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Area forecast discussion 
National Weather Service Spokane Washington
222 PM PDT Tuesday may 5 2015

a low pressure system will result in afternoon convection once
again on Wednesday...mainly over the Cascades and Okanogan Highlands.
The weather will generally be dry and warm through the end of the
workweek, except for a small chance of mountain showers over the
Idaho Panhandle. Temperatures will moderate a bit by the weekend
as another weak disturbance approaches the coast. This feature
will likely pass into the area early next week with a return to
cooler temperatures and a chance of precipitation.


tonight through Wednesday night: an upper level low pressure
system is currently circulating over northwest Washington and southeast
British Columbia. The cold pool aloft and best instability resides across the
northern and western portion of the forecast area. This is where
all of the shower activity has developed through this afternoon
from the Cascades and as far east as the northern Panhandle. Much
of the instability is surfaced based, so I expect a lot of this
shower activity to wane with sunset this evening. I left a slight
chance for thunderstorms through this evening over the higher
terrain of the east slopes of the northern Cascades and over the
Okanogan Highlands, but confidence is low based off the activity
so far this afternoon. Temperatures aloft will continue to cool
through the rest of the evening, so there is still a chance for a
few lightning strikes to be squeezed off. Pressure gradients will
relax this evening with winds waning through tonight. There is
some question of how cold it will get tonight as confidence is low
with how much mid to high level cloud cover hang over the region.
Best chances for freezing temperatures will be across the mountain

The upper level low pressure system will continue to hang out over
the region on Wednesday. It will begin to split tonight as it
moves further onshore. This will result in much of the energy
pushing north (into southern alberta) and south (into or and
northern ca). We will continue to deal with a cold pool aloft and
will likely see very similar shower activity Wednesday as today.
I did leave a slight chance of showers across extreme eastern Washington
and in the Idaho Panhandle since the upper level low (or trough) will
be a bit further east over the region. However, the best
instability will continue to be over the western portion of the
region, especially the northern mtns and over the Cascades. This
is where the best chance for showers will be. There will continue
to be a small thunderstorm threat as well, but does not look
particularly favorable as models indicate only weak instability.

A northernly pressure gradient force will set up across the region
late Wednesday afternoon/evening. This will result in northerly
winds picking up down the Okanogan Valley and through the Purcell
Trench. This should result in showers waning fairly quickly
Wednesday night as drier air filters into the region. /Svh

Thursday through Tuesday...bulk of the energy associated with the
closed low that was in the vicinity and/or overhead of eastern
Washington and northern Idaho early in the workweek digs south in
the large long-wave trof over the western US. This may allow for
minor disturbances rotating counterclockwise around the edges of
the low to brush over the southeast portion of the forecast area
Thursday so minor pops linger in that general area from about the
Blue Mountains up into Shoshone County into early Thursday
evening. Further digging of the low pulls the edges of the low
further away and allows a positively tilted flop-over ridge nosing
in from the west to have greater influence thus Friday and
Saturday is marked with a warming trend and a generally dry
forecast. Forecast uncertainty Sunday and beyond exists between
the models so for now will go with what the European model (ecmwf) hints at which
is continued narrow ridging in the vicinity allowing for forecast
highs to peak Sunday afternoon at the apex of the ridge
amplification and fact the ridge is depicted to get
undercut by disturbances digging through the middle of it on both
sides Sunday night. Monday and Tuesday show a general cooling
trend and increase in pops with the splitting of the ridge coupled
with a wet occluded frontal zone passage Monday into Tuesday. As
far as winds go through this interval the initial set up with the
bulk of energy associated with low positioned to the south allows
for mostly north and northeast winds Thursday through Saturday.
Sunday and beyond the winds are more variable in direction due to
the earlier mentioned loss of ridging and movement of frontal
zones. /Pelatti


18z tafs: an upper level low pressure system will slowly move into
British Columbia and Washington today. This will destabilize the atmosphere with
surface heating through this afternoon. The taf sites will
generally see flat cumulus cover. Better chances for deeper
cumulus with showers will be across the Cascade mtns and over into
the Okanogan Highlands. There is also a chance for thunderstorms
across these areas and mainly near the Canadian border. Winds will
be breezy with gusts between 20-30 kts through the afternoon. /Svh


Preliminary point temps/pops...
Spokane 36 60 39 68 42 71 / 10 20 10 10 0 20
Coeur D'Alene 35 61 37 67 40 69 / 10 20 10 10 0 20
Pullman 32 59 37 67 37 69 / 10 10 0 10 10 20
Lewiston 38 65 42 72 45 74 / 0 10 10 10 10 10
Colville 35 64 38 71 41 74 / 30 30 20 0 0 20
Sandpoint 32 60 34 67 35 67 / 10 20 10 10 0 20
Kellogg 35 59 37 65 37 67 / 10 10 10 10 10 20
Moses Lake 37 66 40 74 45 78 / 20 20 10 0 0 10
Wenatchee 42 64 44 75 47 78 / 20 30 20 0 0 10
Omak 36 65 38 72 42 76 / 20 30 20 0 0 10


Otx watches/warnings/advisories...


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