Last Friday, a surge of westerly winds pushed down the Strait of Juan de Fuca, and at its leading edge there was a band of heavy showers. Such surges are not usual and often occur as a trough of low pressure passes through the region. Here is the surface weather map for Friday at 5 AM and you can see the trough of low pressure offshore (solid lines are isobars, lines of constant pressure, L indicates the center of the trough.
As the trough passes through, a difference of pressure develops along the Strait, with higher pressure to the west and lower pressure to the east. In the constrained Strait, air accelerates from high to low pressure, producing strong westerly winds.
At the leading edge of the westerly winds, there is convergence, with westerly winds on one side and weak winds in front of it. Convergence causes air to pile up and rise, with rising air causing clouds and precipitation. If the air is marginally stable, the vertical motion results in robust convection--towering cumulus and cumulonimbus
Let's look at some wonderful videos produced by Greg Johnson of Skunk Bay Weather showing Friday's surge from a vantage point at the northern part of the Kitsap Peninsula. You will see the surge band coming through and the heavy rain with it. Watch the flags--the wind shift will be clear.
Strait Surge - April 3, 2015 from SkunkBayWeather on Vimeo.
And a shorter sequence is found here:
Surge of Westerly Winds in the Strait - April 3, 2015 from SkunkBayWeather on Vimeo.
Finally, there is a sequence of radar images from the National Weather Service Camano Island radar. Yellows and reds are heavier precipitation. You will see an arch of heavier rain that moves eastward...that is the precipitation associated with the low-level convergence associated with the surge.
The surge on Friday was a garden variety event, with gusts at Port Townsend hitting around 35 knots (about 40 mph)--see graph
But some surges can bring hurricane-force gusts to the eastern Strait, southern Whidbey Is.
and area around Everett and Mukilteo. Like the one in December 1990 that caused millions of dollars of damage to the Washington State Ferry Elwha, which was being repaired in Everett Harbor.