563 
fxus66 kpqr 221603 
afdpqr 


Area forecast discussion 
National Weather Service Portland or 
900 am PDT Thursday may 22 2008 


Synopsis...unsettled weather is expected to continue for the 
forecast area through the weekend as an upper level low pressure 
system lingers over the western United States. A very gradual warming 
trend is expected over the next several days as the coolest air 
associated with our upper low shifts southward into California. 


&& 


Short term...a cool moist air mass over the Pacific northwest 
continues to combine with a vigorous northwesterly jet stream to 
produce scattered showers across the district. This pattern should 
continue throughout the day today. Morning visible imagery shows some 
thin spots in the cloud deck and if some sun gets though in a cool 
moist air mass that will increase instability and showers. 
After today... the strong jet energy digging into the Westside of the 
upper trough is beginning to form a cutoff low over the Great Basin 
today. Models suggest this broad upper level low will essentially 
wobble around the West Coast and Great Basin through early next 
week...mostly to our south. Complicating things are occasional lobes 
of vorticity rotating around the upper low...which will probably 
produce bands of showers moving east to west across Oregon and 
southern Washington through the weekend. Deeper moisture eventually 
becomes entrained into this system this weekend. The 00z GFS actually 
shows some 1 inch-plus precipitable water values over the Willamette 
Valley Sat aftn/evening. Also...models have shown some modest 
instability with Li/S near zero Sat afternoon and will have 
difficulty in the placement of the unstable areas so a slight chance 
of thunderstorms east of the Coast Range Sat/sun afternoons and 
evenings looks reasonable. It could actually begin to feel a bit 
muggy across the district by the end of the weekend if the upper low 
stays south of the forecast area and models are correct in bringing 
this more humid air mass westward across Oregon and Washington. 
Cutoff lows are tough to predict and often times there is large model 
to model and run to run variability in handling them and the details 
associated with them. However there is a fair bit of confidence at 
this point that periods of showers will linger into the 
weekend...with a gradual upward trend in temperatures. Willson 


Long term...no changes. Previous discussion follows... 
no changes. Previous discussion follows... 
a broad upper level low pressure area continues over the northwestern 
U.S. With smaller low pressure areas wobbling around through the 
weekend...and into early next week...keeping some showers in the 
forecast. Will need to watch how details of this pattern under a cool 
upper level low this time of year some minor changes can bring 
convection. On Sunday there are some areas of lifted indices below 
zero indicated north...south and east of the forecast area that could 
be over the forecast area with some minor differences from current 
models. Willson 


&& 


Aviation...northwest onshore flow continues today and tonight. Ceilings this 
morning still MVFR along the coast and into the North Valley...but 
predominantly VFR central and S valley. With the continued onshore 
flow...expect skies to remain broken to overcast today...but ceilings to slowly 
improve to VFR but with pockets remaining of MVFR throughout the 
day...especially over mountains. Ceilings expected to drop back down to 
predominantly MVFR later tonight. 


Kpdx and approaches...MVFR conditions this morning expected to 
become predominantly VFR this afternoon and evening. MVFR conditions 
will likely reform tonight. 
&& 


Marine...pressure gradients between a strong high offshore and low 
pressure inland will strengthen today...bringing winds above small 
craft criteria. Gradients ease some later tonight and early Friday. 
Seas have subsided below 10 feet this morning...but with increasing 
north-northwest winds later today...expect seas to build again to just over 10 
feet. Will drop the Small Craft Advisory for The Bar this evening during the weaker Ebb 
as the North Jetty will limit effects somewhat of a 320 degree swell. 
&& 


Pqr watches/warnings/advisories... 
WA/OR...Small Craft Advisory for wind from Cape Shoalwater 
to Florence through Friday afternoon. 
Small Craft Advisory for hazardous seas Cape Shoalwater 
to Cascade Head this afternoon through Friday evening. 
Small Craft Advisory for hazardous seas Cascade Head to 
Florence tonight through Friday night. 


&& 


Preliminary point probability of precipitation (probability of measurable precipitation) 
AST 22221 pdx 32333 sle 32333 eug 32333 


$$ 


More weather information online at... 
http://weather.Gov/Portland 


This discussion is for northwest Oregon and southwest Washington from 
the Cascade crest to 60 nautical miles offshore. The area is commonly 
referred to as the forecast area.