A 1,000-mile-wide atmospheric river that’s forming in the subtropics will brush San Diego County with light rain Friday and Saturday and bring heavy precipitation to the rest of Southern and Central California, further aiding reservoirs and muting the risk of wildfires, the National Weather Service said on Monday.

The storm is part of a major pattern change that also is expected to produce a second warm, moist atmospheric river that will flow through roughly the same areas on March 14, possibly bringing heavier rain to San Diego, which is experiencing its first wet year in three years.