Answer: Rain in San Diego is anything but average.
It falls mainly in winter and varies a lot from year to year. For example, a very wet winter in 1997-98 was followed by a six-year-long dry spell. That drought was followed by an extremely wet winter in 2004-05. In the winter of 2006-07, San Diego had one of the driest years on record. This winter, everyone expected dry conditions, but it has actually been relatively wet. Mother Nature is full of surprises.
Rainfall in San Diego has been extremely variable over the past several years, but there are no clear signs that rainfall is generally increasing or decreasing. However, even in a wet year, there is not enough local rainfall to supply water to San Diegans, especially during the hot dry summer.
That is why 80 to 90 percent of San Diego water supply is brought in from Northern California and from the Colorado River. Most of this water begins as snow in the Sierra Nevada and Rocky mountains. Snow is a natural reservoir; it accumulates during the winter and then melts slowly during the spring and dry summer. Melting snow flows downhill in streams and rivers and is captured in human-made reservoirs. Snowmelt gives us fresh water in the warm dry season, when water is most needed. San Diego’s water supply would dry up if it were not for the snow that falls on remote mountains.
As climate continues to warm, there will be less and less snow in the Sierras and Rockies. This means that water supply in San Diego — as well as in all of California and the West — is threatened.
So, although San Diego rainfall has not generally increased or decreased, global warming is decreasing our fresh water supply and it will be a huge problem for our water resources in the future.
--Alexander Gershunov, associate research scientist, Climate, Atmospheric Science, and Physical Oceanography Division