You have to have a very strong ridge of high pressure over the interior of eastern British Columbia, usually caused by an arctic airmass sliding south from the polar region.
Then, when lower pressure forms to our south and west, the winds will try to equalize the pressure from the northeast to the southwest.
But the Canadian Coastal Mountains block most of the air. Thus, the Fraser Valley gets howling winds because it's one of the few ways the winds can squeeze through. And when winds constrict to fit into the valley, they accelerate.
Many times, those winds can reach over 50 mph. During one of our most infamous winter storms in December of 1990, the Fraser Winds blasted over 70 mph. Combined with temperatures well below freezing, the Fraser winds can drop wind chills to around -20 or -30.
The Fraser winds can also act as a snow factory. Many times even when the rest of Western Washington was warmed over freezing and the snow has changed to rain, the arctic air pouring out of the Fraser will keep northwestern Whatcom County below freezing, thus keeping it snowing even longer.
Or, if the winds are blowing especially strong, the arctic air can filter further south and west, keeping most of the Puget Sound area in the snow where it would normally be a few degrees warmer.
As an added bonus, the Fraser winds can also create a "Strait Effect" snow, where the arctic air moves over the Rosario Strait and Strait of Juan de Fuca, where it picks up moisture. It then rams into the northeast side of the Olympic Mountains, which causes the air to lift and condense out into heavy snowfall. (It's the opposite effect of the Olympic Rain Shadow, since the air is coming from the northeast instead of the southwest). That can give areas like Sequim and Port Townsend an extra-heavy snowfall that most of the other areas miss out on.
Check The Strength Of The Fraser Winds
On this Web site: www.seawfo.noaa.gov/products/DIENTS, check the column in the top right corner marked "BLI-YWL".
That shows the difference in pressure in millibars between Bellingham and Williams Lake, BC. A negative number means a northeast wind through the Fraser, and positive number means a southwest wind.
It's when that number gets into negative double digits that you see real strong Fraser winds (like between -10 and -13 or greater).