The state is seeing a sharp water divide this year, with lots of rain in the north while the south has stayed dry. A hydrologist explains what’s happening.

Southern California is extremely dry right now, with huge portions of the region having seen less than a quarter-inch of rain in the last eight months. The landscape is parched and vegetation is withered, making the area dangerously susceptible to burning, an unusual situation for January.

After months of anticipation, La Niña has officially arrived, the Climate Prediction Center announced Thursday. But even before its official start, the precursors of La Niña had already set the stage for dangerous fire conditions in Southern California.