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Area forecast discussion 
National Weather Service Spokane Washington
246 PM PST Thursday Feb 26 2015

Synopsis...
a weather system will clip the region through Friday with the best
chance for rain and mountain snow showers in the Cascades. Cooler,
breezy, and dry weather is expected on Saturday. Another weather
system will pass through Monday and Tuesday with wintry weather
possible especially the Idaho Panhandle. Much colder air will
move into the area behind this system with temperatures well below
average for the middle of next week.

&&

Discussion...
tonight through Saturday...there is good model agreement of a
closed low dropping south from Vancouver Island early this evening
south-southeast into the Oregon Cascades on Friday. This low moves
further from the area Friday night with cooler and breezy
conditions Friday night and Saturday. With the low tracking west
and south of the area most of the precipitation will miss eastern
Washington and north Idaho. Exception is the east slopes of the
Cascades where an easterly upslope flow will develop tonight.
Stevens and Blewett passes will have the potential to pick up
light snow accumulations mainly during the overnight hours into
Friday morning. Radar this afternoon also showed isolated showers
over the Idaho Panhandle, Palouse, and Columbia Basin. Overall
this activity will be on the declining trend after sunset with the
loss of daytime heating.

As the low drops south, north- northeast pressure gradients will
be on the increase. This will draw drier air into eastern
Washington and north Idaho tonight through Saturday. This will
keep the shower threat limited mainly to the Cascades, Okanogan
Highlands, Blue Mountains and Camas Prairie on Friday...before
these areas see the shower threat end Friday night.

As pressure gradients tighten Friday and Friday night and 850mb
winds increase to 25-35 kts it will become breezy to locally windy
down the Okanogan Valley into the Moses Lake area, the Purcell
Trench impacting Sandpoint and Coeur D'Alene, as well as the
Spokane area and upper Columbia Basin. Cooler air will also be
drawn into the area. The wind combined with air temperatures in
the mid 20s to mid 30s Saturday morning will lead to wind chill
values in the teens. Jw

Saturday night and sunday: the region will reside between weather
disturbances experiencing dry but cool conditions under shortwave
ridging within a broad northwest flow aloft. The blustery winds of
Saturday will wane through the evening and overnight hours and
skies will generally be clear. Look for an abundance of sunshine
to close out the weekend coupled with light winds Sunday
afternoon. If will be a chilly start to Sunday with most locations
starting off in the teens to twenties, but temperatures look to
creep back toward seasonal readings by the late afternoon topping
out in the 40s.

..potential for light to moderate snow Monday into Monday night
followed by breezy north winds and much cooler temperatures
Tuesday into Tuesday night...

Sunday night through Tuesday night: large scale northwest flow will
remain over the region allowing the next weather disturbance or
shortwave trof to drop into the region. Snow levels will remain
low enough to support snow for most outside the deep basin but
considerable uncertainty remains in place regarding storm
track/evolution and not all locations will receive measurable
precipitation from this event. General characteristics of this
storm suggest a stable regime initially, which is used to moisten
the atmosphere but the potential for the heaviest precipitation
will contain a convective nature, especially Monday evening and
night as a -35c cold pool slides overhead. This suggest the
potential for localized heavy snow showers and amounts. Once this
wave of moisture passes through, look for another push of modified
Arctic air bringing gusty north winds and much cooler temperatures.
Below is the breakdown of confidence levels, snow probabilities,
and additional impacts.

* Snow threat: moderate to high for portions of the Idaho
Panhandle. Low to moderate for eastern Washington east of a
line from Colville to Ritzville to the Idaho/Washington border. Low from
the Cascades to a line from Colville to Ritzville (except along
the Cascade crest)

* wind: gusty north to northeast winds of 15-25 mph with gusts to
45 mph possible through the Okanogan Valley, Purcell Trench, and
across the Columbia Basin Monday night and Tuesday morning.

* Impacts: snow could lead to wintry travel conditions as early as
Sunday night over the Cascade Mountain passes and spread across
remaining mountain passes Monday and Monday night. Snow could
also impact travel in lowlands across eastern Washington and Idaho.
Please note, the threat for accumulating snow on roads decreases
quite a bit during daylight hours this time of year but is
likely to refreeze quickly after sunset. Cold temperatures
arriving Monday night into Tuesday will be the coolest we have
seen since early January with some valley locations potentially
not warming above freezing.

Wednesday and thursday: we will enter another period of quiet weather
around midweek as the offshore ridge begins to migrate inland.
Forecast models are in good agreement and confidence is above
normal. The ridge will flatten somewhat by Thursday allowing some
moisture to stream along the international border but the main
impacts look to be increasing clouds and very little to no
precipitation. This slight buckle within the jet stream will also
cut off the region from northerly flow allowing substantial
moderation of the low to midlevel air mass. Model consensus
indicates 850mb temperatures over northeastern Washington will
moderate from near -10c Wednesday morning to -1c Thursday
afternoon. With light winds at the surface, it is less certain how
this will translate at the surface but GFS does indicate potential
mixing to this level and if that is the case, high temperatures
will likely need to be bumped up. The uncertainty comes with how
cold it will be overnight and whether these diurnal swings are
possible. /Sb



&&

Aviation...
18z tafs: a low pressure system near Vancouver Island will drop
into Oregon Friday morning...with the main influence being an
increase in -ra across central Washington including keat. Easterly
upslope flow will trap abundant amounts of low level moisture into
keat with MVFR conditions likely. IFR stratus can not be ruled out
with low confidence regarding how far the ceilings will lower. This
low will pull drier air into NE Washington and north Idaho tonight
from the northeast, which should limit the threat of fog/stratus
at kgeg/ksff/kcoe/kpuw. Jw



&&

Preliminary point temps/pops...
Spokane 29 46 25 42 20 41 / 10 10 0 0 0 0
Coeur D'Alene 27 46 25 42 19 43 / 10 10 0 0 0 0
Pullman 32 47 26 39 20 41 / 20 10 10 0 0 0
Lewiston 33 52 29 45 22 45 / 20 10 10 0 0 0
Colville 31 47 26 47 22 45 / 20 20 10 10 0 0
Sandpoint 26 41 23 39 18 40 / 10 0 0 0 0 0
Kellogg 27 41 19 36 18 41 / 10 10 10 0 0 0
Moses Lake 35 53 31 50 23 46 / 30 40 10 0 0 0
Wenatchee 38 51 32 52 27 48 / 50 70 20 0 0 0
Omak 34 50 30 50 24 46 / 40 20 20 0 0 0

&&

Otx watches/warnings/advisories...
Idaho...none.
Washington...none.
&&

$$

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