BETHEL ISLAND -- Fire crews continued to water down hot spots Monday after a brush fire the night before on Bethel Island, even as the threat of dry lightning increased the risk for such fires throughout the inland part of the Bay Area.

The National Weather Service issued a red flag warning at midnight Monday for the Bay Area, and it will remain in effect until 3 a.m. Wednesday morning, forecaster Bob Benjamin said.

The warning means that conditions are ripe for large, fast-spreading wild fires.

"We'll be watching this extra carefully, because of the dry thunderstorms," Contra Costa Fire Protection District Capt. Robert Marshall said. "The conditions are dry and hazardous, and they become more so when because Mother Nature is the one who can provide the spark."

The conditions are ideal for such a spark, because an upper level low-pressure system continues to gather moisture, and it has combined with a more high pressure system down lower in the atmosphere to create thunderstorm conditions, Benjamin said.

But because of the heat, and the conditions of the system, any rain that falls will evaporate, Benjamin said. That makes the lightning strikes more dangerous than normal, Benjamin said, because there is no moisture to douse any sparks.

Benjamin said there is about a 20- to 30-percent chance of thunderstorms as the system continues to move in from the Sierra Nevada, "but the moisture will stay aloft. It will not hit the ground."

The hot, dry conditions did not aid fire crews in fighting the blaze on Bethel Island, which began around 8 p.m. at Bethel Island Road near Gateway Road. Calls to East County Fire officials were not returned immediately, but no injuries were reported. The fire, according to media reports, briefly threatened some homes and buildings.

Crews also responded quickly to small vegetation fires in Orinda and Martinez on Monday afternoon. There were no reports of injuries in either blaze; the Orinda fire damaged a tree house and a deck, but the adjacent main home was spared

Contact Rick Hurd at 925-945-4789 and follow him at Twitter.com/3rdERH.