A slow-moving winter storm intensified over California on Friday, triggering the first blizzard warning in parts of the Los Angeles area since 1989 and creating the extraordinary sight of snowflakes swirling around the iconic Hollywood sign.

Snow and freezing rain pushed into the Pacific Coast state from the north, where it dumped about 25cm of powder on Portland, Oregon, earlier in the week.

The storm was expected to strengthen on Friday and linger over California till Saturday, the National Weather Service said.

A massive low-pressure system driven from the Arctic was responsible for the unusual conditions, said Bryan Jackson, a forecaster at the weather service’s Weather Prediction Center in College Park, Maryland. In Southern California, “this is a rare case of a cold, significant storm event”, Jackson said.

In a sight that must have delighted many Angelenos, snowflakes even fell around the iconic Hollywood sign atop Mount Lee in the hills above the city, known for its sunny days and palm trees.

At an elevation of 457 metres, the sign — with its giant, white-block lettering visible for miles around the city — was close to the threshold for the formation of snow during the storm, Jackson said.

Craig Robert Young, an actor who starred in the CW Network’s fantasy show Charmed and TNT’s The Last Ship, lives in Hollywood Hills within eyeshot of the famed sign. He said he was amazed to see snow swirling there.

“I moved here from the UK 20 years ago, and haven’t seen snow since,” said Young, 46. “I actually had a snowball fight.”