Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 061154
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
354 AM PST Tue Dec 6 2016

.SYNOPSIS...
A low pressure system will move through the southeast zones today
and result in light to moderate snow at times for portions of the
Columbia Basin, Palouse, southern Idaho Panhandle, and Blue
mountains. Otherwise conditions will begin to dry from the
northwest with some rather chilly temperatures from this afternoon
through Wednesday night. More snow will be on the way Thursday
into Friday, and this snowy pattern will likely continue into the
weekend.


&&

.DISCUSSION...

...Light to moderate snow across the southern zones today
 then very chilly through the end of the week...

Today and tonight...A pesky surface low is currently spinning
through southeast Washington between the Tri-Cities east to the
Blue mountains, with wrap around moisture bending back to about
Coulee city. It looks like the the 00z GFS has a better handle on
the placement of the low, but most guidance is similar showing the
surface low slowly filling as it drops south and east through the
day. With the surface gradient northerly much drier air filtering
down the Okanogan valley and the Purcell trench. This should put
a quick end to any chance for precipitation north of abut I-90
this morning. Otherwise expect snow to slowly decrease through the
day from the northwest to the southeast. Chances for snow was
bumped up quite a bit through the morning to cover the slower
timing. Additional accumulations of 1-2 across the southern
Palouse, and the Camas Prairie, with 2-3 inches for the Blue
mountains will be possible.

The northerly gradient will increase to 4-5mb this morning then
begin to decrease through the afternoon and evening as the
surface low moves out of the region. Add in pretty good cold air
advection and winds done north-south valleys will be gusty through
the day. These winds will easily spill out onto the Waterville
Plateau, the upper Basin and likely into the Coeur d`Alene metro
area. This will create pretty uncomfortable conditions for people
working or recreating.

The much drier air and clearing skies will result in very good
radiational cooling tonight and combined with cold air advection
down north-south valleys we will have some very chilly overnight
temperatures. Lows from the single digits for the northern valleys
to the lower teens for the southern zones will be likely.

Wednesday and Wednesday night...Weak high pressure will build into
the region on Wednesday for one dry and sunny, but chilly day. By
Wednesday night moisture will begin to move up from the southwest
ahead of the next incoming system. For the Inland northwest this
will mean increasing cloud cover and slightly warmer overnight
lows. Tobin

...WIDESPREAD SNOWFALL THURSDAY AND THURSDAY NIGHT...

Thursday through Saturday night: Snow appears to be likely for all
areas beginning as early as Thursday. Snow is likely to continue
across the Inland Northwest through Thursday night. A low pressure
circulation is progged to spin up out in the eastern Pacific at
the base of an upper level trough of lower pressure at around 40N
latitude and between 140W and 150W longitude Tuesday afternoon.
The surface low will undergo further cyclogenesis in the left
exit region of a 110 +kt jet streak. There is still a lot of
uncertainty with how deep the surface low will get. Models are in
general agreement with the track. They all take the low northeast
and track it off the coast of WA. There are some subtle
differences with how far offshore the low will remain. The 00Z ECMWF
is much weaker with the surface low compared to the 00Z NAM, GFS
and Canadian. A warm front will swing up across the region from
the southwest, but there are some timing difference amongst the
models with this detail as well. The GFS is about 6 hours faster
than all the other models. I prefer the timing off of the ECMWF
model which is much more in line with the Canadian and NAM
solutions. The precipitation field looks most reasonable as well.
I went with a blend of mostly a non-GFS solution with precip, but
blended a little bit more toward the ECMWF.

With so many model differences, it is difficult to have much
confidence is snow amounts at this point. A general 2-5 inch swath
of snow can be expected with the warm front in the valleys;
however, some model members show amounts up as high as 6 to 10
inches. Tough to forecast amounts that high with out much surface
development over the region with the low remaining offshore. Snow
ratios will be high at around 15:1 to 17:1 due to the cold air
mass in place. This will result in dry snow and there could be
some blowing and drifting of snow with some breezy winds out of
the north Thursday into Thursday evening.

Chances for precip will continue into the weekend. Snow levels
look to lift above the L-C valley into Friday though with snow
possibly switching over the rain. This may be the case on the
Palouse as well, but most other valley locations look to remain
cold enough for snow. Best chance for precip Friday into the
weekend will be along the Cascade crest and into the ID Panhandle
with westerly flow resulting in favorable orographics. Even so,
any ripples that push across will be capable of generating precip
in portions of the basin. Temperatures will be modifying a bit
through this period, especially as winds begin to switch more
southerly Friday into the weekend. /SVH

Sunday and Monday: We stay in an active pattern through the
beginning of next week as a few weak disturbances move through.
Under zonal flow and with plenty of moisture available this will
favor snow showers in the typical orographically enhanced zones
including the Cascade Crest and Idaho Panhandle mountains while
the lee of the Cascades and Basin will be shadowed from most of
the precip. Temps remain cold with highs in the mid 30s and snow
levels generally under 1000 ft. /bwilliams


&&

.AVIATION...
12Z TAFS: A surface low is slowly tracking east and south through
the Columbia Basin this morning and will slowly move out of the
region later this afternoon. Plenty of moisture wrapping around
this low is resulting in widespread light to moderate
snow...mainly south of highway 2. The snow is expected to decrease
from north to south through the day. KGEG-KCOE-KSFF are just far
enough north of the low for a broken mid level deck and may see a
short showers early this morning...otherwise VFR conditions
through 12z Wed. For the remainder KMWH-KEAT -SHSN through 15-18z
with MVFR and short lived IFR cigs/vsby. Conditions will improve
18-20z to VFR...but there will be a chance for fog/stratus
development near KMWH 06-12z. KPUW-KLWS are in the thick of the
circulation with SHSN resulting in wildly fluctuating conditions
between MVFR-IFR and possibly LIFR through 20z or later. Then
some improvement later in the afternoon. Tobin



&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        26  10  20  11  23  23 /  20   0   0   0  50  90
Coeur d`Alene  24   7  20   8  23  22 /  10   0   0   0  50  90
Pullman        28  11  25  15  26  26 /  70   0   0   0  70  90
Lewiston       31  15  25  16  31  31 /  70  10   0   0  70  70
Colville       26  11  21   9  23  21 /   0   0   0   0  50  90
Sandpoint      21   8  16   5  21  20 /   0   0   0   0  30  90
Kellogg        21   5  16   4  21  21 /  20  10  10   0  50 100
Moses Lake     31  12  26  16  25  23 /  60   0   0   0  60  90
Wenatchee      30  16  25  15  23  23 /  40   0   0  10  70 100
Omak           27  10  23  11  23  22 /   0   0   0   0  60  90

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Winter Weather Advisory until 1 PM PST this afternoon for Idaho
     Palouse.

WA...Winter Weather Advisory until 1 PM PST this afternoon for Lower
     Garfield and Asotin Counties-Washington Palouse.

     Winter Weather Advisory until 7 AM PST this morning for
     Waterville Plateau-Wenatchee Area.

     Winter Weather Advisory until 10 AM PST this morning for Moses
     Lake Area.

     Winter Storm Warning until 10 AM PST this morning for Upper
     Columbia Basin.

&&

$$


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