Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 032340
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
440 PM PDT THU SEP 3 2015

.SYNOPSIS...
A cool and showery weather pattern is expected for the remainder
of the work week as a cold storm system moves by the area.
Widespread rain is expected in the Panhandle and portions of Eastern
Washington on Saturday. Drier and warmer weather is expected by
early next week.

&&

.DISCUSSION...
Tonight through Saturday...An upper level trough/closed low will
impact the Inland NW through Saturday with several aspects of this
system to monitor. The center of a closed low will track from near
Vancouver Island today before dropping south into western Oregon
on Friday...before tracking across central and NE Oregon Fri night
and finally into central Idaho Saturday.

Tonight/Friday:

...Patchy frost tonight northern valleys...

Initially with the low tracking west of the region tonight into
Friday the best instability will lie over the Cascades where
scattered showers and a slight chance of thunderstorms is
expected. The highest coverage of showers will occur during peak
heating hours. A dry slot coming around the low will track across
Eastern Washington tonight into Friday which combined with light
winds will result in strong radiational cooling for the northern
valleys. Colder pockets such as Republic, Chewelah, Deer Park,
Springdale, and Priest Lake will likely drop to near the freezing
mark. Also as the low drops into western Oregon on Friday
northerly pressure gradients increase which will result in breezy
winds down the Okanogan Valley and Purcell Trench (Sandpoint,
Coeur D`Alene areas). This will result in elevated fire weather
concerns for the Okanogan Valley but not to the degree where
highlights are warranted.

Friday night/Saturday:

...Wetting rain likely across Eastern Washington and North
Idaho...

As the closed low tracks across Oregon Friday night into Central
Idaho on Saturday a deformation axis sets up over Eastern
Washington and North Idaho resulting in a swath of steady rain.
Differences remain however between the models regarding the
precise strength and placement of this rain band. The ECMWF and
GEM models have a deeper and further north track giving Eastern
Washington and the north Idaho Panhandle the brunt of the rain
mainly along and east of a line from Colville to Ritzville...with
as much as 0.50-1.25 inches of qpf. The GFS is a bit further south
and weaker with the low with the Idaho Panhandle getting the
brunt with .25-.50 inches of qpf with lighter amounts over
Eastern WA. With the ECMWF consistent with its solution over the
past 3 runs and with the SREF supporting a further north low track
forecast favors the wetter solution for Eastern WA. That said
forecast temperatures were cooled and rainfall amounts were
increased. There may also be a convective component to the precip
amounts Saturday afternoon which could influence rain amounts.
Omak, Grand Coulee, and Moses Lake westward to the Cascade crest
are expected to be too far west to receive rain from this
stratiform event. JW

Saturday night through early Tuesday morning...Longwave trot with
various synoptic size and smaller systems passing through it mark
this forecast interval with below normal temperatures and various
pops and QPF amounts to assigned to systems making the trek
through the area. Significant systems to note include one exiting
extreme Eastern Washington and North Idaho Saturday night which is
followed up with brief shortwave ridging for Sunday ahead of yet
another mesoscale system that is over the Washington Cascades and
close vicinity at that same time. That system passes to the south
and east early Sunday Evening and exits late in the day Monday.
Monday afternoon through Tuesday morning the long-wave trof
translates east enough to allow more of a drier northwest/north
flow ahead of a positively tilted ridge extending from off the
coast up through Southern British Columbia which allows for a
lowering of pops and some warming in temperatures. /Pelatti


Tues through Thurs: A dry, quick moving, frontal passage Tues
will produce broken to overcast skies...followed by swift westerly
steering flow. With a dry air mass accompanying these features,
it`ll be tough to get any pcpn, especially given the lack of deep
instability. This westerly flow is, however, favorable for gusty
winds in the Cascade gaps. Frontal passage number two will reach
the Pac Nw Thurs with conditions similar to the Tues front. bZ

&&

.AVIATION...
00Z TAFS: A low pressure system over western Washington will drop
south into Oregon tonight. This will result in scattered showers
along with a slight chance of thunderstorms mainly in the Cascades
and near the Canadian border through this evening before activity
decreases after sunset. Convection is expected mainly over the
high terrain with the TAF sites remaining dry with a continuation
of VFR conditions. Winds will shift to the north overnight with
stronger gusty winds expected down the Okanogan valley (KOMK) and
less so down the Purcell Trench (KCOE). The next precipitation
expected to affect the TAF sites will begin late Friday. /EK

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        43  70  47  58  46  68 /   0   0  50  70  50  10
Coeur d`Alene  40  70  47  60  46  67 /   0   0  50  70  50  10
Pullman        37  69  49  56  43  65 /   0  10  70  70  50  10
Lewiston       48  74  51  63  52  72 /  10  10  70  60  50  10
Colville       38  72  48  65  43  70 /  10   0  40  70  30  10
Sandpoint      37  68  47  61  42  65 /  10  10  50  70  50  20
Kellogg        39  66  46  57  46  64 /  10  30  60  70  50  20
Moses Lake     43  74  53  75  47  76 /   0   0  10  10  10  10
Wenatchee      50  74  51  77  57  76 /  20  10  10  10  10  10
Omak           45  73  48  77  47  76 /  30  10  10  10  10  10

&&

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

$$



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