Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 182148
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
248 PM PDT Wed Oct 18 2017

.SYNOPSIS...

Moderate precipitation will continue to fall tonight into Thursday
as a low pressure system slowly sags through the region. A cold
front will follow on Friday for showers, gusty winds and cooler
temperatures that will last through the weekend and into Monday. A
return to warmer temperatures and drier conditions is expected next
week.

&&

.DISCUSSION...


Thursday: The cold front will make very slow progress through
eastern Washington and north Idaho Thursday and Thursday night.
The models are in good agreement that the large low currently off
the BC coast will shear northward and a secondary shortwave will
cause the trough along the coast to dig southward. The end result
will be a very slow moving frontal system that will likely bring
an all day rain to northern Idaho Panhandle and northeast
Washington. The Spokane metro area will likely experience rain for
most of the day as well. The East Slopes of the Cascades and the
Wenatchee/Moses Lake area will have a good shot of some light
rain, but amounts should be less than a tenth of an inch as strong
mid-level southwesterlies shadow a chunk of central Washington.

By Thursday night, the GFS, ECMWF, and NAM push the shearing cold
front into the central and southern Idaho Panhandle. Despite the
front stretching apart, there should be enough deep layer moisture
and orographic forcing to produce a tenth to two-thirds of an inch
of rain for the mountainous zones of the central and southern
Panhandle Thursday night. Look for snow levels to fall into the
4000-5000ft range over the Cascades Thursday night and down to
6000ft in the Panhandle as the cold front limps eastward.

Friday: The Palouse, Basin, and West Plains will be windy on
Fri behind the cold front. Sustained southwest winds of 15 to 20
mph are expected with gusts up to 30mph over the open wheat
country. Precipitation will become more showery under the cold
upper trough with snow levels in the 4000-5000ft range. The high
terrain along the Cascade Crest will accumulate several inches of
snow Thursday night into Friday while the Panhandle mountains will
likely experience transient snow showers capable of light
accumulations that will likely melt during the afternoon. /GKoch

Friday night through Sunday night: Unsettled weather continues
with cold conditionally unstable airmass shows an exit to the east
with the jet stream to the west and south of the forecast area,
thus low snow levels result from the resulting steep lapse rates
at 700-500mb with snow likely reaching valley floors at times but
with a snow level signaling realistic accumulations of snow above
3000 feet Friday night into Saturday morning. The cold
conditionally airmass then moves to the east and the jet stream
then moves either overhead or slightly to the north of the
forecast area Saturday afternoon through Sunday night to allow
snow levels to rise to near 6000 feet. At its peak the
precipitable water content streaming into the baroclinic band
running near parallel to the jet stream has values near 1.5 inches
through its core that taper down to near an inch on the east side
of the Cascade Crest Saturday afternoon and evening and as such
this portion of the forecast merits the highest pops and qpf of
the seven day forecast.


Monday through Wednesday: Ridge of high pressure amplifies and
pushes the jet stream and the moisture running parallel to it to the
north and away from Eastern Washington and Northern Idaho allowing
for a decrease in pops and sky cover Monday that transitions to a
generally dry and mostly clear forecast for Tuesday and Wednesday
coupled with a warming trend that takes temperatures back slightly
above seasonal normals and lighter winds in comparison to those
occurring over the weekend. /Pelatti

&&

.AVIATION...

18Z TAFS: The addition of low level wind shear to the Spokane and
Coeur d`Alene TAFS is the biggest change to the latest forecast.
The NAM and GFS are in good agreement developing a 50kt jet at
850mb overnight ahead of a slow moving cold front. The core of
this low level jet is expected to set up from the Dalles to Moses
Lake to Spokane. Given the higher elevation of Spokane and CDA, it
looks like the belt of strong winds will be within 2000 feet of
the surface. At this time, we did not include low level wind shear
at Moses Lake or Wenatchee since it the strongest shear may be
just above the 2000 feet criteria. Bands of rain will also
accompany this slow moving system. Warm frontal rains this
afternoon/evening and more rain with the cold front Thu morning.
/GKoch



&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        48  56  42  53  36  49 /  60 100  90  50  20  70
Coeur d`Alene  46  56  42  52  36  47 /  70 100 100  60  30  70
Pullman        46  60  42  52  37  49 /  10  50 100  50  40  70
Lewiston       48  65  46  59  40  54 /  10  10  90  40  20  60
Colville       47  56  44  55  34  47 / 100 100  90  40  30  80
Sandpoint      45  51  42  51  33  45 / 100 100 100  80  40  70
Kellogg        45  58  41  46  35  44 /  40  50 100  70  70  60
Moses Lake     50  60  43  61  35  53 /  40  60  40  20  10  80
Wenatchee      46  58  41  56  37  50 /  40  50  30  20  10  90
Omak           45  58  41  58  32  48 /  70  80  70  30  10  70

&&

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

&&

$$



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