Sonoma County elections officials said about 66,000 ballots remain uncounted as of Thursday afternoon, an updated total that includes mostly mail-in ballots dropped off at polling places or mailed in too late to be tallied on Election Day.

More ballots are expected to arrive by mail Friday at the county’s main elections office in Santa Rosa, said Deva Proto, the incoming clerk-recorder-assessor and elections chief. Proto said the U.S. Post Office delivered about 800 ballots Thursday.

Proto said elections staff have not yet begun to tally the remaining ballots toward election results. An extensive process of signature verification and ballot sorting must be completed first.

“We have to verify the signatures before we can even start removing ballots from the envelopes,” Proto said.

It will likely be several days before the blue mail-in ballot envelopes are opened and their votes registered, she said. That means final outcomes for several tight city council and school board races could remain undecided well into this month. A prime example is the District 4 contest for Santa Rosa City Council, where Victoria Fleming leads Dorothy Beattie by 169 votes.

No estimate was available for how many untallied ballots exist in that contest or others across the county.

Of the estimated 66,000 untallied ballots, some 57,000 are vote-by-mail ballots. The bulk of the remainder are provisional ballots cast by voters who thought they were eligible but were not showing on the precinct roster, or by vote-by-mail voters who did not have their mail-in ballot. About 500 are conditional ballots cast by voters who registered late.

Thursday’s estimates do not include ballots expected to arrive by mail at the county elections office Friday, Proto said.

Late-arriving mail-in ballots slow down the vote counting process because it takes much longer to certify them. Votes cast Tuesday are certified on the spot by polling place workers.

In Sonoma County, 202,000 mail-in ballots were sent to voters. Of these, only 96,622 mail-in ballots were returned early enough to be counted on Election Day. Only about 36,000 votes were cast by people entering a precinct polling booth.

In Mendocino County, elections officials reported late Thursday that there were 16,730 mail-in and 1,011 provisional ballots that have yet to be counted. Mendocino County Assessor-County Clerk-Recorder Susan Ranochak said there were 3,288 uncounted ballots in the Board of Supervisors’ 3rd District and 3,210 in the 5th District.

Ranochak said there were 2,900 uncounted ballots in Ukiah, 776 in Willits and 1,300 in Fort Bragg.

Proto said due to the large number of voters in Sonoma County, uncounted ballots have not yet been sorted by precinct or region. She said election workers began checking signatures on Thursday.

You can reach Staff Writer Martin Espinoza at 707-521-5213 or martin.espinoza@pressdemocrat.com. On Twitter @renofish.