A major Pacific storm hammered California on Tuesday, bringing desperately needed rain to a drought-parched state that has seen very little precipitation over the past year but also carrying the threat of flooding and mudslides.

The sub-tropical storm, which moved onshore before dawn after barreling across the eastern Pacific Ocean, had dropped 3 inches of rain on some areas by mid-afternoon, prompting the National Weather Service to issue flash flood watches for mountain and foothill areas left scarred by wildfire.

"The last time it rained this much was back in late February, so its been nine months since we've seen this type of rain," National Weather Service meteorologist Eric Boldt said.

California has been in the grip of a record-shattering, multi-year drought that has forced officials to sharply reduce water supplies to farms and prompting drastic conservation measures statewide.

Prior to Tuesday's storm, downtown Los Angeles had recorded a total of only 5.89 inches of rain for 2014, compared to the 15 inches the region receives in a typical year.

Boldt said the storm, the first of California's winter season that typically begins in December, will be welcome in drought-ravaged parts of the state but on its own won't make a significant difference.

"It's kind of like a drop in the bucket. If we had 10 more storms like this it would be perfect to get us on the right path, he said. "The drought (resulted from) three years of below-normal rainfall, we need several seasons of above-normal rainfall to make up for that."

He said the heaviest rain had been reported in Ventura and Santa Barbara Counties, where some 3 inches were reported by 3 p.m. on Tuesday afternoon, but Los Angeles and San Bernardino counties were also getting hit hard as the day went on.

Flash flood warnings were issued for areas across the state that were burned during a particularly intense wildfire season and residents below denuded hillsides were advised to be alert to mudslides.

Boldt said that no major flooding or mudslides had yet been reported as of late Tuesday afternoon.