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California: a Place of Extremes
With 840 miles of coastline; the highest and lowest points in the continental U.S.; more national forests than any other state, the tallest, largest, and oldest trees on earth and three deserts, California's topography and geography are diverse – perhaps even extreme. And, our weather is no less so. California holds world records for both the hottest day (134 degrees) and hottest rain (115 degrees) and U.S. snow records for both single storms (15.75 feet) and single season (74 feet). It's not just extreme events but the variability in annual precipitation and physical location that make the impacts of California’s weather so diverse.
Graphic illustrates the percentage of year to year variation above or below the long-term average. Please note: each dot represents a defined local area, increasing accuracy of information. (Graphic: Dr. Michael Dettinger, USGS)
Precipitation in most of the country varies year to year 10 to 30 percent above or below the local long-term average. In most of California, precipitation varies year to year 30 to 70 percent above or below the local long-term average (see illustration at right). Additionally, California’s physical location makes it subject to three very different types of storm phenomena: the North American monsoon season, storms from the northern Pacific Ocean, and atmospheric rivers.
North American Monsoon
Precipitation in the Inland Empire region of California is more influenced by the North American monsoon season than weather patterns along California’s coast. The North American monsoon season occurs during the summer throughout the southwest when moisture flows northward from the Pacific Ocean and the Gulf of Mexico, producing a radical change in moisture conditions. Storms caused by this weather pattern are usually brief, extremely intense, and can cause flash flooding and mud slides – especially in burn areas.
From the north coast to the northeastern border of the state and into the Central Valley, California is subject to arctic storms that come from the northern Pacific Ocean and blanket the Cascade and Sierra Nevada mountains with snow during the winter, contributing significantly to the state’s water supply in the form of snowpack. Typically, these storms are colder, carry less moisture and can have very high wind speeds .
California is also susceptible to atmospheric river storms (also called Pineapple Express). These storms are called atmospheric rivers because of the extreme amount of moisture they carry. Typically originating in the southwest, tropical area of the Pacific Ocean, atmospheric rivers are usually several long, narrow bands of warm moisture that get picked-up by passing winter storms. Atmospheric river storms are characterized by warm, heavy rainfall ahead of the cold front. This warm rain prevents snowpack from forming below 8,000 feet.
Though not all atmospheric rivers cause flooding and they are an important element in California’s water supply, extended atmospheric river storms can contribute to loss of established snowpack and cause extreme flooding. Forty-two atmospheric rivers have impacted the state between 1997 and 2006. Seven caused significant flooding in the Russian River watershed, and the more widespread “New Year’s Day Flood” of 1997 caused more than a billion dollars in damage.
California is experiencing one of the worst droughts on record, yet since 2012 the state has experienced at least $52.3 million dollars in flood damage and four-flood related fatalities. The state’s physical location along the edge of the continent makes it subject to three very different types of storm phenomena, and the extreme variation in our annual precipitation means that every Californian must be both water-wise about conservation and aware that given the right conditions, flooding can happen any time of the year.