Area Forecast Discussion
Issued by NWS Boise, ID

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FXUS65 KBOI 170341

Area Forecast Discussion
National Weather Service Boise ID
841 PM MST Sat Dec 16 2017

.DISCUSSION...No update to the forecast for tonight. A dry,
stable air mass is moving in aloft as seen on the evening Boise
weather balloon data. In the low-levels, converging flow in the
Western Magic Valley is maintaining lower cloud cover a few light
snow showers. This activity will diminish through 06Z this
evening and little if any accumulation is expected from the shower
activity. Winds weaken overnight with mostly clear skies,
resulting in cold low temperatures Sunday morning. Fog remains a
possibility in sheltered valleys or areas where a northwest wind
acts as an upslope wind.


.AVIATION...VFR for most locations with some patchy fog causing MVFR
conditions in lower valleys and sheltered mountain passes. Quick
burn-off of fog after sunrise with increase in mid-upper level cloud
cover through evening hours. Surface winds, variable 5-15 kts. Winds
aloft to 10k feet MSL, north-northwest 10-30 kts.


SHORT TERM...Skies have cleared and the inversion has been scoured
by gusty west/northwest surface winds.  Winds will diminish this
evening, and under clear skies temps will become quite cold,
especially where fresh snow cover remains.  Patchy fog will form
in the valleys late tonight and Sunday morning.  Warm-frontal
mid/high clouds will come in from the west Sunday, mainly over
northern areas, and there will be a chance of light snow in
northern mountains late Sunday through Monday.  There will also
be warming in all areas at higher elevations resulting in a new
inversion in the cold valleys.  This inversion will last until a
storm comes in from the Gulf of Alaska Wednesday.

LONG TERM...Monday night through Saturday...A warm-frontal boundary
will lay draped along the northern tier of Baker to Valley Counties
through early Tuesday morning before it begins to lift north. During
this time period, precipitation will continue along the front with
snow levels near 4000 feet; this translates to snow in the mountains
and a wintry mix in the mountain valleys. As the boundary pulls
north Tuesday, warm air will surge across the region from the south
warming temperatures to a few degrees above normal and bringing a
brief break in cloud coverage and precipitation. A deep trough off
the Pacific coastline will dig southeast bringing widespread
precipitation to the region beginning overnight Tuesday and
spreading across southwest Idaho by daybreak Wednesday. As snow
levels fall with an associated cold front, and temperatures return
to normal for Wednesday`s high, rain will first transition to a
wintry mix and then to snow by evening-time along the Snake River
Plain. However, by this time, most of the precipitation will have
moved east of the region and a cold northerly wind will remain
aloft in its stead. Temperatures will drop a few degrees below
normal Thursday, and drop again by a few degrees for the beginning
of the weekend. Conditions look to remain generally dry during
this period as the ridge builds back toward the north and east.





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