Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 202158
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
258 PM PDT Fri Oct 20 2017

.SYNOPSIS...
A windy and wet weekend is in store for the Inland Northwest.
Widespread rain and high mountain snow will develop Saturday
morning over the Cascades and spread through eastern Washington
and north Idaho through the day. Sunday will be quite windy with
gusts of 40 to 50 mph across portions of the open wheat country of
Washington and the Idaho Palouse. Mild weather will return Monday
and Tuesday before more wind and precipitation returns Wednesday.


&&

.DISCUSSION...

...SNOW TO IMPACT CASCADE MOUNTAIN PASSES SATURDAY...

...GUSTY WINDS SATURDAY NIGHT AND SUNDAY SOUTHEAST WASHINGTON...

Tonight through Saturday night: An active autumn weather pattern
continues during this time interval as one trof of low pressure
passes through tonight followed by a moist laden warm front
passage Saturday and Saturday night. In addition to widespread
rain and mountain snows...strong winds will be possible for some
areas.

With the cold trof passing through the remainder of this afternoon
and evening...convective showers will remain possible across
Northeastern Washington, North Idaho Panhandle, and Cascade Crest.
Activity across NE WA and N ID will be light with a mix of
rain/snow down to elevations of 2500 feet. Activity along the
Cascade Crest will have the potential to be heavier, especially
within a narrow band commonly known as the Puget Sound Convergence
Zone. Models have been consistent with the idea of this forming.
Moderate snowfall will be possible under this band which has a
decent shot of falling over sections of Hwy 2 around Stevens Pass
toward Coles Corner. The Seattle office has a winter weather
advisory out to address these concerns developing on the windward
side of the mountain. Breezy southwest winds currently in place
will remain breezy through the evening then weaken as they back
toward the southeast.

On the heels of the today`s trof is an atmospheric river and next
frontal system. There will be enough of a break tonight between
these systems to allow for clearing skies and sufficient
radiational cooling. This will allow snow levels to fall closer to
the 2500 foot level for many areas along the Hwy 20 corridor.
Widespread precipitation starts around the Cascades early Saturday
morning then spreads across all of Ern WA and N ID by afternoon.
Blewett, Stevens, Washington, and Loup Loup Passes are likely
expected to have snow and slick travel conditions. As such, a
winter weather advisory has been issued for above 3500 feet.
Carrying lower confidence but something to monitor will be
temperature trends for locations like Plain, Stehekin, and Upper
Methow Valley. Bufkit data via the NAM suggest some snow but with
borderline temperature profiles for accumulations. Further east
along Hwy 20 over Sherman Pass and I-90 for Lookout Pass, snow is
expected however later timing of precipitation suggest a lower
threat for snow-covered roads. Sherman will need to be monitored a
bit closer than Lookout due to its higher elevation. Snow levels
Saturday evening/night will climb quite rapidly rising above all
passes.

Winds will also be an issue Saturday night/Sunday time-frame.
Strongest winds will focus over the eastern third of the Columbia
Basin into the WA/ID Palouse, and Blue Mountains. This includes
the West Plains and adjacent areas of Idaho. Winds increase from
the south Saturday evening but steady rains will keep a potent
lowlevel jet of 50-60kts from descending to the surface. Higher
elevations in the Blue Mtns, Steptoe Butte, and into the Central
Panhandle Mountains stand a better shot for isolated gusts of this
nature. At this time, this carries low confidence. After 10PM
with the passage of the warm front, winds will really crank up
with sustained speeds increasing into the 25-32 mph range in these
aforementioned areas of SE WA. The low-level jet remains just a
few thousand feet off the deck but confidence remains low that
these speeds will make it to the surface and model guidance
largely agrees at this point. Nonetheless, sustained speeds at 30
mph can have moderate impacts and I will have the night shift look
into the potential for wind highlights. /sb

Sunday: Windy conditions will persist through late afternoon on
Sun. Deep low pressure will be over southern Alberta in the
morning and race across the Canadian Prairies toward Manitoba and
Saskatchewan by afternoon. Strong cold advection combined with a
potent jet stream overhead will contribute efficient momentum
transport to the ground. The GFS progs a 130kt jet at 250mb along
the Washington Canadian border on Sunday. Our gust potential may
peak in the morning hours along and just behind a passing cold
front. With 850mb winds of 50 to 60kts, there will be the
potential for sustained winds of 25 to 30mph with gusts of 40 to
50mph early in the day. Fortunately, the early morning frontal
passage may limit our mixing potential. If this front were to pass
during the afternoon, the gust potential could exceed 50mph.

How does Sunday`s event look compared to the wind we had on
Tuesday? At this time, it looks like the belt of strongest winds
look to be confined to areas along and south of Highway 2. We may
see some gusts comparable to Tuesday`s event in places like
Pullman, Spokane, Coeur d`Alene, Ritzville, and Waterville.
However, places like Colville, Cusick, Sandpoint, and Bonners
Ferry probably won`t see the magnitude of wind Sunday that we saw
on Tuesday given the anticipated low track.

Monday and Tuesday: A high pressure ridge will return to the
Pacific Northwest. We are looking for a couple of dry days with
light winds. Mild temperatures too.

Wednesday through Friday: The GFS and ECMWF forecast another windy
day for Wed. If the models hold true, this vigorous cold front
will hit us from the north. At this time, the probability for
strongest winds appear to be down the Okanogan Valley across the
Waterville Plateau into the Moses Lake area. We are still 5 or 6
days away so the forecast could change considerably, but pattern
recognition suggests the potential for sustained north winds of 20
to 30mph with gusts to 40 mph for Omak, Oroville and Douglas with
a bit less for Waterville, Ephrata, and Moses Lake. Light rain and
high mountain snow will likely accompany Wednesday`s frontal
passage. Thursday and Friday will likely be considerably cooler as
winds diminish and a very dry Canadian air mass settles over the
region. /GKoch

&&

.AVIATION...

18Z TAFS: Band of steady rain will shift east into Montana.
Residual moisture and favored boundary layer flow will keep MVFR
cigs arnd the eastern terminals 18-20z then lift above 3K AGL.
Atmosphere becoming increasingly unstable this aftn under a -28C
cold pool promoting convective showers across NE WA, N ID, and
Cascade Crest. A few lightning strikes will be possible 20-23Z.
Gusty westerly winds to impact most terminals through the day.
Winds decrease some after 00z but will remain steady until backing
to the south/southeast ahead a moist laden warm front. May have a
few hours of MVFR cigs prior to the wind switch. Next frontal
system to bring top-down moistening Sat morning with the onset of
pcpn 15-19z in a general west to east fashion. /sb


&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        36  46  45  60  40  56 /  10  90 100  20  10   0
Coeur d`Alene  36  45  41  59  40  55 /  10  80 100  40  10   0
Pullman        36  47  45  60  43  58 /  10 100 100  90  20   0
Lewiston       39  53  49  65  45  62 /  10  80  80  60  10   0
Colville       33  44  39  60  35  57 /  10 100 100  20  10   0
Sandpoint      33  44  38  55  34  53 /  10  70 100  90  10   0
Kellogg        35  42  39  52  38  52 /  30  70 100 100  30  10
Moses Lake     37  50  48  65  38  61 /   0 100  80  10   0   0
Wenatchee      37  47  41  62  40  60 /  10 100  80  10  10   0
Omak           33  47  38  60  34  58 /   0 100 100  10  10   0

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...Winter Weather Advisory from 5 AM to 7 PM PDT Saturday above
     3500 feet for East Slopes Northern Cascades.


&&

$$



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