VanIsle Circ

The annual cycle of weather is important to understand. The large-scale weather around Vancouver Island is controlled primarily by the existence and position of the Pacific High, a semi-stable region of higher pressure off the west coast of North America, which builds in strength and moves northward in the warmer periods, and weakens and moves southward in cooler periods.

Late Fall and Winter

Ten to fifteen low pressure systems roll past or over the island per month. The lows may reach 960 millibars (28.3 inches Hg), with hurricane force winds. The prevailing wind direction is from the SE, usually with precipitation which can be heavy, especially on the western coast of Vancouver Island.

Spring Equinox through the Summer

The redevelopment of the Pacific High is usually fairly fixed by Spring Equinox, with its northern limits already affecting 48° N. This relatively stable structure will tend to push approaching lows ashore well north of the region. While the high is in place most locations will have a high percentage of light winds or calms, and little precipitation.

The West Coast of Vancouver Island experiences a predictable daily pattern under the influence of the Pacific High: calm nights, with fog or low clouds moving on shore. As the day progresses, moderate north west breezes develop, dissipating fog and clouds and creating a low chop. As night comes on the breezes drop, and calms return.

The Juan de Fuca Strait has a different but predictable daily pattern. As the inland regions of the BC province warm, they develop a large-scale low pressure system, drawing particularly up the Fraser River Valley. This causes moderate to gale force westerly winds within the strait, strongest near the eastern entrance and often accompanied by well-developed wave patterns. The system can be synergistic with summertime westerly wind patterns as to have overnight ‘lulls’ where the wind speed drops to 20 knots, and picking up during the day into the upper 30s and low 40s. for days at a time.

Low pressure storm systems do make it past the Pacific High, usually two to six weeks apart. They are forecast by a falling barometer, a swing in wind direction and intensity, and usually precipitation. The summer lows are generally not so intense, perhaps as low as 985 millibars (29.1 inches Hg) but more usually around 990-995 (29.2-29.3.)

Summer prevailing winds are most commonly from the west, SW-W-NW, with winds primarily NW along the northern inside coast of the island and the entire outside coast while the Georgia Basin is more variable and the Juan de Fuca primarily strong westerlies. In advance of a summer low pressure system, winds start from the S or SW and back to SE as the front approaches. The strongest winds and highest seas generally occur just ahead of the front, and as it passes wind will veer to NW and may last one or two days.

Fall Equinox

Sometime around the autumnal equinox the Pacific High will begin to weaken and retreat southward. As its protective shield is withdrawn, the winter pattern of lows will begin to assert itself bringing wave after wave of moist low pressure over the region.