THIS ARTICLE republished from The Conversation under a Creative Commons license.

Dry conditions across Southern California in early January 2025 set the stage for a series of deadly wind-driven wildfires burned thousands of houses and other structures in the Los Angeles area.

Ming Pana hydrologist at the University of California-San Diego’s Center for Western Weather and Water Extremestracks the state’s water supply. He puts Southern California’s drought in perspective with charts and maps.

How Dry Is Southern California Today?

In early January, soil moisture in most of Southern California in the lower 2 percent of historical records for that day in the region. That is extremely low.

On January 8, 2025, soil moisture, measured to about 40 inches (100 cm), was at a historic low of 2 percent for that day in areas around Los Angeles.

Illustration: NASA

California hydrologists are looking hard at the sky starting in Octoberwhen California’s water year begins.

The state has little rain from May through September, so late fall and winter are critical to filling reservoirs and building snowpack to provide water. California is dependent the Sierra snowpack accounts for about a third of its freshwater supply.

However, Southern California is starting the 2024-25 water year relatively dry. The region got some rain from an atmospheric river in November, but not much. After that, most of the atmospheric rivers that hit the West Coast from October to January turned northward to Washington, Oregon, and Northern California.

If the air is hot and dry, transpiration and evaporation also absorbs water from plants and soil. That leaves dry vegetation that can yield fuel flying embers to spread the firesas seen in the Los Angeles area in early January.

Water year data from October 1, 2024, to January 7, 2025, show precipitation levels and anomalies from the 1991–2020 average.

Illustration: Center for Western Weather and Water Extremes