nail
down. The 06Z NAM has radically changed from the 00Z, bringing the
snow farther north into the Treasure Valley and, more importantly,
dramatically increased amounts for the western Magic Valley. The
06Z GFS also changed in similar fashion, but not as drastically. I
have decided to move in that direction, and it changes the
advisories slightly, making them last longer in the western Magic
Valley and in southern Twin Falls County. The far east end of the
Upper Treasure Valley zone is very close, but have left it out for
now.

Temperatures today will be held down by precip, cloud cover, and
northwest winds as they develop and strengthen during the day.
Therefore have reduced temps from previous forecast. Lows tonight
will be colder than we have seen in a while over most of the area,
as colder air will be brought in on those northwest winds.
However, those same northwest winds will stay up during the night,
increasing mixing and not allowing for ideal cooling. Therefore,
have increased lows in most areas. Highs Tuesday will be cooler,
with highs struggling to reach freezing anywhere in the CWA.

.LONG TERM...Tuesday night through Sunday...Northerly flow will
develop over the region Tuesday night as an upper level trough
moves east and a ridge of high pressure builds north along the
west coast. The northerly flow will bring cold air to the region,
keeping temperatures well below normal. Even though the northerly
flow will be dry, there will be a weak systems moving through the
flow for a slight chance of snow showers over higher terrain
Tuesday night and Wednesday. Models still showing a weak upper
level system trying to break through the ridge Wednesday night and
Thursday, but the path of this system remains uncertain along with
where precipitation with this system will be. Will continue the
trend of slight chance pops over Southeast Oregon along the Nevada
border and the mountains of southwest Idaho Thursday. By Friday
the axis of the upper level ridge will be over the forecast area
for dry conditions, temperatures well below normal and development
of valley inversions. The ridge of high pressure will remain over
the region through the weekend.

&&

.AVIATION...Generally VFR this morning with local MVFR/IFR
conditions in valley fog and isolated snow showers. After 18Z
showers south of a KBNO-KMUO-KJER line will obscure mountains
along with widespread MVFR and areas of IFR conditions. There will
also be reduced visibility south of KTWF due to blowing and
drifting snow. North of the KBNO-KMUO-KJER line will be mostly VFR
with areas of MVFR and isolated IFR conditions in snow. Surface
winds 5-15 knots with gusts around 25 knots south of a KMUO-KJER
line. Winds aloft southwesterly 20-30 knots up through 10K feet
MSL.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Winter Weather Advisory until 11 AM MST Tuesday IDZ016-030.
     Winter Weather Advisory until 11 PM MST this evening IDZ015-029.
OR...None.

&&

$$

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SHORT TERM...SP
LONG TERM....JA
AVIATION.....JA



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