Area Forecast Discussion
Issued by NWS Boise, ID

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FXUS65 KBOI 210249

Area Forecast Discussion
National Weather Service Boise ID
849 PM MDT Fri Oct 20 2017

.DISCUSSION...Scattered showers will continue this evening as the
upper level trough moves across the region. Satellite imagery at
02z shows the back edge of the trough over Harney county. The
trough is expected to exit the region by midnight MDT ending the
showers. Next system is quickly approaching from the west with
satellite showing the moisture plume just off the Oregon Coast.
Patchy fog could develop overnight if skies can clear long
enough. At the moment that doesn`t seem likely. Precipitation will
spread eastward along a warm front Saturday morning reaching
Burns, Oregon around 13z, Baker City around 17z and Southwest
Idaho around 19z. Temperatures profiles across Southeast Oregon
Saturday morning look cold enough for precipitation to start as
very wet snow with 850 MB temperatures hovering around 0 C making
for a tricky snowfall forecast. It`s probable that accumulating
snow will occur with the heaviest precipitation bands where the
weaker bands remain as rain. Snow will change to rain with the
frontal passage late Saturday morning. The Burns area could see
up to 2 inches of wet snow Saturday morning that will likely melt
off by Saturday afternoon. Expect rain below 4000 feet elsewhere
with snow level rising to 6000-8000 feet by Saturday evening.
Treasure valley will see steady rain beginning around Noon MDT and
lasting through Saturday evening. Temperatures will reach the mid
to upper 40s Saturday in the lower valleys. Made adjustments to
forecast through Saturday Evening. Update will be out shortly.


.AVIATION...Generally VFR through 12Z as some clearing occurs.
Isolated to scattered showers across the higher elevations through
12Z, mainly over the central Idaho mountains. An incoming warm front
will spread precipitation across the region as early as 12Z in SE
Oregon and 15Z in SW Idaho. VFR conditions will deteriorate to low
VFR to MVFR, IFR in sheltered mountain valleys. Precip will begin as
SN or RASN at first, then switch to RA by the afternoon, except near
KMYL. Snow levels begin around 5kft MSL, increasing to 7-8kft MSL in
the evening. Snow levels over the central Idaho mountains will be
slower to rise. Mountains will be obscured. Surface winds: west to
northwest 10 kts or less, increasing to 15-20 kts KBNO-KTWF and
south. Winds aloft at 10kft MSL: west 20-25 kts, increasing to
30-40 kts around 18Z.

Sunday Outlook...Showers will shrink north into the higher
elevations by Sunday with mountains continuing to be obscured.
Snow levels will rise to 8000-10000 ft MSL by Sunday afternoon.
Patchy valley fog possible. Surface winds southeast to southwest
5-15 kts.


SHORT TERM...Tonight through Sunday...Rain and snow associated
with the frontal passage this morning continues to weaken across
the area. Lingering shower activity mainly noted across
southwestern Idaho. Much cooler temperatures observed across the
area today, resulting in some periods of snowfall for McCall and
Twin Falls. Brief improvement expected this evening, with a
potential for pockets of fog on Saturday morning. Another moisture
push expected Saturday, continuing the cooler temperatures and
the presence of rain and mountain snow across the area. Activity
looks most favorable Saturday afternoon and evening for
southwestern Idaho. Moisture will begin to lift more north Sunday
morning with improving conditions noted for the southern two
thirds of the area. All areas anticipated to see warming
temperatures, with the southern two thirds seeing values return to
around normal.

LONG TERM...Sunday night through Wednesday night...Moist
westerly flow aloft will continue Sunday night, with a chance of
showers over Baker County Oregon and the west central Idaho and
Boise Mountains. Models have been consistently showing a high
amplitude upper level ridge building over the west coast on
Monday, providing clear skies and slightly above-normal
temperatures for our area through Tuesday night. On Wednesday an
upper level trough from the Gulf of Alaska will weaken as it moves
into British Columbia and encounters the ridge. The trough will
slide southeast across northern Idaho and western Montana
Wednesday afternoon and evening as the ridge rebuilds offshore.
Precipitation with this system is expected to stay mainly north
and east of our area, but there will be a chance of showers over
the west central Idaho mountains Wednesday night.

Thursday and Friday...An upper level trough dips down through
northern Idaho and Montana early Thursday morning. The associated
precipitation with this trough just clips our northern zones
bringing chances of precipitation for the West Central Mountains.
Cooler temperatures aloft will drop snow levels Thursday to around
5000 feet for northern zones, but these levels will climb back
above 9000 feet by the weekend.





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