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Area forecast discussion 
National Weather Service Spokane Washington 
239 am PDT Friday Jul 10 2009 


Synopsis... 
warmer weather will return to the inland northwest today... 
tomorrow...and Sunday. A low pressure system off the coast of 
Oregon will move across the region on Sunday...bringing the 
potential for showers and thunderstorms. Showery and unsettled 
conditions are expected to linger into Monday. Drier...more 
typical July weather will develop Tuesday and persist through 
the end of next week. 


&& 


Discussion... 


Today and tonight...the upper flow pattern along the Pacific coast 
of North America resembles a Rex block. A ridge of high pressure 
is anchored over the Gulf of Alaska...and an upper level low has 
become established off the coast of Oregon. The state of 
Washington will essentially be caught in between the branches of 
the polar jet today and tomorrow. The northern branch will bisect 
Alberta and the southern branch associated with the Oregon low 
will be well south of the Evergreen state. We will monitor a 
cluster of showers moving almost due north into west central 
Oregon as of 09z. Both the NAM and GFS depict moderate q-vector 
convergence between 700-300mb in the vicinity of this light 
precipitation. The forcing is not forecast by either model to 
spread north today or tonight. The upper flow over Washington is 
largely confluent and does not look as if much synoptic forcing 
will be available for precipitation. Diurnally driven instability 
showers do not look promising either. The middle levels are expected 
to warm sufficiently that deep layer instability will be 
significantly decreased. There may be enough shallow instability 
to produce isolated afternoon and evening convection over the 
mountains along the Canadian border...and over the ridges of the 
Cascades. However coverage will be noticeably less than the last 
couple of days. Afternoon temperatures across the region will 
likely be 5 to 8 degrees warmer today than yesterday as the 
airmass continues to modify. /Gkoch 


Saturday through Sunday night...a rather Flat Ridge will get pushed 
up over the region as an upper level low off the Oregon coast starts 
to move east. Southerly flow ahead of the trough will push 850mb 
temperatures to the 22 to 24c range. This will translate to valley 
temperatures in the 90s. The thermally induced surface pressure 
trough will remain over the inland northwest on Sunday but an 
increase in clouds will keep maximum temperatures a few degrees cooler 
on Sunday but still above seasonal normals. Showers will spread east 
across the region Saturday night...increasing in coverage on Sunday. 
Models are bringing moisture from the remnants of Tropical Storm 
Blanca northward where it will be captured by the circulation of the 
upper low as it approaches the Oregon coast. The latest GFS guidance 
brings precipitable waters  of 150 to 200 percent of normal across the inland 
northwest by Sunday. Models are consistent in placing a bulls-eye of 
quantitative precipitation forecast to our southeast zones. As the low moves ashore the best upper 
divergence will be over southeast Washington and north Idaho. In 
addition...a strong vorticity maximum in the cusp of the trough will slide 
over the region. Bulk shear in the 0 to 6 km layer will increase on 
Sunday with values between 35 and 50 knots..the highest of which 
will reside over the southeast portion of the forecast area. The 
threat for showers and thunderstorms will increase across the 
inland northwest on Sunday and there is the potential for some of 
these storms to become severe. 


Monday through Thursday...the trough will move east and the best 
chance for showers and afternoon thunderstorms will be across the 
eastern zones as flow shifts to the west. After brief shortwave 
ridging on Tuesday...progressive westerly flow takes over with 
several weak impulses riding through the flow. This looks to be a 
generally dry period for the valleys with scattered afternoon 
showers and thunderstorms for the higher elevations. /Kelch 


&& 


Aviation... 
-shra over the mountains have basically dissipated from the loss of 
PM heating. Ragged cumulus should also dissipate within the next 
couple of hours and dry air will dominate the middle-levels overnight. 
The result will be clear skies over most locations through morning 
with only scattered middle and high clouds Friday afternoon. /Fries 






&& 


Preliminary point temps/pops... 
Spokane 83 57 90 62 86 58 / 0 0 0 10 30 40 
Coeur D'Alene 82 52 89 57 84 56 / 10 10 0 10 30 40 
Pullman 83 48 90 58 84 54 / 0 0 0 10 40 40 
Lewiston 89 59 97 67 90 62 / 0 0 0 10 30 30 
Colville 89 51 92 57 90 55 / 10 10 10 10 30 40 
Sandpoint 81 45 86 52 84 53 / 10 10 0 10 30 50 
Kellogg 81 53 87 58 84 55 / 10 10 10 10 30 50 
Moses Lake 88 57 94 63 88 58 / 0 0 0 10 20 20 
Wenatchee 88 61 93 68 85 62 / 0 10 10 10 30 20 
Omak 90 55 94 60 90 57 / 10 10 10 10 30 30 


&& 


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


$$ 

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