Weather

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afdpqr 


Area forecast discussion 
National Weather Service Portland or 
848 am PDT Thursday Aug 28 2008 


Synopsis...a warm front will brush across Washington today. An east-West Ridge of 
high pressure over central Oregon will lift north towards the Washington 
border later this afternoon and into southern Washington tonight. A 
trough of low pressure will build up to the S Oregon coast tonight. High 
pressure off northwest Oregon weakens Friday afternoon with thermal 
trough shifting to eastern Oregon. Pacific front approaches northern 
coast late Friday and moves across area Friday night. Upper trough 
deepens off Pacific northwest coast Saturday as jetstream drops to 
southern Oregon. Upper trough moves over area Saturday night and into 
eastern Oregon Sunday. Upper trough shifts to rockies Monday with 
high pressure building offshore. Weaker front and trough brush across 
area Tuesday. Northwest flow over area Wednesday. 


&& 
Short term...warm front over western Washington this morning is weakened by 
models during the day today. Surface observations suggest rain 
extends no further S than Columbia River along the Washington coast. Surface 
ridge extending east-west across Oregon this morning is expected to lift 
slightly north today towards the Washington state line...weakening the onshore 
flow in the far north. 


Remainder of short term discussion unchanged... 
SW Washington. High pressure lifts a little further north tonight with thermal trough 
developing off S Oregon coast. Clouds lift little more north overnight 
though northern coast still fairly cloudy. Thermal trough shifts into 
eastern Oregon Friday afternoon. Onshore flow increases by afternoon. Temperatures 
start off warmer Friday then today but with increasing onshore flow 
temperatures should balance to about the same. Front approaches northern 
coast late with some precipitation threat and temperatures cooling some from 
today. Front moves across area Friday night with remains over far cental 
Cascades early Sat. GFS looks light with precipitation as trough 
pushing front fairly strong for this time of year and jetstream of 90 
to 100 knots. Will go solid precipitation threat northern coast Friday evening 
moving to Cascades later Friday night. Quantitative precipitation forecast mainly .25 inch or less 
coastal areas and northern Cascades. Inland valleys less though some 
moisture likely with drizzle or light rain. Upper trough deepens off 
Pacific northwest coast Sat. Strong west-northwest flow aloft should keep convective 
showers well east and north of area. With strong west onshore flow north will bring 
shower threat to northern coast and SW Washington Sat. Temperatures drop around 10 
degrees Sat from Friday with cooler air mass. 
Kosovitz 


Long term...no changes. Previous discussion follows... 
upper trough moves over area Sat night as jet drops into 
northern California. Air mass till fairly dry but with onshore flow except some 
showers northern coast and Cascades and some threat northern valleys. 
Freezing levels down to lowest levels so far this season. Coldest air 
aloft moves over area Sun morning then shifts to eastern Oregon. Not 
good timing for thunderstorms sun though expect instability shower threat for 
northern two thirds of area. No further changes made to extended. 
Kosovitz 


&& 


Aviation...another frontal band moving into Washington today should give 
MVFR ceilings to kast...with a threat of some -dz as well...then 
ceilings likely to lift back to VFR during the afternoon. 
Inland...north of Salem conditions should stay VFR through Thursday 
morning but with some VFR ceiling moving in. South of ksle...clear 
skies with local IFR ceilings in valleys of Cascade foothill southeast of eug. 


Kpdx and approaches...VFR today but with ceiling above 5000 feet as 
frontal clouds over Washington brush the area. 
&& 


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 hazardous seas Cape Shoalwater 
to Florence through this morning. 
Small Craft Advisory for rough Columbia River bar 
late this afternoon. 
&& 


Preliminary point probability of precipitation (probability of measurable precipitation) 
AST 11363 pdx -0-31 sle 00-31 eug 00031 


$$ 


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.