Area Forecast Discussion
Issued by NWS Boise, ID

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FXUS65 KBOI 070400

Area Forecast Discussion
National Weather Service Boise ID
900 PM MST Tue Dec 6 2016

.DISCUSSION...Fog has already begun to form in the Treasure
Valley west of Boise, including Caldwell and Nampa. Skies have
trended mostly clear in this area, and combined with light winds
and today`s snowfall, have allowed the fog to develop. The fog
will probably expand to other valleys through the night where the
skies have cleared and winds are light. Not sure at this time
whether the fog will be dense. For now will update to expand the
patchy fog to include the Upper Weiser Basin and the valleys
surrounding Idaho City. Temperatures will be even colder tonight
compared to last night with sub-zero readings expected in the
higher elevations, and single digits to teens in the valleys. Cold
dry weather is expected on Wednesday with mostly clear skies and
light winds under a northwesterly flow aloft.


.AVIATION...Decreasing clouds. With recent snow and colder
temperatures will see development of valley fog. Fog will produce
terrain obscuration along with IFR and local LIFR conditions.
Surface winds light and variable. Winds aloft northwesterly 10-20
knots up through 10K feet MSL.


SHORT TERM...Tonight through Thursday...A shortwave is currently
rotating out of the area with conditions expected to continue
clearing out as the evening wears on. Snow showers that are
currently ongoing in the Magic Valley should taper off near
sunset. As the clearing progresses overnight, low temperatures for
tomorrow morning are expected to plummet into the teens in the
Snake River Valley and single digits or less in the mountains. Fog
is possible as well tomorrow morning in the usual basins near
Baker City and Burns, OR and in the Long Valley. Patchy fog in
the Treasure Valley is in the forecast too, but confidence is not
as high as the previously mentioned locations. The existing snow
cover in the Treasure Valley, that is not expected to melt today,
is lending itself to the possibility of fog and low temperatures
being slightly below most guidance. Wednesday is expected to be
dry with high temperatures remaining around 10 degrees below
normal. For Thursday, the forecast becomes much more challenging.
An upper low off the coast of British Columbia will be pushing in
a warm front with a decent moisture tap into the area. Confidence
is high that all locations outside the Snake River Valley will see
possibly advisory level snowfall. However, in the Snake River
Valley, a persistent Southeast flow through the day on Thursday
will attempt to scour out the existing cold air that will settle
in tomorrow. The extent to which that SE flow gets to the surface,
when and what speeds will be experienced at the surface are
proving to be a bit of a challenge because small timing
differences exist in models. The current forecast is considering
the warm front overrunning the colder air in the valley for most
of Thursday producing snowfall of up to 2 inches, however, models
vary from less than an inch to 4 inches for this time period. By
Thursday evening the warmer air will begin to manifest itself in
the snow levels with a gradient from SW to NE of around 6000 to
1000 respectively. By this time, most of the QPF is expected to be
pushed into the Idaho Central Mountains with snow accumulations
continuing there. With the uncertainty of snowfall in the Snake
River Valley, and whether or not the 2 inch advisory threshold
will be met or not, no headlines will be put out at this time, but
it will be closely monitored.

LONG TERM...Friday through Monday...Unsettled weather will be the
rule late week through the weekend. A series of Pacific weather
systems will impact the region with a nearly continuous threat of
precipitation for most areas...especially the mountains. The first
will arrive late Friday into Saturday and the second late Sunday
into Monday. Initially on Friday snow levels will remain above
valley floors, but by late Saturday into Monday snow will be
possible in all locations across southeast Oregon and southwest
Idaho. During this period significant amounts of snow are expected
in the mountains and anyone with travel in the area should plan
for winter conditions and slick roadways. Temperatures are
expected to top out in the upper 30s to around 40 for valley
areas, with lows in the teens to 20s.

Monday Night through Saturday...Models in fair agreement in the
next surge of moisture pushing into eastern Oregon by Monday
morning. The moisture push will spread eastward through the day,
with shower activity potentially impacting much of the forecast
area. Mild daytime temperatures on Monday will keep most snowfall
out of the area valleys, though a rain-snow mix will be possible.
The duration of this upcoming event varies by model solution, with
the GFS being the least aggressive with precipitation coverage and
duration, and the Canadian being the most aggressive. Current
forecast trending more towards the moist solutions, with shower
activity remaining possible through Tuesday night.





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