Area Forecast Discussion
Issued by NWS Spokane, WA

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542
FXUS66 KOTX 121028
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
228 AM PST Tue Dec 12 2017

.SYNOPSIS...
Strong high pressure will persist through Thursday night. The
lowlands of eastern Washington and north Idaho will continue to be
covered by low clouds while the mountains remain mild and mostly
clear. Chances for wintry precipitation increase Friday morning.
Unsettled weather expected through next weekend and early next
week with a good chance of light snow returning to the mountains.

&&

.DISCUSSION...
Today and tomorrow...
The most significant change in our weather pattern over the last
week is an upper level disturbance rotating around the northern
periphery of the blocking ridge of high pressure. At this hour,
the circulation is just moving onshore over central BC...this is a
bit farther north than models were indicating this time yesterday.
Models are in agreement that this will apex later this morning and
then slide down the backside of the ridge toward far eastern
Montana Tuesday night.

This feature is already spreading increased high level clouds over
the Cascades. Models are indicating the mid and high level clouds
will continue to moisten through the morning hours as it also
shifts east into eastern WA and north ID. This is the cause of the
biggest changes to the ongoing forecast....first added the
potential for intermittent light freezing drizzle or flurries and
secondly, high elevations that have been cloud free will now have
a broken cloud deck at multiple levels.

As we`ve been seeing the last couple of nights, the very moist low
levels just need a small touch of lift or help from a mid level
deck to produce light freezing drizzle or flurries. The models are
actually picking up on the phenomenon first in the valleys of the
north Cascades then spreading to the Okanogan Valley and other
valleys of northeast WA later today. Then overnight Tuesday night
into Wednesday morning, there is again the chance in the lower
Columbia Basin. Confidence is low that this will have any impact
but added the wording anyway since any accumulation could result
in locally slippery roads.

The other minor changes were to refine the cloud coverage during
and after this wave moves past the region. This will have very
little impact for a majority of the region...just minor tweaking.
Otherwise the ongoing forecast was right on point through
Wednesday. /AB

Wednesday night through Thursday night: The strong ridge of high
pressure will prevail through Thursday night with widespread low
clouds, patchy freezing fog and light winds. Mountains will remain
milder than the valleys. Dry conditions continue.

Friday through Monday: A pattern change will arrive by Friday
morning, a bit later than previous forecast. The ridge of high
pressure flattens by a low pressure trough rolling across southern
Canada. A weak frontal boundary limps east of the Cascades with
light precipitation for Friday. Snow levels will be on the rise
although sub freezing temperatures found in the valleys, from the
lee of the central Cascades, across the Columbia Basin into the
Palouse and LC Valley. This area has the potential of a wintry mix
of precipitation with a a slight chance of freezing rain. By
afternoon, low level mixing increases with west to southwest winds
and surface temperatures will moderate leading to more valley rain
and mountain snow for Friday afternoon and night. A new
development in the models is the ridge of high pressure amplifying
off the coast. This leads to a northwesterly flow aloft and a
better chance of orographic precipitation across southeast
Washington and north Idaho. By Saturday, the ridge presses inland
albeit dirty with more mid level moisture streaming overnight.
Light orographic snow will continue in the mountains. Chances
increase for Sunday and Sunday night with the passage of a weak
impulse across southern BC leading to the potential for more light
snow in the mountains. Monday looks drier in the west northwest
flow aloft and it`s based on the timing and track of the next low
pressure trough that will take shape by the middle of next week.
Temperatures will trend milder into the weekend and early next
week. /rfox.

&&

.AVIATION...
06Z TAFS: Low layer of stratus filling in the lowlands and valleys
with the tops generally around 1500 feet AGL or less to change
little into Tuesday. Some patchy fog noted around the edges of the
stratus as well with the primarily impact being the low IFR ceilings
caused by the stratus. A repeat of northeast winds tomorrow may
allow for another retreat of stratus along the north and east
edges and allow for breaks during the afternoon and early evening
at KGEG, KSFF, KCOE, and KPUW  /Pelatti

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        28  25  31  25  30  27 /   0   0   0   0   0   0
Coeur d`Alene  31  25  33  25  32  27 /   0   0   0   0   0   0
Pullman        31  25  33  23  33  27 /   0   0   0   0   0   0
Lewiston       30  27  34  25  34  30 /   0   0   0   0   0   0
Colville       30  28  31  28  30  28 /   0   0   0   0   0  10
Sandpoint      32  20  33  17  31  25 /   0   0   0   0   0   0
Kellogg        31  21  31  19  32  26 /   0   0   0   0   0   0
Moses Lake     33  28  33  30  32  27 /   0   0   0   0   0  10
Wenatchee      32  28  32  29  32  28 /   0   0   0   0   0  10
Omak           32  29  33  30  32  29 /   0   0   0   0   0  10

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Air Stagnation Advisory until 10 AM PST Friday for Central
     Panhandle Mountains-Coeur d`Alene Area-Idaho Palouse-Lewis
     and Southern Nez Perce Counties-Lewiston Area-Northern
     Panhandle.

WA...Air Stagnation Advisory until 10 AM PST Friday for East Slopes
     Northern Cascades-Lower Garfield and Asotin Counties-Moses
     Lake Area-Northeast Blue Mountains-Northeast Mountains-
     Okanogan Highlands-Okanogan Valley-Spokane Area-Upper
     Columbia Basin-Washington Palouse-Waterville Plateau-
     Wenatchee Area.


&&

$$



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