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Area forecast discussion 
National Weather Service Portland or 
244 PM PDT Thursday Aug 28 2008 


Synopsis...surface high pressure over northern Oregon will weaken 
overnight. A cold front coming in off the Pacific will move southeast 
through the forecast area late Friday and Friday night. An upper level 
trough will follow the front...digging down over the Pacific 
northwest for the weekend. 


&& 
Short term...satellite shows clouds have remained entrenched over 
western Washington into early afternoon as a warm front slowly decays...while 
most of western Oregon remains sunny with a northerly flow into the 
thermally induced trough on the S Oregon coast. This pattern expected 
to evolve only slowly tonight...before a cold front drops southeast into the 
region late Friday. Models push this front through most of the area Friday 
night. Satellite pictures from the front today support model 
depiction of mainly just low level moisture accompanying the front. 
Lack of deep moisture will be the limiting ingredient for 
precipitation chances Friday and Friday night...with best chances near the 
coast late Friday...and in the Cascades late Friday night. 


A cold upper trough follows the front Sat and sun...with models 
indicating 500 mb temperatures around -22. Model cross sections indicate 
moisture continues to be limited to low levels. Even with the cold 
air aloft...bufr soundings look at best marginally unstable. Due to 
the depth of the trough will keep mention of showers in late Sat 
through sun...but will limit to slight chance or chance wording. 
After a seasonably warm day Friday...looks like a return to cooler than 
normal temperatures for the weekend. 


Long term...remnants of a weak disturbance should be exiting the 
forecast area on Sunday night keeping the forecast area dry through 
Tuesday when a second weak disturbance translates over extreme northwest 
Oregon. Guidance varies substantially on the remainder of the 
extended...and really do not believe any of the operational 
solutions are applicable. So decided to trend towards GFS ensembles 
bringing a Flat Ridge over Vancouver Island. In light of 
this...dropped probability of precipitation below climatology and kept temperatures slightly cooler than 
normal. Wolfe 
&& 


Aviation...VFR prevaling except for the kast area with MVFR ceilings 
will linger into the evening and possibly overnight. The central 
coast has a threat of IFR fog or stratus later tonight and Friday 
morning but improving for afternoon. Next system approaching 
appears the it will bring ceilings around 4000-5000 feet inland during 
the day. 


Kpdx and approaches...VFR tonight and Friday. Increasing clouds Friday 
morning with ceilings around 5000 feet after 18z or so. 
&& 


Marine...a narrow surface ridge continues along a SW to NE line 
through the middle of the coastal waters today...inbetween an 
intensifying thermal trough over northern California and another weak frontal 
trough shifting into Washington. A broad ridge develops west of the coastal 
waters late in the week with fairly robust northwest flow across the 
region...behind a weak trough which drops southeast through the area late 
Friday. Seas...a swell train generated by a Pacific storm over the NE 
Pacific will continue seas around 10 feet through this morning... 
then seas will slowly drop off tonight and Friday. 
&& 


Pqr watches/warnings/advisories... 
or/WA...Small Craft Advisory for rough Columbia River bar 
late this afternoon. 
&& 


Preliminary point probability of precipitation (probability of measurable precipitation) 
AST 144332 pdx 003122 sle 003122 eug 003122 


$$ 


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.