THIS ARTICLE IS 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 that burned thousands of homes and other structures in the Los Angeles area.
Ming Pan, a hydrologist at the University of California-San Diego’s Center for Western Weather and Water Extremes, tracks the state’s water supplies. He put Southern California’s dryness into perspective using charts and maps.
How Dry Is Southern California Right Now?
In early January, the soil moisture in much of Southern California was in the bottom 2 percent of historical records for that day in the region. That’s extremely low.
Hydrologists in California watch the sky very closely starting in October, when California’s water year begins.
The state gets very little rain from May through September, so late fall and winter are crucial to fill reservoirs and to build up the snowpack to provide water. California relies on the Sierra snowpack for about one-third of its freshwater supply.
However, Southern California started out the 2024–25 water year pretty 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 into January veered northward into Washington, Oregon, and Northern California.
When the air is warm and dry, transpiration and evaporation also suck water out of the plants and soil. That leaves dry vegetation that can provide fuel for flying embers to spread wildfires, as the Los Angeles area saw in early January.
