California wildfire season: Now at 'pretty extreme' risk

* Fire season continues to 'lengthen'
* More engines on standby
* Choppers fly longer distances for water

After four years of severe drought, firefighters in California are preparing for what could be a dangerous wildfire season because of bone-dry grasses and an accumulation of dead, combustible trees.

Cal Fire's assistant southern region chief, Thom Porter, told CNBC on Thursday that the state's fire risk is "pretty extreme. For this time of year it's unprecedented. And we have staffed up in order to maintain our initial attack capability—and we've kept engines on all winter." (Tweet This)

According to Cal Fire, firefighters have already responded to well more than 150 wildfires this year, surpassing the number the state had last year by this time. The biggest one so far in 2015 is a brush fire in northeast Riverside County that started on Saturday and has scorched about 1,049 acres. The so-called Highway Fire was 96 percent contained late Thursday. At its height, nearly 660 firefighters battled the flames.

Statewide, the traditional wildfire season usually starts in June; Southern California's season sometimes begins earlier due to the region's hot, dry Santa Ana winds, which can increase the fire danger.

Ventura County Fire Department trainees in drills ahead of the wildfire season.
Jeff Daniels | CNBC
Ventura County Fire Department trainees in drills ahead of the wildfire season.

Anthony LeRoy Westerling, who studies climate and wildfires at the University of California at Merced, said the wildfire season has "continued to lengthen and expand" over recent years.

Cal Fire kept approximately 70 fire engines on standby through the winter months this year due to the increased fire risk. "We've never done that in our entire history," said Porter. "We're ramping up early this year."

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"All areas of the state are hiring firefighters, training, and staffing all of our engines and air resources up to two months early this year," he said.

The Ventura County Fire Department, for example, is bringing on additional hand crews to prepare for wildfires—and this week the department held drills with dozens of new trainees to simulate the physical demands of fighting brush fires.

"We're going to seven-day staffing and bringing on those additional resources that we use to carry us out through the fire season," said Ventura County fire Capt. Brendan Ripley.

Meanwhile, the drought also has created additional challenges for fire agencies in the state, given that some regular water supplies used to fight wildfires are dry.

"We're finding that those stock bonds we normally have full of water are either puddles or nonexistent at all," said Porter. Helicopters that lift water to dump on fires "are going to have to go longer distances to pick up water to more stable water supplies—reservoirs and big lakes. And even those are dropping to the point, in some cases, where we're going to look at building additional road lengths just to get down where the water is for our fire engines and those kind of things."

—CNBC's Brian Sullivan contributed to this report.