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Area forecast discussion 
National Weather Service Spokane Washington
243 am PDT Tuesday may 5 2015

Synopsis...breezy conditions are expected today behind the cold
front passage from last night. A low pressure system tracking near
the region will result in a chance of showers and a slight chance
of thunderstorms today and Wednesday...mainly over the Cascades
and Okanogan Highlands during the afternoon and evening. 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.

&&

Discussion...
today through Wednesday...water vapor satellite shows a closed low
over Vancouver Island with a very pronounced dry slot over central
and northeast Washington. Ahead of this dry slot a band of high
based showers was evident on radar from the Camas Prairie to the
central Panhandle mountains. Models agree that as the low drops
southeast this area of elevated convection will move east of the
area. This low dragged a cold front through last night and early
this morning...with breezy winds expected today behind the front
in a well mixed air mass.

Models show the closed low moving over northwest Washington and southern
British Columbia this afternoon. This will result in showers over the Cascades
and Okanogan Highlands. Uncapped cape values of 200-400 j/kg could
trigger isolated thunderstorms as well. Meanwhile the dry slot
over eastern Washington and north Idaho should prevent showers
from developing except for possible the high terrain near the
Canadian border. Over the Wenatchee area and western basin, the
environment will become more conducive to showers by this evening
as a 500mb cold pool of -28 to -30c tracks over the area. However
downslope west winds off the Cascades and a very dry low level air
mass with dew points in the low to mid 20s should keep this
isolated.

For tonight showers are expected to decrease as day time heating
is lost and a dry low level air mass remains. Isolated showers are
possible near the low center (mainly the cascades) otherwise most
areas should be dry. On Wednesday as the low drops south across
southern Washington and northern Oregon models have backed off a
bit on the instability over eastern Washington and north Idaho.
The dry low level air mass will also limit shower activity. Models
currently depict the best chance of convection over the Cascades
and Okanogan Highlands where showers today should help raise
surface dew points allowing for more instability on Wednesday.
Scattered showers and isolated thunderstorm development is
expected during peak heating. Elsewhere the slight chance of
thunderstorms was removed from the forecast and pop's were
lowered. Jw

Wednesday night: the atmosphere will begin to stabilize as the
low located in central Oregon is drops south. The chances for
thunderstorms have weakened from the previous run. The highest
cape values are in the 200-400 range located along the Cascades
and the northern mountains. These locations will have a slight
chance for lightning into early Thursday. Lapse rates are still in
the 8 range but drier air moving into the region will make it hard
for any showers to develop over the Columbia Basin.

Thursday through saturday: with ridging and dry, warm air moving
into the region, this period is expected to have no significant
weather. Winds will be breezy on Friday as a shortwave trough
pushes through the region creating some gusts of around 22 miles per hour.
Outside of that, temperatures will be around the upper 60s to
upper 70s. /Jdc

Sunday through Tuesday...the models are beginning to change their
tune a little for the latter half of the weekend. Whereas
yesterdays runs showed a weakening negative tilted trough moving
through the Cascades on Sunday, now most have backed off on that
idea. The majority view suggests the inland northwest will remain
fixed beneath a narrow but strong ridge pinched between an
offshore trough and one over the prairie provinces. We will follow
this notion which will support a dry forecast with more
unseasonably warm temperatures. As for specifics, it looks like
850 mb temps will be similar to what we experienced yesterday.
That equates to widespread valley highs in the mid 70s to mid 80s.

Our confidence drops off precipitously for Monday and Tuesday. The
00z run of the GFS takes the aforementioned offshore trough and
drops it down the West Coast. This would leave our region dry with
continued unseasonable warmth. Meanwhile the European model (ecmwf) and to lesser
extent the Canadian take the offshore trough through the region
as a weakening negative tilted shortwave. This view is shared by
the 00z GFS ensemble mean and will be favored for our forecasts.
What this means is our warm and dry weather will come to a gradual
end. If we go with the ec or Canadian timing it looks like a
moderate strength cold front will pass through the area either
Monday or Monday evening. The greatest precipitation threat with
the front will impact locations near the Cascade crest as well as
over the Idaho Panhandle, however just about anywhere could see
some light precipitation. Once the front passes the threat of
precipitation will lower considerably over the Columbia Basin,
Wenatchee area, and Okanogan Valley as drier air moves in from the
west. Meanwhile it's hard to determine what the chances for
precipitation will be elsewhere. Yesterdays ec run placed a deep
upper low along the southeast Washington/NE or border and placed a deformation
band of precipitation over the southern half of our forecast area.
Meanwhile the latest ec and Canadian suggest it will be a much
more progressive system with the core of the upper level trough
remaining off the coast. The 00z GFS ensemble also supports this
idea to a certain extent and we will hedge the forecast in this
direction. Fx

&&

Aviation...
06z tafs: the pressure gradient is tightening up around the region
thanks to the passing cold front. Gusty southwest to northwest
winds will persist overnight for most locations. All taf sites
will see an increase in winds late Tuesday morning as daytime
heating mixes down some gustier conditions. Still not expecting
any significant precip changes at any locations. The focus and
challenge in the next 24 hours is the magnitude and timing of the
wind. By Tuesday evening, winds will come down very quickly
leading to a pleasant VFR for all taf locations.
/Ab



&&

Preliminary point temps/pops...
Spokane 61 36 60 39 68 42 / 0 10 20 10 10 0
Coeur D'Alene 61 35 61 37 67 40 / 0 10 20 10 10 10
Pullman 60 34 59 37 67 37 / 10 10 20 10 10 10
Lewiston 67 39 65 42 72 45 / 10 0 20 10 10 10
Colville 65 37 64 38 71 41 / 20 30 30 20 0 0
Sandpoint 63 33 60 34 67 35 / 0 10 20 10 10 10
Kellogg 59 35 59 37 65 37 / 10 10 20 10 20 10
Moses Lake 67 38 66 40 74 45 / 10 20 20 20 0 0
Wenatchee 66 43 64 44 75 47 / 10 20 30 20 0 0
Omak 65 36 65 38 72 42 / 20 20 20 20 0 0

&&

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

$$

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