Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 010102

Area Forecast Discussion
National Weather Service Spokane WA
310 PM PST MON NOV 30 2015

Low clouds and areas of fog will be common in valleys. Strong
high pressure will contribute to air stagnation and cloudy skies
in the lowlands through Tuesday. A weak frontal system is
expected to bring a chance of light wintry precipitation Tuesday
night into Wednesday...with a wetter system moving in wednesday
afternoon into early thursday. The combination of the two weather
systems will likely produce light to moderate snow near the
Cascades and over the northern washington mountains. There will
also be a threat of freezing rain across western portions of the
Columbia basin, Wenatchee valley and okanogan valley. wet but
warmer weather is expected from thursday afternoon and into the



Tonight through Tuesday...A pair of weak upper level disturbances
pass over the ridge of high pressure over the area but do little
other than to force a bit more snow flurries out of the stratus
and add a bit more cloud cover for the mountains and elevations
above 4000 feet MSL which are poking above the low stratus clouds
which remain entrenched over the valleys and lowlands below 4000
feet MS. Having more flurries occurrence usually means that the
fog will remain less prevalent and ceilings will rise a little bit
and there will be a very slight warming trend in daytime high
temperatures for Tuesday relative to those of today. /Pelatti

...light to moderate snows possible over portions of the Inland
Northwest as well as areas of freezing rain between Tuesday night
and Thursday...

.Tuesday night through Thursday...Confidence is quite high that the
stable ridge which has sat over the Inland NW for the past week
or so will begin to break down or shift into western Montana. This
will result in an increasing threat of precipitation as the mid
level flow pattern turns increasingly southwesterly and brings
Pacific moisture into the region. The first threat of
precipitation arrives early in the evening at the Cascade Crest
and moves steadily eastward into Wednesday morning. This feature
is fairly progressive and expected to weaken into the ridge over
Montana. Consequently the chances for the heaviest precipitation
will likely occur over northern and western portions of the
forecast area, including the Cascades. Precipitation amounts won`t
be all that heavy with the heaviest amounts expected to occur over
Chelan, Okanogan and Douglas counties, where anywhere from
0.10-0.20 inches of liquid is expected. Initially the
precipitation will fall as all snow, however a slug of above
freezing air is expected to slowly wedge its way into the southern
portions of the Columbia Basin and perhaps the Wenatchee area by
Wednesday morning. This could bring some light freezing rain to
locations such as Moses Lake, Ritzville, and Vantage as the warm
wedge rides over the sub-freezing air near the ground. This would
likely impact much of I-90 across this area so travel problems are
certainly possible. Farther north all the precipitation will fall
as snow with 1 to 3 inches possible over the northern valleys with
an less than inch over the vast majority of the region. Overall
this is a fairly minor system but the threat of ice could produce
moderate impacts.

The bigger problem with the forecast is the second system which
is expected to move in from the southwest in the form of a warm
front on Wednesday. This system is expected to pack much more
moisture and potentially more problems. The 12z GFS and 12z NAM
brought the warm front into the lower Basin, and Wenatchee area
before sunset and push it slowly north toward the Canadian border
by Thursday morning. Meanwhile the ECMWF and Canadian models hold
off any threat of significant precipitation until Thursday
morning. This is a big difference to say the least and would have
large impacts on the area weather. If the GFS and NAM solutions
were correct, we`d be looking at moderate precipitation amounts
with the sub-freezing air holding stubbornly near the ground.
There is the other question as to how long would this sub-freezing
air remain near the ground as the winds aloft increase steadily.
Near the Cascades we expect the freezing air to remain the longest
and deepest as there is a good offshore pressure gradient which
will reinforce the cold-air damming. Moderate snow amounts would
be possible especially over Okanogan, Chelan, and Douglas
counties. The warm wedge of air aloft would also transition the
snow to freezing rain especially for the Columbia Basin, Wenatchee
area, Waterville Plateau, and Okanogan Valley. The warm air aloft
could also move into the northern Washington and Idaho Valleys
late Wednesday night resulting in a smaller risk of snow changing
to freezing rain. Farther east and south the transition from snow
to rain would be more steady, so for places like Spokane, Pullman
and Ritzville we`d be looking at snow amounts less than an inch
with more liquid than snow. Of course this is assuming the NAM and
GFS solutions are correct. That is a BIG if at this point.

The ECMWF, Canadian, and new 18z GFS hint at a much slower arrival
of the warm front with little if any precipitation until early
Thursday morning, We would still see the possibility of some mixed
winter precipitation ahead of the warm front including in the
Spokane area but the switch would occur fairly quick. All models
concur that 850 mb temps and consequently snow levels skyrocket as
the front comes in so significant snows won`t be the problem if
the latter solutions verify. By Thursday afternoon, 850 mb temps
soar to well above zero for all locations except near the
Cascades, so rain would become the primary precipitation type.
Needless to say the confidence for this forecast is quite low. We
will hold off on winter highlights for now and focus our efforts
via social media and weather stories. fx

Friday through Monday...Models are similar showing a mean long
wave trough over the western states through the weekend with
several disturbances moving through the general flow. It`s in the
finer details where we start to see some differences in timing and
strength of each wave.

For the wave on Friday it looks like another wet system through
the day and into Friday evening. An embedded upper level trough
will move through the region. The lower east slopes and deep basin
will likely see some shadow effect off the Cascades but still
could pick up a hundredth or two of precipitation. For the
mountains there will be the possibility of between a quarter to a
half inch of precipitation. Snow levels will result in
precipitation as rain below 3000 feet and rain above, with 3-5
inches of snow possible. The exception would be the Methow valley
where cooler air is expected o be locked in that valley. 10-15 mph
southwest winds will allow good mixing with temperatures rising
into mid 30s to mid 40s.

Saturday through Monday...the confidence in the forecast is rather
low at this time. Model guidance is similar showing that several
waves will move through the mean trough, however the timing and
strength of each individual wave is quite suspect. So this portion
of the forecast is pretty generalized. At this time it looks likes
the best chance for precipitation will be Saturday night and
Sunday. Warm air advection in southwest flow will allow snow
levels to increase to between 3k-4k feet so precipitation would be
as valley rain and mountain snow. Temperatures will be on the warm
side of normal. Stay we get closer to the weekend we
will have a better idea and just what the weather will hold. Tobin


00Z TAFS: Stratus with ceiling btwn 800-1400 AGL will continue to influence
the aviation area through 00z Wednesday.  A weak wave overtopping the
low clouds will bring the potential for some lifting and additional
periods of light snow in the form of flurries or snow grains.  The top
of the stratus will generally be between 4500-5000 ft MSL or roughly
2500-3000 feet AGL creating wdsprd mtn obscrns with only the higher
peaks extending into VFR skies.  There has been some clearing vcnty KLWS
but expect this to fill back tonight.  Exact timing carries moderate
uncertainty.  A much wetter storm system spreads clouds into the region
aft 00z Wed with a continued threat for IFR/VFR conditions, much deeper
cloud layer, and potential for light snow.  /sb


Spokane        18  30  25  35  33  43 /   0  10  20  60  50  30
Coeur d`Alene  17  32  25  36  34  45 /   0   0  20  50  50  40
Pullman        20  33  27  37  33  48 /   0  10  20  60  40  30
Lewiston       22  36  28  41  35  50 /   0  10  10  40  30  20
Colville       17  30  23  35  32  35 /   0  10  50  70  70  70
Sandpoint      16  30  23  34  32  37 /   0   0  20  60  50  50
Kellogg        16  31  23  35  33  40 /   0   0  10  50  50  20
Moses Lake     18  31  26  33  33  40 /  10  10  60  50  60  40
Wenatchee      19  29  24  33  32  34 /  10  10  80  60  80  50
Omak           17  29  24  34  32  34 /  10  10  80  50  90  70


ID...Air Stagnation Advisory until noon PST Friday for Central
     Panhandle Mountains-Coeur d`Alene Area-Idaho Palouse-Lewis
     and Southern Nez Perce Counties-Lewiston Area-Northern

WA...Air Stagnation Advisory until noon 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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