With both presidential nominations virtually locked up and few burning issues on the ballot, about half of Sonoma County's voters are expected to sit out Tuesday's election, officials and observers say.

Voter turnout will be 50 to 53 percent, county elections chief Janice Atkinson predicted Monday, as absentee ballots continued coming in at a below-average rate.

"There aren't enough compelling issues on the ballot to draw people out," said Atkinson, who has been involved in county elections for nearly 40 years.

Some local experts put the turnout even lower, in the high 40 percent range, and Atkinson conceded: "They could be right."

Three supervisorial races, two congressional seats and four state legislative contests are in play, along with just two statewide ballot measures.

With President Barack Obama and Republican Mitt Romney almost certain to meet in November, the primary election's main event won't motivate many voters, Atkinson and the experts said.

In 2008, when California held its presidential primary in February, Sonoma County voter turnout was 76 percent.

Participation in the June, 2010, gubernatorial primary was 51 percent.

Atkinson said her forecast was buttressed, in part, by the slow return of absentee ballots, officially known as vote-by-mail ballots.

Of the 163,115 absentee ballots mailed to voters, just 63,894 valid ballots — 39 percent — had been returned on Monday, she said.

The day before the June, 2010, primary election, 45 percent of absentee ballots had been returned.

Nearly two-thirds (65 percent) of the county's 249,597 registered voters use mail ballots.

Mendocino County reported 11,594 absentee ballots, about 32 percent of those issued, had been returned as of Monday. About 76 percent of Mendocino County's 47,574 registered voters cast mail ballots.

Lake County reported less than a third of absentee ballots had been cast. About 57 percent of Lake County's 33,553 registered voters cast mail-in ballots.

In Sonoma County, all of the valid absentee ballots received through Monday will be counted and included in the first results posted at 8 p.m. Tuesday, Atkinson said.

As ballots begin arriving from polling places, starting shortly after 9 p.m., updates will be posted, and all regular ballots should be counted by 1 a.m. Wednesday.

But the outcome of close races will likely remain uncertain, owing to an estimated 25,000 to 35,000 absentee ballots expected to arrive in the mail or be turned in at polling places on Tuesday, Atkinson said.

Those votes will not be counted and no subsequent updates will be posted until the final, official election results are released, three to four weeks after election day, she said.

"We tell the candidates (in close races) to take a vacation," Atkinson said.

Staff Writer Glenda Anderson contributed to this report.