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The Snows of California

An Analysis of California Snowfall in the 21st Century

Snow is vital to the environmental health of the western United States. Unlike rainfall, it doesn't come as one burst that's here and then quickly gone. Instead, it tends to accrue over the winter and slowly melt as the temperatures rise in the spring. The melt typically continues into the early summer, providing a steady, consistent level of water in mountain watersheds. This is particularly important for a state like California, which receives almost no summer rainfall.

Snow comes in a variety of forms. It can be light and airy, almost dry, or it can heavy and dense. For example, light, fluffy snow that measures four feet deep might actually have the same volume of water as heavier, denser snow that is two feet deep.To account for this variation a measure called snow water equivalence is used to normalize the amount of liquid water in a given level of snow.

Compare the average snow/water levels by year and elevation. The general trend is towards drier conditions, but wet years are interspersed with drier years. Notice that 2011 was a very wet year across all elevations, while 2015 was extremely dry.

Snow Average by Year (in inches)

Snow Average by Date (in inches)

Watersheds

More moisture tends to fall the further north you go in California, resulting in more opportunity for snowfall. This is partially offset by the fact that the Sierra Nevada tend to increase in elevation the further south you go. So that any precipitation that does fall in the south is more likely to fall as snow. Moisture also increases as elevation increases. As air rises it grows cooler and drier, dumping any available moisture. Parts of the Sierras receive the equivalent of 50 inches of rainfall a year. Contrast this with Los Angeles which typically receives about 15 inches of rainfall in an average year.

Nonetheless, the southern Sierra has been particularly lacking in moisture over the last few years. Far fewer storms than normal have come in off the Pacific, along with higher temperatures, have resulted in a decreased snow pack.

The April 1st average is a key metric in California snow measurement. It represents the point of maximum snow accumulation. While it is possible for further snowfall to occur snow levels tend to decline from this point forward. The percentage of April 1st averages below is a measure of the average of all stations at the given elevation. While not directly comparing April to April levels, it serves as a useful measure to compare relative snow levels from year to year across elevations.

Select a basin to see its location, snow and snow/water equivalent levels over the 21st century.

American River Basin (Snow Average by Year in Inches)

Snow and snow/water equivalent levels are in inches.

Snow percentage is percentage of April 1st average, as discussed above.

Note: All basin maps should be considered the approximate extent of the given watershed.

Note: The Sacramento River is one of California's major rivers. Many of the other rivers in the state eventually terminate into it. The basin map given is for the Sacramento's headwaters.

Data Sources