Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 180547

Area Forecast Discussion
National Weather Service Spokane WA
947 PM PST Fri Nov 17 2017

Saturday will be dry but with high clouds filtering the sun
and near average temperatures. Precipitation chances will
increase Sunday afternoon into Sunday night. Moderate to heavy
mountain snow is expected Sunday night over the Cascades and
Monday in the Idaho Panhandle mountains. A mild and wet storm
system Tuesday into Wednesday will bring another round of rain to
our region.


Tonight through Monday...The Pacific satellite loop this
afternoon reveals a rather complex field of motion upstream of the
forecast area. Features of note include an upper low dropping out
of the Gulf of Alaska pushing a moderate fetch of Pacific
moisture into British Columbia...a second upper low south of it in
the eastern Pacific...which is directing a deeper moisture fetch
up it`s eastern flank. These moisture feeds are connected withing
a deformation region between the two low circulations and it
appears the northern moderate fetch is being wrenched off of the
deeper fetch in this area. Meanwhile the forecast area is sitting
under westerly flow producing a noticeable rain shadow over
eastern Washington and lingering upslope snow showers into the
Idaho Panhandle mountains.

How all these elements interact in the next few days will bring an
increasingly active weather pattern to the region. The latest
model fields of motion are is decent agreement so there is
fairly high confidence in the broad scenario.

Tonight and Saturday the forecasts area will reside under a weak
upper ridge...but the northern moderate moisture feed will bring
significant high clouds especially on Saturday and may be deep
enough to promote some more snow showers over the high terrain of
the Idaho Panhandle. Otherwise a generally seasonably cool and dry
day will occur on Saturday with less wind but thick high clouds
filtering the sun for much of the region.

After that the Gulf of Alaska trough will dig further off the
coast and absorb the secondary upper low and take charge of
steering the main deep moisture feed into the northwestern US.
Thus will begin a general deterioration. For the southeastern
reaches of the forecast area and north Idaho Sunday will remain
dry but with even more and thicker clouds with bases lowering as
this fetch begins to moisten the atmosphere from top to bottom.
The Cascades and northern Washington mountainous zones will see
mountain snow and valley rain developing during the day.

Mountain snow and valley/basin rain will overspread the entire
forecast area Sunday night...with a very robust moisture feed
favorably directed into the forecast area for a sustained period
of steady precipitation through Monday. This looks like an event
that will require some winter weather highlights for the mountains
and travelers planning on crossing regional passes should take
note and follow refinements in the forecast this weekend. /Fugazzi

Monday night through Friday: Storm system responsible for the wet
and snowy weather Sunday night/Monday moves treks away to the
southeast and in turn its connection to subtropical moisture makes a
similar exit away from the area. The situation gets a bit
complicated however due to model differences as the GFS allows for a
distinct 12 hour or so break due to transient shortwave ridging
while the more favored ECMWF passes yet another disturbance through
the area. Will continue the strategy utilized in the previous
forecast by utilizing a blend of the two which will focus the higher
pops primarily over the southern portion of the forecast area Monday
night. A well maintained significant subtropical moisture feed off
the coast gets better resolved Tuesday and by Tuesday afternoon
there are hints from earlier models that the tip of the moisture
stream may make enough progress eastward to stream moisture northeast
and over the North Cascades and some distance into the Columbia
Basin and Okanogan Highlands. The plume has a good north to south
orientation to it such that by Tuesday night into Wednesday there
will be minimal to no downsloping effect in the lee of the Cascades
when a low pressure system tied into the plume passes through. This
will result in substantial precipitation with very high snow levels
(as high as 7-9 thousand feet MSL) and warm temperatures. This warm
wet rainfall will aid in compacting the mountain snow at higher
elevations and may work to erode a good amount of it from about 4500
feet MSL and below. In fact the seven day temperature forecast
should peak with Wednesdays high temperature. The rain and the
resulting mountain runoff is likely to result in higher streamflow
Wednesday and Thursday but there is enough snow in the mountains to
absorb a significant quantity of the snow and slow the runoff to
just result in a slight bump in terms of streamflow increase but
nothing to suggest any main stem river flooding. Pops remain
somewhat high for Wednesday night into Thursday as there models hint
at additional disturbances digging into the weakening remnants of
the plume remaining off the coast but going with the idea that the
moisture plume is pretty much worn out at this point in time the
expectation is some slight cooling and lowering of snow levels
should take place. Will show a further decrease in pops coupled with
some warming for Thursday night into Friday as models hint at
disturbances digging southward in the large offshore trof that could
further interrupt/divert any moisture stream positioned between the
large offshore trof and narrow inland ridging which would limit the
moisture any disturbances passing through Eastern Washington and
Northern Idaho would have to work with. /Pelatti


06Z TAFs: A flat ridge will persist into Saturday with abundant
mid and high level clouds passing through the region. A moist
boundary layer over NE Washington and N Idaho will lead to areas
of MVFR conditions due to stratus through Saturday morning. There
is a chance of KGEG/KSFF/KCOE being impacted from the stratus
but because of the abundance of mid level cloud cover and larger
5-10F temp/dew point spreads as of 05z opted to keep conditions


Spokane        32  43  30  44  36  44 /   0   0   0  20  90  40
Coeur d`Alene  31  41  30  43  35  42 /  10   0   0  20  90  50
Pullman        33  44  32  47  36  46 /  10   0   0  10 100  60
Lewiston       36  49  32  50  40  51 /  10   0   0  10  90  60
Colville       29  39  28  40  33  41 /   0   0  10  50 100  50
Sandpoint      29  39  27  41  34  39 /  10  10   0  20 100  60
Kellogg        31  38  26  42  32  39 /  20  10   0  10 100  80
Moses Lake     31  46  29  46  35  48 /   0   0   0  20  60  10
Wenatchee      33  44  31  43  33  45 /  10   0   0  30  50  10
Omak           31  41  30  40  31  42 /   0   0  10  50  70  10




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