After a brief respite, soggy weather returned to Southern California on Wednesday thanks to an atmospheric river that unleashed heavy rain across the region. Atmospheric rivers are concentrated streams of water vapor in the middle and lower levels of the atmosphere. These obstacles force the atmospheric rivers into higher altitudes, where they cool and start shedding their burden of moisture. Some atmospheric rivers are weak and produce beneficial rain, and some are larger and more powerful, causing extreme rain, floods and mudslides. On average, 30% to 50% of the West Coast’s annual precipitation comes from a few atmospheric rivers each year.

Source: Los Angeles Times December 04, 2019 18:54 UTC