Windswept rains may keep voter turnout low today, as Cambridge residents head to the polls to select from among 31 candidates for City Council and School Committee, election officials predicted yesterday.

Nearly 47,000 registered voters, including 1250 Harvard students, are eligible to cast ballots for the nine council and six school board spots. Three referendum questions dealing with nerve gas testing, pornography, and Harvard real estate sales are also on today's ballot.

The National Weather Service last night called for steady showers and gusty winds during most of election day, which meteorologists said may be heavy enough to cause flooding in some low-lying areas.

Voter turnout will "depend on the intensity of the bad weather," said Edward J. Samp, an election commissioner. "If we get deluged, we may only get 50 percent [of the registered voters].

An unusually large number of the city's voters, 66 percent, turned out in the last municipal election two years ago, mostly because of the so-called Nuclear Free Cambridge ballot question, which drew national attention. This year the anti-pornography referendum generated the most controversy in this otherwise sleepy race for public office.

Because Cambridge selects its officials by a complicated system of proportional representation, the unofficial results of today's tally will not be available until tomorrow night, and official results will follow in six days, election commisioners said yesterday.

Power Struggle

At stake in today's race is the tenuous balance between conservative and liberal members of the city council, which is currently split between the liberal Cambridge Civic Association (CCA) and four neighborhood-oriented Independents.

Although incumbent Alfred E. Vellucci has held the swing-vote for the past few years, observers say the shift of a single seat could significantly alter city policies, particularly regarding the much-debated issue of rent control.

The current balance between the two city council factions will remain intact after today's balloting, observers said yesterday, but two incumbents could be unseated before the day is over.

Politicians said yesterday they will be watching to see how voters react to the emergence of a new state of moderate candidates running under the banner of Coalition '85. Two Independents, Sheila T. Russell and William J. Walsh--both of whom are running for the first time--are expected to do the best among the challengers.

Of the three ballot questions, passage of a Question 3--a binding referendum which would allow the victims of pornography to sue the makers and distributors of such materials--is the most uncertain.

Polls will remain open today between 8 a.m. and 8 p.m. throughout the city's 55 precincts.

On election eve, the city council last night adjourned with unusual speed to give incumbents one last night of campaign preparation. Candidates said they were planning for the worst weather situation and were getting more volunteers to drive people confined to their homes to the polls.

Following a 10-year election eve tradition, City Councilor Thomas W. Danehy said he wrapped up his campaign last night by knocking on the door of the same North Cambridge household on Gold Star Road and then going to get his car washed.

But three-time mayor Alfred E. Vellucci said he spent yesterday running last-minute errands and stocking up on salami, provolone cheese, and motadella for a victory party tonight at the the Italian cultural center in East Cambridge.

"I have to feed the people who work for me at the polls," said the 70-year-old politician.

When polls close tonight, all the ballot boxes will be taken to the Longfellow School on Broadway St. where 125 workers will begin the ritualistic vote count by hand tomorrow morning