US Firefighters have found a badly burned body in a transient camp engulfed by raging wildfires, the first death from the blazes that have ravaged large parts of southern California.

Cooling temperatures fuelled hopes on Friday that thousands of people evacuated could begin returning to their homes, threatened by three days of fires.

Strong seasonal Santa Ana winds whipped up so-called “firenados” – columns of rapidly rotating flames – at the height of the crisis, which saw thousands of firefighters swarm to tackle up to nine separate blazes.

There had been no reported fatalities across San Diego County until late on Thursday when authorities in the beach resort of Carlsbad confirmed a body had been found.

“During a hot spot check, firefighters were alerted to a transient encampment … on checking the area, (they) located a badly burned body,” said a statement.

One of the worst hit areas was San Marcos, 24km inland from Carlsbad, which is north of San Diego.

On Friday, the fire there had more than doubled in size overnight to over 1200 hectares.

But cooling temperatures meant firefighters were more hopeful of bringing that blaze and others under control.

California and other western US states are routinely hit with wildfires during the summer and autumn, but blazes have occurred earlier in the year in recent times. So many blazes at once in only mid-May is very rare.

A number of decades-old records for mid-May temperatures have been broken or equalled.

Downtown LA sizzled in temperatures of 38 degrees Celsius on Thursday, tying a record dating back to 1970.