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000
FXUS66 KOTX 080540
AFDOTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SPOKANE WA
939 PM PST Sat Dec 7 2013

.SYNOPSIS...
Tonight will be the coldest night of this arctic outbreak, and
many locations will dip below zero. An upper level disturbance
will bring a chance of light snow to north Idaho and extreme
Eastern Washington on Sunday. The threat for snow will increase
Wednesday into Thursday as Pacific moisture moves into the region.


&&

.DISCUSSION...
Tonight through Sunday...Bitterly cold temperatures are expected
tonight with most areas expected to drop below zero. Satellite
imagery shows a strong blocking ridge in the northern Pacific
along 140W with a continuation of northerly flow. Models are in
general agreement that a weak system over northern British
Columbia and Alberta will drop south clipping extreme Eastern
Washington and north Idaho on Sunday. This timing suggests that
most areas will see sufficient clearing tonight which combined
with clear skies this evening and winds becoming light will allow for
strong radiational cooling, especially snow covered valleys. There
are several factors however that will likely affect the temperature
forecast tonight depending on location.

Mid and high clouds will increase from north to south beginning near
the Canadian border late this evening which will keep temperatures
from dropping to their full potential. The Kelowna- Omak pressure
gradients as of 2 pm was also still at 5mb suggesting that northerly
winds may not completely die off this evening in the Okanogan Valley
which may keep low temperatures from falling as quickly. A few
pockets of stratus near the Blue Mountains could also interfere with
radiational cooling around Pomeroy, Lewiston, and Pullman but this
is not expected to have big impacts on low temperatures with NAM
model Boundary layer progs not favorable for an expansion of
stratus. If Pullman stays clear until the mid and high clouds reach
this area late tonight then lows could drop below -10F with snow
cover accelerating surface cooling this evening.

As the weak clipper system drops into the area Sunday GFS and
ECMWF suggest nothing more than flurries for extreme Eastern
Washington and the Idaho Panhandle while the NAM and SREF
solutions show much more moistening of the lower levels with a
3-6 hour period of light snow. Given that an air mass this cold
won`t take much moisture for light snow to develop forecast leans
towards the NAM/SREF solutions. With cold temperatures tonight and
then increased cloud cover tomorrow high temperatures will remain
very cold especially over the Idaho Panhandle with valley highs
struggling to surpass 10-12F. JW

Sunday night through Tuesday...Northerly flow over the inland
Northwest for Sunday night into Monday will gradually transition
into northwesterly Monday afternoon and Tuesday. This is the
result of the high amplitude ridge beginning to flatten and move
east in response to an upstream weather system moving through the
gulf of Alaska. A series of weak disturbances in the north-northwest
flow will mean a continuation of mainly mountain snow
showers...with temperatures beginning to warm 4-5 degrees each
day. But temps will remain below seasonal normals.

Northwest flow will put the focus of light snow across the Cascade
crest...but a much better chance in the orographically favored
northeast mountains of WA...the Idaho Panhandle and to a lesser
degree the Camas Prairie. None of these weak waves will be very
strong..but with the help of orographics the atmosphere should be
able to squeeze out some light snow. Moisture is not overly
abundant but the dendritic layer is well saturated and snow
production should be moderate or better. What this scenario looks
like will be a fairly high pop but low QPF. Most of the higher
elevation of the Panhandle ...northeast mountains and near the
crest of the Cascades may pick up 1-3 inches of snow Monday and
Tuesday...with a dusting to maybe a half inch at the lower
elevations of the Panhandle zones. Otherwise most of the forecast
area will remain dry. winds will be light and mainly out of the
south. /Tobin

Tuesday night through Saturday...The Inland Northwest will be in a
rather active weather pattern as a series of shortwave disturbances
move through the region. The first of these will bring some light
snow to the region Tuesday night into Wednesday. A second stronger
frontal system with a good tap into Pacific moisture will affect the
Inland Northwest on Thursday. Warm air advection in the mid levels
will bring a period of overrunning precipitation to the forecast
area, with the threat of sleet or freezing rain for parts of the
Columbia Basin and Lewis-Clark valley. The northern half of the
forecast area will likely see snow levels remain at valley floors
through the work week and possibly the weekend as well. Another wet
frontal system is set to arrive for the weekend. The large scale
weather pattern will bring west to southwest flow to the region with
a resulting moderation in temperatures. The trend will be back to
more seasonal temperatures, but for mid December that means highs in
the 30s and lows in the 20s. /Kelch

&&

.AVIATION...
06Z TAFS: A weak disturbance will drop southward across the region
out of Canada on Sunday. There will be a chance for some light snow
and MVFR cigs at KGEG, KSFF, KCOE between 16-21Z and at KPUW and
KLWS between 18-02Z; MVFR cigs and vis will also be possible with
the snow, but confidence is low. A very dry boundary layer and weak
lift may result in only some mid level cloud cover, but it will not
take much to squeeze out a little bit of snow in this arctic air
mass. /SVH


&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        -2  12   3  18  10  24 /   0  40  10  10  20  30
Coeur d`Alene  -3  11   2  20  11  24 /   0  50  10  30  30  30
Pullman        -8  11   0  20  12  26 /   0  40  10  10  20  40
Lewiston        2  16   4  24  17  29 /   0  40  10  10  10  20
Colville        1  16   4  20   9  25 /  10  20  20  20  20  20
Sandpoint      -4  12   2  19  10  22 /   0  50  20  40  40  30
Kellogg        -4  10   1  18  11  23 /   0  50  20  40  40  50
Moses Lake      1  18   1  21  10  26 /   0   0  10  10  10  10
Wenatchee       2  18   5  21  14  27 /   0   0  10  10  10  10
Omak            2  17   3  21  11  25 /   0   0  10  10  10  10

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...None.
&&

$$