Thanks to georger I was able to see some of Tom's data on wind speed. According to that, the wind direction and wind speed were remarkably consistent from Seattle down to Portland - even at higher altitudes. This isn't anything new. Tom posted this a couple of years ago. What's interesting is that the wind direction was less from the southwest and more from the south. In fact, I'd say the wind was blowing at an angle of 250 or 260 degrees rather than the commonly held 220. Other historical data from Weather Underground support this: the wind was blowing from the south around 6 pm and had slowly shifted to the south-southwest by 10 pm.
Wind speed was light at 10,000 feet but jumped dramatically at 8,000 feet before tailing off quite quickly the closer you get to the ground.
Not sure if this really affects much, but interesting to me nonetheless.
Chaucer, I hate to have to ask you this, but have you ever heard of a "compass rose"? If not, here is a brief tutorial for you.
True North (which is not to be confused with Magnetic North) is labeled 000 degrees or 360 degrees which is the same thing here. Going around a circle in a clockwise direction (oddly enough this is the same direction all the hands on a clock rotate, although I have seen exceptions to this), 090 degrees is labeled East, 180 degrees is labeled South, 270 degrees is labeled West, and 360 degrees is labeled North as is 000 degrees.
In your post above, you list the winds as coming from a more westerly direction, not more southerly direction, than the data supports. Your claims about the wind velocity are not accurate either.
A close look at the radiosonde data shows that the winds at 10,000 feet above sea level are essentially from 225 degrees, which is the southwest, and the velocity at that altitude is about 35 Knots. The winds at lower altitudes do not decrease as fast as you suggest.
The FAA has excellent publications, which are downloadable and free, on their web page. Some of those publications relate to aircraft navigation and some to aviation weather. I suggest that you download them and give them a good study. You might learn something. Good luck.