Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 312326
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
426 PM PDT Wed Aug 31 2016

.SYNOPSIS...
Our summer weather will come to an end as a chilly low pressure
system arrives Thursday and Friday. Widespread rain is expected
over north Idaho and northeast Washington Friday. The Labor Day
weekend will be cooler than average with scattered afternoon
showers over the mountains.

&&

.DISCUSSION...
Tonight through Friday...Latest suite of model guidance is now
in pretty good agreement leading to high confidence of a general
deterioration into an active...increasingly wet and much cooler
pattern for the end of the work week. Tonight will be a relatively
quiet period...but satellite suggests another minor out-rider wave
best resolved on the water vapor channel moving up the southwest
flow currently over southwest Oregon. The current disturbance over
the Idaho Panhandle will exit late this afternoon...and then this
next wave will take a similar track overnight. Not much is
expected to happen with this wave...however it is a weakly
dynamic disturbance running into a surface thermal trough with
weak mid level instability and thus the possibility of sprinkles
and a stray thunderstorm cannot be ruled out affecting the Idaho
Panhandle overnight. Otherwise a benign and mild overnight period
is expected other than some stray showers near the cascade crest.

Thursday through Friday brings the dramatic change. The deep
upper low circulating off the Pacific coast of Canada will finally
eject inland carrying a moist cold front across the Cascades late
Thursday and pushing though the rest of the region Thursday night
and Friday. A good Pacific moisture feed and strong frontal
forcing and dynamic support will bring a period of dense/frequent
rain showers to the entire forecast area...even the normally dry
Cascades lee areas Thursday evening before post frontal westerly
flow allows some tapering Friday morning. Over the eastern half of
the forecast area a quarter to 1/2 inch of rain is possible late
Thursday night through Friday...with a few hundredths to a tenth
of an inch Thursday night in the deep basin. Friday will be a
showery...breezy and unstable day in the cusp of the trough with
dense hit-and-miss showers and garden variety thunderstorms over
pretty much the entire forecast area.

Temperatures during this transition period will drop noticeably on
Thursday back into the lower to mid 70s most locations and then
plummet further into the solidly unseasonable 60s on Friday...with
some areas repeatedly run over by showers especially in the
northern valleys and Idaho Panhandle struggling to break 60.
/Fugazzi

Fri Nt through Wed: Two important short-wave troughs will impact
the Labor Day weekend... the first Fri Nt and the second Sat Nt
through Mon morning. The biggest changes we made to the fcst was
to increase pcpn chances substantially with both of these features,
especially for the Idaho Panhandle. Our confidence is also
higher, at least for now given better model agreement, that
anywhere from one-half inch to one inch of rain is possible for
the E. Slopes of the Cascades, and the Bitteroots, Selkirk and
Cabinet mtns of NE Wa and N. Idaho. The heavier amnts won`t be
widespread for lower elevations, but could vary significantly due
to the convective nature of the pcpn and the uncertainty as to
where the upper trough, vort max, jet speed max, and steep mid-
level lapse rates track. Breaks in the clouds could also enhance
heating and the thunder potential each afternoon. Heavy rain are
possible also given the somewhat slow storm movement. Temps could
be about 10F below normal for many towns this weekend, especially
mtn valley towns. bz

&&

.AVIATION...
00Z TAFS: moist southwest flow aloft ahead of an approaching
trough will bring a weak mid level disturbances through the
eastern TAF sites tonight with mid level cloud decks. The main
trough will approach the region on Thursday with thickening high
clouds and precipitation beginning near the Cascade crest by 00Z
Friday. VFR conditons are expected at all TAF sites through 00Z
Friday.
/MJF

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        56  74  51  64  47  67 /  10  10  70  70  60  20
Coeur d`Alene  54  74  50  62  46  65 /  10  10  70  80  60  20
Pullman        52  74  48  64  44  66 /  20  10  60  60  40  10
Lewiston       58  80  55  72  52  74 /  10  10  60  60  30  10
Colville       53  73  47  62  44  70 /  10  10  70  80  60  40
Sandpoint      52  74  47  58  45  64 /  10  10  70  90  70  40
Kellogg        51  73  46  58  45  64 /  20  10  70  90  70  30
Moses Lake     54  76  51  70  46  74 /  10  10  40  40  20  10
Wenatchee      55  72  54  69  51  72 /  10  10  60  40  20  10
Omak           52  75  51  68  46  72 /  10  20  60  60  30  20

&&

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

$$



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