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Area forecast discussion 
National Weather Service Spokane Washington
557 PM PDT Mon Aug 29 2016

Synopsis...
an approaching weather system will slowly cool the region through
Thursday and bring an increasing potential for showers and
thunderstorms as the week wears on. This upper level trough will
move into the region late in the week, ushering in cooler
temperatures and showery conditions for the beginning of the
Holiday weekend.

&&

Discussion...
tonight through Saturday...the driving factor of the weather for
the remainder of the work week and into next weekend will be the
satellite depicted deep upper low pressure system off the
Canadian coast this afternoon. Latest models are all in agreement
through roughly Thursday in digging this upper low further south
and then beginning to eject it inland. Model agreement
deteriorates beyond Thursday regarding the strength of the low as
it swings through the forecast area and the speed of transit. This
creates some uncertainty for the upcoming Holiday weekend.

In the mean time...confidence is increasing that the Tuesday
through Thursday approach period will be characterized by a slow
moderation of high temperatures off of today's maximum by three to
5 degrees each afternoon...as well as periodic waves in the pre-
trough southerly flow bringing at least a small chance of nocturnal
thunderstorms Tuesday evening and again on Wednesday evening and
overnight to much of the region. These out-riding waves will be
rather weak and any elevated convection will likely be
isolated...with a better chance over the Cascades tomorrow
evening. Daytime break periods will be generally dry and mostly
sunny...although with varying degrees of mid level altocumulus castellanus and high
clouds filtering the sun at times.

From Thursday Onward the models do agree on ejecting the closed
low or trough into the forecast area and confidence is high for a
general cool down to below normal and comfortable mid 70s high
temperatures. The uncertainty lays in the extent and
aggressiveness of hit-and-miss showers and thunderstorms affecting
the region associated with better defined waves and cold fronts
as the trough encroaches and passes. The GFS offers the greatest
potential for widespread showers and wet thunderstorms as it
maintains the integrity of the closed low as it tracks straight
over the region Friday and Saturday. The ec is the most timid
depicting an open trough with the dynamics and cold pool
instability staying to the north over British Columbia with only the southern
fringe of the trough sweeping through Washington and north
Idaho...good for a cooling and breezy cold front passage with some
showers along the front on Friday...but quicker to pass to the
east and offer a drier beginning of the Holiday weekend. The
Canadian model lays between the two extremes.

Therefore...given this uncertainty the forecast will exploit the
greatest agreement elements and go hog-wild with the cooling
trend...but pops each day Thursday through Saturday will be mid-
range for hit-and-miss activity with the best chance across the
northern mountains. /Fugazzi

Saturday night through Monday... by Sunday we transition to
the back side of the upper level trough and northwest flow
by Monday/Labor Day. This will spell an overall drying trend and
very gradual moderating of temperatures. However, with no major
ridging in site, and rather the potential for another trough
digging in beyond the extended forecast, temperatures will likely
remain near or slightly below normal. Additionally, a few weak
disturbances running through the northwest flow could lead to
some small mountain shower chances. /Kalin

&&

Aviation...
00z tafs: skies will generally clear this evening as a weather
disturbance moves east into Montana. Mid and high level clouds
will move in from the south Ard 12z. Clouds will become thicker
through the morning hours. Ard 00z Wed there will be a threat of
ts for all the eastern taf sites. Will likely be adding this to
the 06z taf issuance.

&&

Preliminary point temps/pops...
Spokane 61 86 59 81 57 73 / 10 10 20 10 10 20
Coeur D'Alene 56 87 56 81 54 73 / 10 10 20 10 20 20
Pullman 54 84 53 80 51 73 / 10 10 20 10 10 10
Lewiston 63 90 60 87 59 80 / 10 10 20 10 10 10
Colville 52 89 53 83 52 76 / 10 10 20 10 20 20
Sandpoint 50 86 52 81 51 75 / 10 10 20 10 20 20
Kellogg 54 86 53 81 51 73 / 10 10 20 10 20 20
Moses Lake 57 87 55 83 54 78 / 10 0 10 10 10 10
Wenatchee 62 86 61 81 58 76 / 10 10 10 10 10 20
Omak 55 90 56 85 52 79 / 10 10 20 10 20 20

&&

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

$$

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