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Area forecast discussion 
National Weather Service Spokane Washington
431 am PDT Tuesday Apr 21 2015

Synopsis...

Today will likely be the warmest day in the next 7 to 10 days as
strong high pressure remains over the region for one last day. A
cold front will arrive Tuesday night bringing cooler weather and
breezy conditions into Wednesday. A cool upper trough is expected
to bring showery conditions Friday and Saturday. A stronger
weather disturbance will result in widespread and possibly
moderate precipitation Sunday and lingering into the new week.



&&

Discussion...
today through Wednesday...the ridge axis, located over central
Washington as of 2 am, will be nudged east today as an upper
trough swings toward the coast today. The trough will pass
through the region tonight into Wednesday, bringing a chance of
showers to portions of the forecast area. Models are now trending
toward the GFS solution of a closed low tracking across the
forecast area, leaving the European model (ecmwf) as the outlier with an open wave
clipping the northern zones. Pops have been trended toward the
consensus but this will not be a slam dunk for showers across
central Washington. Moisture should reach the Cascade crest late this
afternoon or early this evening and afternoon instability may be
enough to produce a thunderstorm or two. The cold front weakens as
it crosses the Cascades and encounters dry air while westerly flow
acts to rain shadow the east slopes and basin. There does appear
to be a reinforcing shot of moisture moving up from the south that
will bring a chance of showers to the NE blue mts overnight. The
best chance for showers will be on Wednesday as the trough
dynamics interact with upslope flow for the rising terrain east of
the basin. There will also be a chance of thunderstorms for the
Idaho Panhandle as the cold pool aloft destabilizes the
atmosphere.

The breakdown of the ridge will also result in gusty winds for
the Cascade valleys starting this afternoon as the cooler more
dense marine air invades The Gap valleys. Gusty winds will spread
to the Waterville Plateau this evening and continue overnight. The
eastern basin and Palouse will see breezy conditions with the
frontal passage Wednesday. Generally expect sustained winds of 15
to 25 mph with gusts of 30 to 40 mph. Relative humidity should be
recovering by the time the winds increase since it looks like the
marine layer will be deep enough for some of the moisture to make
it over the Cascades.

Temperatures will continue the warming trend today then cool
significantly for Wednesday with the trough passage when most
valley locations will be in the upper 50s to mid 60s. /Kelch

Wednesday night through Friday the forecast will follow a blend of
the ECMWF/Canadian/NAM. A very wrapped up closed low will track
northeast across the north Idaho Panhandle and points further
north and east. Meanwhile another deep closed system will drop
south along the b.C. Coast Thursday, then fill as the low moves
along the Canadian border. This low will tap into slightly deeper
Pacific moisture. The cold core Wednesday evening associated with
the closed low will increase lapse rates and destabilize the lower
levels. This will result in some active showers across the
northeast zones Wednesday evening. Surfaces based cape is present
but not overly deep, but there may be enough instability to
support a few lightning strikes through the evening hours.
Thunderstorms were already in the forecast, but were trimmed back
to mainly Boundary County up near the Canadian border. Showers
will remain in the forecast with the secondary low, mainly across
the north Idaho Panhandle, and stretching as far west as the
northeast Washington mountains up near the Canadian border. A second area
of showers will be along the Cascade crest. Temperatures will cool
off into the mid 50s to mid 60s, which are very close to seasonal
averages.

Friday night and Saturday the region will be in between weather
systems with weak and short lived high pressure tracking across
the region. Some light orographic mountain showers will be
possible for the Panhandle mountains and across the Cascades,
otherwise expect a period of dry but continued cool weather.

Saturday night through Tuesday a much deeper upper level low will
move into the eastern Pacific and stall. At this time it appears
as if the low may not move inland until mid-week if not later, but
there is much uncertainty in the model solutions through this
period. What we do know is the southwesterly flow will tap into a
very deep tropical atmospheric river Saturday night that looks
like it will linger through at least Tuesday. Precipitable waters go up to
around an inch which is well above normal. A deep southwest
moisture tap of this magnitude will not need much in the way of a
lifting process to wring out any and all moisture across the
region. It looks like we will be in for an extended period of
moderate stratiform precipitation Sunday through Tuesday.
Southwest flow will warm temperatures, but these will likely be
inhibited somewhat by cloud cover and widespread rain and not
reach potential. South-southwest winds will become breezy Saturday
night and Sunday and remain elevated at least through Monday.
Tobin.

&&

Aviation...
12z tafs: high pressure will result in VFR conditions through 12z
Wednesday. A deepening marine layer west of the Cascade mtns will
result in breezy northwesterly winds at keat late Tuesday
afternoon. Gusty southwest winds will develop after 06z Wednesday
in the kgeg/ksff/kpuw areas. /Ek



&&

Preliminary point temps/pops...
Spokane 74 49 59 36 60 41 / 10 10 20 10 10 20
Coeur D'Alene 73 43 60 34 60 37 / 10 10 30 20 10 20
Pullman 72 45 57 34 60 39 / 10 10 30 10 10 10
Lewiston 79 50 64 38 66 44 / 0 10 30 10 10 10
Colville 77 47 63 37 63 39 / 10 10 50 30 10 20
Sandpoint 72 40 59 33 59 34 / 10 10 30 30 20 20
Kellogg 71 43 57 34 60 36 / 10 10 40 20 10 20
Moses Lake 79 49 66 37 65 40 / 0 10 10 0 10 10
Wenatchee 77 53 65 41 64 44 / 10 10 10 10 10 10
Omak 78 47 64 35 64 38 / 10 10 30 10 10 10

&&

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

$$

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