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Area forecast discussion 
National Weather Service Spokane Washington
351 am PST Thursday Dec 18 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...
today through Friday night...a weak cold front and accompanying
trough is moving through the forecast early this morning and will
be followed by a weak ridge of high pressure today. Radar is
indicating rain and snow showers across the east and southeast
zones this morning as the front moves out of the area. Road
temperatures are warm enough that any snow that does fall should
melt. The exception will be over Lookout Pass where they could get
an inch or two through the early morning. Up-sloping flow will
keep rain and snow showers across the Panhandle through the day
with light accumulations expected, otherwise we should see a brief
break in the weather today. Expect areas of fog and stratus to
linger over the lower elevations and northern valleys through the
morning.

As mentioned above the break will be brief as another wet weather
system will move in off the Pacific tonight and track across the
forecast area on Friday. Moisture is plentiful with pwat's rising
to almost 200 percent of normal. South-southeast lower level flow
overnight will overcome the shadow effect off the Cascades and all
areas will see measurable precipitation. The cold front will track
across the region during the day Friday and result in a drying
trend from west to east Friday afternoon.

This same southerly flow will allow warm air to rapidly advect
into the region through the night. So we are expecting mainly
valley rain and mountain snow. The challenge is the cooler air in
the Cascade valleys...chiefly the Methow Valley and the Okanogan
Valley. Model guidance has been to cool in these valleys the past
few days and it looks like the 00z runs are following that trend.
That does not mean that these valleys will not see snow, just
possibly not as much as expected. This could easily result in a
busted forecast for these valleys. For now it looks like the
Methow Valley will pick up 1-3 inches of snow. The current models
sure want to scour out the remainder of the valleys and the
Waterville Plateau as the warm air move through the area. These
areas could still pick up 1-2 inches above 1500-2000 feet before
turning over to rain. For the mountains 2-5 inches are likely with
a few inches more up near the crest of the Cascades. For the
Wenatchee valley and along the Columbia River across to Spokane
and Coeur D'Alene the precipitation could start as wet snow or a
rain/snow mix before turning over to all rain through the night.

Some showers will linger across the Cascade crest and up-sloping
into the Panhandle will keep a pretty good chance of rain and snow
showers Friday night. Otherwise expect drier conditions for the
remainder of the forecast area. Tobin

Saturday through Sunday night...one very wet period. An atmospheric
river will take aim on the Pacific northwest. The GFS and European model (ecmwf) show
agreement of moderate isentropic lift spreading across the region
Saturday morning with a stream of plentiful moisture for most of
the weekend. Pwats of over an inch pool off the coast with up to
0.75" of inch making it east of the Cascades. Again precipitation
type will be tricky. Snow levels will be stubborn in the northern
valleys, especially in the Cascade valleys where some light to
moderate snow is possible. Warming increases Saturday night into
Sunday, making it a mainly rain/High Mountain snow event by Sunday
afternoon. In this transition, there is a small chance of a wintry
mix with pockets of freezing rain in a few of the Cascade valleys.
The 850mb 0c line pushes to the Canadian border on Sunday. Sunday
appears to be the warmest day with widespread 40s and a few 50s
possible. Quantitative precipitation forecast looks juicy with a 0.25" to 0.40" possible from
Saturday afternoon into Sunday morning. Gusty winds will increase
Saturday night into Sunday evening with speeds 25-40 kts across
the Columbia Basin. Gusty winds will continue in the mountains
through night, especially in the Cascades.

Monday through Wednesday...a cold front slides through the region
early Monday allowing the atmospheric river to sag southward and
cooler drier air to filter in from the north. The bulk of the
precipitation will continue to be found near the Cascade crest and
central Panhandle mountains through Monday through Monday evening
and then gradually taper off. A few lingering showers are possible
by early Monday morning across the northern mountains, but for the
most part partial clearing can be expected will allow temperatures
to cool late Monday night and into Tuesday. With the northwest
flow aloft, a disturbance is expected to drop down from the Gulf
of Alaska. In the 00z model runs this feature looks to swing wide
and drops west and south of the region, with the main brunt of the
precipitation found across Oregon. Opted to continue a drier
forecast through mid week with the best chance of snow falling
near the Cascade crest, Blue Mountains and central Panhandle
mountains. / Rfox.

&&

Aviation...
12z tafs: light rain and snow showers will be possible across the
eastern taf sites through 15-16z this morning with MVFR/VFR
conditions for kgeg-kcoe corridor and kpuw...VFR conditions at
klws. Improvements in both cigs/vsby expected after 18-20z.
Further to the west the southeast gradient will keep cigs/vsby
mainly VFR with conditions slipping to IFR at times through the
morning. These conditions should also improve after 20z. Another
weather system will increase the moisture and chances for
precipitaion beginning at keat around 03z and spreading east.
Although the best chance for the eastern taf sites may hold off
until after 12z. This will result in another deterioration in
cigs/vsby. Precipitation as a rain/snow mix at keat but turning
to quickly to -ra...otherwise precip as -ra. Tobin

&&

Preliminary point temps/pops...
Spokane 40 36 40 32 39 37 / 30 40 90 10 80 90
Coeur D'Alene 40 34 40 32 39 37 / 60 20 100 20 70 100
Pullman 42 35 43 31 43 41 / 40 40 90 10 70 90
Lewiston 44 35 46 32 45 44 / 40 20 80 10 50 80
Colville 37 34 38 33 37 36 / 30 60 100 20 80 100
Sandpoint 37 34 37 32 37 36 / 60 40 100 30 60 100
Kellogg 38 32 37 30 37 35 / 70 30 100 40 50 100
Moses Lake 42 36 42 32 40 37 / 10 70 70 10 80 80
Wenatchee 39 33 40 31 38 32 / 10 80 50 10 90 80
Omak 38 33 37 30 35 32 / 20 90 80 10 80 90

&&

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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