Umpqua Bank, based in Portland, has five branches in Clark County. Umpqua assumed the former Bank of Clark County’s deposits after bank regulators shut down the Vancouver-based bank in 2009. Sterling, with 11 branches in Clark County, completed its acquision of the former First Independent Bank, which had been based in Vancouver, in 2012.NEW YORK — Umpqua Holdings Corp., Oregon’s biggest bank, is in advanced talks to buy Sterling Financial Corp., the lender backed by Warburg Pincus and Thomas H. Lee Partners, said people with knowledge of the discussions.The deal may be valued at $1.8 billion, said one of the people, who asked not to be named because the process is private. The talks may still collapse and there’s no guarantee a transaction will occur.Buying Sterling would almost double the size of Portland- based Umpqua to more than $20 billion in assets and nearly 400 branches, according to data compiled by Bloomberg. The move makes strategic and financial sense because it would help Umpqua expand in California, said Joseph Morford, an analyst with RBC Capital Markets in San Francisco. The bank could save money by closing some Sterling branches, he said.“Given some of the overlap, we suspect it should be very accretive to earnings,” said Morford,Warburg Pincus and Thomas H. Lee each held more than 20 percent of Sterling’s common stock as of June, data compiled by Bloomberg show. Spokesmen for Sterling, Warburg Pincus and Thomas H. Lee declined to comment. A representative at Umpqua didn’t reply to a request for comment.