The weird weather was caused by a "cut-off low" which separated from the jet stream that usually moves storms from west to east. Cut-off low pressure areas tend to move slowly and erratically, and this one was drifting around in the Pacific for a week before it finally moved over land on Friday (third link). They are fairly common off the West Coast in the winter but less so in the summer, and they tend to move slowly and erratically. The counterclockwise circulation around the recent cut-off low brought tropical moisture unusually far north for several days, replacing the usual cool winds off the Pacific. The result was weather more typical of Tornado Alley than the Willamette Valley, especially on Thursday, when over a quarter of the severe-weather spotter reports nationwide were from Oregon (see fourth link for map and list).
Source(s):
http://amsglossary.allenpress.com/glossary/search?id=cut-off-low1
http://www.theweatherprediction.com/blocking/
http://www.accuweather.com/mt-news-blogs.asp?blog=clark&partner=accuwea...
http://www.spc.noaa.gov/climo/reports/090604_rpts.html


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