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Area forecast discussion 
National Weather Service Spokane Washington
356 PM PST Wednesday Dec 17 2014

Synopsis...
a more active weather pattern will continue through the end of the
week with periods of light snow in the mountains...and rain or
snow in the valleys. Lower elevations from Interstate 90 south
will see mainly rain...while areas in the Cascades and near the
Canadian border will remain mainly snow. A much stronger weather
system over the weekend will usher in warmer temperatures with
sharply rising snow levels...as well as the potential for windy
conditions.

&&

Discussion...

Tonight and tomorrow...a number of small weak disturbances running
across eastern Washington and northern Idaho through tonight and
tomorrow. They are very difficult to pick out in some of the
models as there is too much clutter in the flow associated with
the weakening baroclinic band upon where these disturbances are
located near that is currently passing through the area. They
haven't been showing up too well in the radar either as they have
been far enough away for most of the day for the base elevation
beam to get a return on as well. Best bet to find these features
has been the infrared satellite imagery on a fast rock and picking out
the small scale rotation and cloud top cooling. One such feature
is moving northeast this afternoon and is causing precipitation
just east of Moses Lake up to Omak and some points to the north
and east. A second feature, appearing to almost be a southern part
of the splitting/weakening baroclinic band has brought light rain
to Lewiston. All of these disturbances translate to the northeast
and east at varying times tonight and tomorrow and as such the
pops and quantitative precipitation forecast show a general favoring of locations to the north and
east (including locations near Spokane down to pullman) to receive
some precipitation but as expected the amounts are not too
significant...still it doesn't take too much liquid to pool on
pavement that in turn cools overnight to allow for slick
conditions in spots similar to what occurred this morning.
/Pelatti

Thursday night through Sunday...an active weather pattern is
expected especially over the weekend with two systems to impacts
the inland northwest during this forecast period.

Thursday night/friday: a large upper low seen on infrared
satellite imagery in the Gulf of Alaska will drop southeast
sending a cold front across the area. The timing of the front
tracks into central Washington Thursday night during the overnight
hours...reaching eastern Washington and north Idaho Friday
morning. A north-south oriented jet parallel to the front and
southerly low/mid level flow will minimize downslope flow with all
areas likely to receive some precipitation. Inspection of model
soundings suggest this to be a mainly valley rain or rain/snow
mix...with snow in the mountains. 850mb winds backing to the south
will aid in bring up warmer air from the south. However a wedge of
cooler air trapped along the east slopes of the Cascades will
result in lower snow levels over north central Washington with the
potential for 2-3 inches of snow accumulation above 1000-1500
feet. This includes the Methow Valley, plain, Waterville Plateau,
higher portions of Omak, and Republic. The Wenatchee area and
lower portions of the Okanogan and Columbia River valleys are
expected to see mainly a rain/snow mix or wet non-accumulating
snow. The mountains will generally see 2-5 inches of snow with the
Cascades seeing the highest amounts.

Saturday through Sunday...a much stronger system is expected this
weekend bringing a wide variety of expected weather. The models
continue to show an atmospheric river as a subtropical moisture
fetch becomes aimed at the area. The leading edge of this moisture
will enter on Saturday in the form of a warm front. Strong
isentropic lift will result in the development of widespread
rain and snow. 850mb winds backing to the south-southeast will
favor upslope flow into the northern mountains, with cooler air
remaining banked up into the Cascades, Okanogan Valley, and
Okanogan Highlands. Models suggest the potential for moderate to
locally heavy snow accumulations in these areas along and ahead of
the warm front before snow levels rise from south to north
Saturday night into Sunday morning. Snow levels will be fast to
rise from Interstate 90 south with precip expected to fall as
mainly valley rain and mountain snow. The valleys north of Spokane
such as Colville, Metaline Falls, Priest Lake, and Bonners Ferry
may start off as snow on Saturday with light accumulations...before
changing to rain Saturday night. As the warmer air aloft pokes
into the Cascades Saturday night...the valleys will likely have a
hard time mixing out with freezing rain possible especially south
of Lake Chelan in places like plain and Leavenworth.

On Sunday the atmospheric river shifts south near the
Washington/Oregon border with strong westerly flow over the area.
This will lead to a strong rain shadow east of the Cascades with
precipitation likely confined mainly to the Cascade crest, Idaho
Panhandle, and southeast Washington. 850mb winds of 25-35 kts and
possible sun breaks across the Columbia Basin...and 850mb temps of
3-6c could lead to very mild temperatures along with breezy to
windy conditions. For now went a bit above MOS guidance which
usually does not go warm enough in these situations. Jw

Sunday nt through Christmas eve: following the passage of the windy
and wet short-wave trough that will move through broad upper
ridging Sunday nt, what's left of a stalled cold frontal boundary
will remain over nrn Oregon and extreme srn WA tues and keep the
threat of pcpn in the fcst for the Lewiston area as well as the Camas
Prairie. Where this front stalls will be critical as far as the
Christmas eve fcst, because it is this boundary that will combine
with a significant short-wave trough and vort Max moving into the
region as it translates southeast down the British Columbia coast. Deep cyclogenesis is
fcst by most model guidance over ern Oregon Wednesday as the flow
backs and increases lift over the aforementioned boundary. As
pressure gradients increase, cold air will then begin to move into
ern WA from srn BC. Though it's unusual even to see modest amnts
of agreement among models as to the placement of this deep low 7
days out, there still are enough differences how far south the
low will track that still make our level of confidence low as to where
potentially heavy snow bands may set up Christmas eve. The trend,
however, has been slightly south which would keep the higher snow
threat out of nrn WA. Stay tuned... bz

&&

Aviation...
00z tafs: a weak weather disturbance is generating light precip
across eastern Washington and in the Idaho Panhandle late this afternoon.
This wave will begin to exit over the course of this evening, but
will be followed by another overnight with more light precip to
fill in across the region. Low level winds will also veer through
tonight, which will result in more favorable upslope flow for
stratus formation. Expect cigs/vis to deteriorate tonight at
kgeg, ksff, kcoe and kpuw with IFR/MVFR conditions and localized
LIFR conditions possible. /Svh

&&

Preliminary point temps/pops...
Spokane 32 37 33 39 31 40 / 50 30 40 90 10 70
Coeur D'Alene 33 38 32 39 32 39 / 60 60 20 100 20 70
Pullman 33 43 34 42 32 43 / 50 40 40 90 10 70
Lewiston 35 45 36 46 33 47 / 40 30 20 70 10 40
Colville 32 37 34 38 32 38 / 60 30 60 100 10 80
Sandpoint 32 37 32 36 31 38 / 60 60 30 100 40 60
Kellogg 32 38 32 36 31 36 / 70 70 20 100 40 60
Moses Lake 29 39 36 42 30 39 / 20 10 70 70 10 70
Wenatchee 31 37 33 40 30 34 / 30 10 70 60 10 70
Omak 27 34 32 37 30 33 / 40 10 80 80 10 80

&&

Otx watches/warnings/advisories...
Idaho...none.
Washington...air stagnation advisory until noon PST Friday for east slopes
northern Cascades-Moses Lake area-northeast mountains-
Okanogan Highlands-Okanogan Valley-upper Columbia Basin-
Waterville Plateau-Wenatchee area.

&&

$$

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