A weather system swirling above Northern California is capable of producing fierce lightning storms that, combined with tinder-dry vegetation, could spark a fresh round of wildfires in a region where firefighting resources have already been stretched thin, forecasters warned Tuesday.
The potentially destructive combination prompted the National Weather Service to put much of the northernmost area of the state under a red-flag warning, which is in effect from 1 p.m. to 11 p.m. Southern Oregon was also on high alert as the unstable weather system moved north.
In recent weeks, lightning bolts have been blamed for dozens of brush fires in state and national forest lands in Central and Northern California. Gov. Jerry Brown declared a state of emergency over the weekend, citing the effects of the fires, including taxing firefighting resources.
The abundance of thunderstorms has been caused by an unusual flow of monsoonal moisture from the southeast. The storms have sparked wildfires in some areas while delivering powerful bursts of rain that on Sunday sent rocks and mud cascading through mountain towns in San Bernardino County, killing one man and destroying several homes.
Changes in ocean temperature thousands of miles away have delivered the atypical summer weather to Southern California, driving up humidity and causing sporadic destruction.
Warm equatorial water in the Pacific, from mainland Mexico to Peru, normally pumps monsoonal air up the Sea of Cortez into the Southwest, with mountains blocking it from the coastal plains of Southern California.
But this year, the ocean temperatures are higher than normal, climatologists say, producing a more powerful "tropical wave" that made it all the way to the coast.
Meanwhile, tens of thousands of acres continue to burn across California. Two of the biggest wildfires -- the Bald and Eiler fires in the Lassen National Forest -- have burned nearly 70,000 acres combined, according to the U.S. Forest Service.
Other large wildfires have also pockmarked the Sierra, Klamath and Tahoe national forests, as well as Yosemite National Park. Dozens of homes have been lost as flames threaten hundreds more, prompting evacuations in some parts of the state.