Spatially and temporally coherent northwesterly wind characterizes the climate of coastal California. Coastal waters experience strong, frequent and persistent northwesterly surface wind, especially in spring and summer. Even though land-falling extra-tropical cyclones commonly occur in winter in Northern California, highest monthly mean wind speeds occur in late spring. Monthly mean wind speeds are greatest in summer for some Central and Southern California coastal waters. In spring and summer, wind measured at buoys 20-220 km (10-120 nm) offshore the California coast from Oregon to the Mexican border can exceed 10 m/s (20 knots) for days to weeks at a time. Wind speeds occasionally exceed 15 m/s (30 knots) from the northwest producing hazardous seas that pose significant risk to commercial, recreational, scientific, and naval operations.

Strong and persistent surface northwesterly wind along the California coast in spring and summer can be traced to large-scale forcing. Synoptic and seasonal changes in the large-scale pressure pattern often lead to an intensification of the surface pressure gradient, which drives enhanced northerly wind flow along the coast. Northwesterly wind events continue from spring through summer despite substantial changes in surface and upper atmospheric circulation patterns. Spring wind events are associated with a relatively strong, zonal upper-level jet over California and passage of frontal storms into the continental US well north of California. Summer wind events involve subtle adjustments to the position of the quasi-stationary Eastern North Pacific High pressure system and the thermal trough over the extreme southwest US. The large-scale surface and upper-level circulation associated with the onset, peak, and decay of wind events in each season is highlighted using National Centers of Environmental Prediciton (NCEP) Reanalysis fields. A companion paper presents a climatology of spring and summer wind speed and direction along the California coast based on coastal and offshore buoy data over the last 20+ years.