California’s weather patterns are defined by its Mediterranean climate, characterized by hot, dry summers and mild, wet winters. The “rainy season” refers to the period when the vast majority of the state’s annual precipitation occurs, a seasonal concentration fundamental to its ecosystems and water resources.
Typical Timing
The rainy season in California generally extends from late October or November through April or May. Approximately 75% of California’s yearly rainfall occurs between November and March, with 90% falling between October 1 and April 30.
The peak months for rainfall are December, January, and February, which collectively account for about half of the annual precipitation total. While these months are the wettest, the season is marked by an increased likelihood of rain, not constant downpours every day. The exact start and end dates of the rainy season, along with the total amount of precipitation, can vary significantly from one year to the next.
Regional Variations
California’s substantial size and diverse topography lead to regional differences in its rainy season patterns. Northern California generally experiences a longer and wetter rainy season compared to the southern parts of the state. Prevailing winds and the polar jet stream contribute to higher rainfall totals in the north, with some areas receiving over 50 inches of precipitation annually. The wet season in Northern California typically spans from October through April.
Southern California, in contrast, has a shorter and less intense rainy season, which often begins later in the year. For instance, Los Angeles receives 92% of its annual precipitation between November 1 and April 30. Despite the generally drier conditions, Southern California can still experience intense rainfall events, sometimes with continuous rain for several days.
Mountainous regions, particularly the Sierra Nevada, exhibit distinct precipitation patterns, with much of it falling as snow. Precipitation in these areas increases with elevation and latitude, and the western slopes receive significantly more moisture due to orographic lift, while eastern slopes lie in a rain shadow. The accumulated snowpack in the Sierra Nevada is a major source of water for the entire state. The primary snow accumulation season in the mountains runs from December through March.
Characteristics of the Season
During California’s rainy season, precipitation often arrives in distinct storm systems rather than continuous, prolonged periods of drizzle. These storm events are frequently interspersed with significant dry periods. A few larger storms, typically five to seven, contribute the majority of the precipitation that falls during the wet months.
A major contributor to heavy rainfall events in California is the phenomenon known as atmospheric rivers. These are long, narrow bands of highly concentrated water vapor that extend from tropical regions. When these atmospheric rivers make landfall, they can deliver substantial amounts of moisture, leading to record rainfall totals, widespread flooding, and even landslides. The precipitation from these events is essential for replenishing California’s water supply.