Extreme Precipitation over the West Coast of North America: Is There a Trend?

Heavy precipitation and resulting flooding are the most serious weather-related hazards over the west coast of North America. This presentation analyzes the trends in heavy precipitation events for the period 1950 through 2009 by examining the decadal distributions of the top sixty and twenty two-day precipitation events for a collection of stations along the coastal zone of the U.S. and British Columbia, as well as the decadal distribution of daily maximum discharge for unregulated rivers from northern California to Washington State. During the past sixty years there has been a modest increase in heavy precipitation events over southern and central coastal California, a decline in heavy events from northern California through the central Oregon coast, a substantial increase in major events over Washington, and a modest increase over coastal British Columbia. Most of the trends are not significantly different than zero at the 95% level. The trends in maximum daily discharge of unregulated rivers is consistent with the above pattern, with declining discharges over the past three decades south of 45°N and increases north of that latitude. The above-observed trends in heavy rainfall and maximum discharge are compared to the future patterns indicated by general circulation models under various global warming scenarios.