A huge wildfire has defied containment efforts and jumped a highway, opening a new front for firefighters in California.

The week-old Rocky fire leaped across highway 20 in multiple places and scorched hundreds of acres of brush on Tuesday, expanding its total range to 65,000 acres. It is 12% contained.

“We were really working hard to keep it south of highway 20 but the fire spotted ahead,” said Daniel Berlant, a spokesman for the California Department of Forestry and Fire Protection. “It has grown at such an explosive rate. The speed is really unprecedented and shows just how dry the conditions are.”

The blaze is the worst of 21 wildfires scorching parched woodland across the state. Some 9,000 firefighters backed by aircraft are battling to contain the fires but four years of drought have created tinderbox conditions.

Thunderstorms expected this week may cool temperatures and increase humidity but they are also likely to bring wind and lightning, which has caused many of the recent fires. “It’s definitely a concern,” said Berlant.

The Rocky fire, which started near Clear Lake, about 120 miles north-west of San Francisco, has confounded computer models by growing at breakneck speed, at one point consuming 20,000 acres in just five hours.

Here's the latest perimeter map of the 65,000 acre #RockyFire. Google Interactive fire map: https://t.co/qvszLSO6iP pic.twitter.com/IiAApD1jng — CAL FIRE PIO Berlant (@CALFIRE_PIO) August 4, 2015

Standard containment methods – dropping fire retardant from the air, ploughing ditches and earthworks to create break lines and setting controlled fires to consume vegetation and other fuel – have proved no match for the extremely dry conditions.

Even at night and during slack winds the fire has aggressively expanded. “To see a fire burn like that without significant wind factor is historical,” said Berlant.

It has destroyed an estimated 50 structures, threatened another 6,300, and forced the evacuation of more than 13,000 people.

Cooler temperatures on Monday gave only a brief respite.

The California Department of Forestry and Fire Protection issued an advisory on Tuesday urging residents to stay vigilant and obey evacuation orders and road closures. “Fire activity and direction can change at any time, be prepared and stay informed. With the elevated fire danger, we are asking everyone to use extreme caution.”

On Sunday, California secured extra funding from the Federal Emergency Management Agency (Fema) to help cover the costs of local, state and tribal agencies fighting the blaze. Last week Governor Jerry Brown declared a state of emergency for California and activated the state’s national guard to help with disaster recovery.

Fire crews are also battling blazes in Oregon and Washington.

Investigations are continuing into the death of David Ruhl, a 38-year-old fire captain from South Dakota who died last Thursday while scouting the Frog fire in Modoc County, on the California-Oregon border.

The married father of two was driving alone and became trapped when erratic winds fanned flames in an unexpected way. Ruhl left his home state to help protect one of California’s majestic forests, Brown said in a statement. “We extend our deepest condolences to his family.”