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Area forecast discussion 
National Weather Service Spokane Washington
434 PM PDT Thursday Sep 18 2014

Synopsis...
a weak cold front will bring the potential for widely scattered
showers this evening. A strong high pressure ridge will return
Friday and promote warm and dry weather through Monday. High
temperatures Sunday and Monday should be ten degrees above
normal. The breakdown of this ridge...and a return to cooler
conditions...will begin Tuesday. However...precipitation is not
expected until Wednesday for most communities.

&&

Discussion...
tonight and Friday...a weak cold front is moving through the
Columbia Basin this afternoon. Radar and satellite suggests
shower activity is spotty and light...and this is not expected to
be a significant precipitation producer for the area.
Still...there is plenty of moisture available and visible
satellite indicates substantial sunbreaks which should decrease
stability through the rest of the afternoon and into the early
evening leading to scattered showers and possibly a stray
thunderstorm over northeast Washington and particularly the Idaho
Panhandle where orographic forcing will play a part in generating
lift. Otherwise in general this frontal passage will be
characterized by breezy conditions developing over the Columbia
Basin as a tightening pressure gradient forms over eastern
Washington in the wake of the cold front.

Beyond this evening models are in decent agreement in building a
ridge over the region...and this building ridge will severely
weaken the next already weak system visible on satellite over
Vancouver Island as it transits the region tonight and Friday
morning...probably resulting in only mid level clouds for the
area. By Friday afternoon the ridge will assert itself more
aggressively with a general clearing trend. High temperatures
Friday will probably achieve near or slightly cooler than today's
highs...remaining slightly above normal for this time of year.
Winds will remain noticeable and out of the west/southwest but
only locally breezy. /Fugazzi

Friday night through Monday morning...ridge of high pressure moves
back into the area and amplifies considerably allowing for a dry
forecast with a warming trend. Forecast temperatures remain on the
warm side of what would be considered normal for this time of
year. Ridge axis placement aloft translates from eastern
Washington Friday night to well into central Montana on Monday
morning. The surface pressure gradient seems to be the weakest
Friday night which may allow for a repeat of late night and early
morning low clouds and fog in the more sheltered northern valleys.
/Pelatti

Monday through thursday: this period will mark the transition period
from dry and warm conditions under an upper-level ridge toward
cooler and wetter fall-like conditions under an upper-level
trough. Overall, models are in decent agreement with the changes
forthcoming but how quick this transition occurs still carries
moderate uncertainty. Confidence is above normal that the ridge
will remain in place through Monday afternoon continuing to
deliver dry weather and above normal warmth as 850mb temperatures
top out near 20 celsius. The first blow to the ridge will come
through Monday night into Tuesday. A bulk of the energy associated
with this wave will be deflected to the north into southern British Columbia and
although the disturbance will tap into some decent moisture, the
region will see very little precipitation with this feature.
Current thoughts support increased clouds and spotty sprinkles. A
few locations in the northern mountains will have the best chance
for a few hundredths. A deeper trof will shift from the ern pac
into the region late Tuesday through Thursday. This is when the
ridge will be scrubbed out of the pac northwest and precipitation chances
will be on the rise. The Cascades and northern mountains are a
sure bet for precipitation with the leading baroclinic band slowly
trickling across Washington allowing southerly midlevel flow to setup. In
fact, with the flow parallel to the meridional front, models have
slowed the onset of precipitation into the Idaho Panhandle until
Wednesday evening...perhaps even latter. The basin should also get
some rain out of the system but amounts are far from certain. The
slower frontal passage will keep the moisture over any location
longer but also favor weaker lift along the front. Additionally,
given some minor timing differences, we have held off on incr pops
until our confidence is higher. Following Monday which will
feature temperatures in the 80s, temperatures will gradually fall
a few degrees each day returning to more seasonal 60s to 70s by
Thursday. /Sb

&&

Aviation...
00z tafs: an upper trough migrates east tonight, while high
pressure starts to build in Friday. Look for cloud cover, a few
showers, particularly toward the northeast Washington mtns and north ID,
and breezy conditions. Mainly Coe/puw/lws have a shower risk. The
winds and shower threat wanes after 02-06z, with gradual clearing
into Friday. Generally, VFR conditions are expected. Yet there is
some potential for stratus to develop late overnight or early
Friday morning (09-16z) from the Palouse north to the Spokane/c'da
area. Some guidance shows some moisture in the boundary levels and
a south to southwesterly flow, which could favor some upslope
stratus development. However confidence is low at this time, but
this will be monitored for possible brief MVFR/IFR cigs. /J. Cote'

&&

Preliminary point temps/pops...
Spokane 55 75 48 81 52 84 / 20 0 0 0 0 0
Coeur D'Alene 54 75 46 80 49 84 / 30 10 0 0 0 0
Pullman 54 76 45 81 49 87 / 20 0 0 0 0 0
Lewiston 61 82 53 86 55 89 / 10 0 0 0 0 0
Colville 52 79 45 84 47 86 / 30 10 0 0 0 0
Sandpoint 50 72 39 78 44 79 / 30 10 10 0 0 0
Kellogg 52 71 46 78 48 81 / 30 10 0 0 0 0
Moses Lake 57 83 49 85 53 88 / 0 0 0 0 0 0
Wenatchee 61 83 54 84 56 87 / 10 0 0 0 0 0
Omak 56 81 48 85 49 87 / 10 0 0 0 0 0

&&

Otx watches/warnings/advisories...
Idaho...none.
Washington...none.
&&

$$

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