Sentence:  During the preceding afternoon a heavy North Pacific fog had blown in … Scudding eastward from the ocean, it had crept up and over the redwood-studded crests of the Coast Range mountains, ..., <WSD>billowing</WSD> steadily eastward, it had rolled up the western slopes of the Siskiyou Range, ... 
Meaning of the word marked with <WSD> tags:  To surge or roll in billows.