The chairman of the Senate District appropriations subcommittee said yesterday that it will be two weeks before District of Columbia officials will know how much money will be available for a key portion of Mayor Marion Barry's summer jobs for youth program.

The project is scheduled to begin in just three weeks.

Sen. Patrick J. Leahy (D-Vt.), the subcommittee chairman, said at a hastily called early morning hearings on the $11 million request that city officials should be prepared to establish priorities in the event all of the money requested is not approved.

"It is conceivable that the full amount might not be appropriated," Leahy told Budget Director Gladys W. Mack and Matthew F. Shannon, acting director of the D.C. Department of Labor.

After hearing, Leahy told a reporter that uncertainties about the Barry administration's ability to provide 30,000 summer jobs and a desire - especially in the House - to hold the line on government spending could lead to reductions in the city's request.

"You saw what happened to the other budgets," Leahy said, in an apparent reference to the House District appropriations subcommittee's decision last month to approve for the city a 1980 federal payment that was the lowest in 15 years. "I would assume that there may have already been raised some questions as to whether 30,000 jobs could be put together.

"There's this feeling in the Congress about balanced budgets and all. Unfortunately, a lot of good programs have been funded in the past. And the District is not one of the favorite budgets up here," Leahy said.

The job program proposal considered yesterday is part of a five-pronged effort by Barry to keep a campaign promise to provide 30,000 summer jobs for people 14 to 21 years of age. The city provided 15,000 summer jobs last year.

The proposal referred to by Leahy, originally designed to provide 11,100 summer jobs but altered by the City Council, would allocate $7.2 million to provide 8,564 of the promised 30,000 jobs at either $2.65 or $2.90 an hour.The remaining $2.8 million would fund 1,500 year-round jobs for youths and adults as well as provide administrative funds for the job program.

Barry's plan for 30,000 jobs includes the expected creation of 13,500 jobs through already allocated federal funds, 2,000 jobs from the city's business community, 2,000 jobs from the federal government and 1,000 jobs in D.C. government.

A week ago, Shannon said that the effort could fall short by as many as 6,000 jobs because of so-far unfulfilled promises from the business community, some minority entrepreneurs, various federal agencies and the City Council's action.

Yesterday he told Leahy, "We are working full speed ahead to employ 30,000 youths this summer." But Shannon did not say how close to the goal the effort might come.

Leahy said yesterday that the special job funding request, which is part of a proposed $76.5 million supplement to the District's fiscal 1979 operating budget, would have to be handled differently that usual because it was delivered to Congress after House Appropriations Committee had already passed the full federal 1979 budget supplement.

Ordinarily, the Senate and House pass separate versions of District spending requests and then iron out the differences in conference. In this instance, the first indication of how much money the city will receive will probably be given by the Senate, which will insert the District package into the full appropriations bill once it is forwarded from the House.

Leahy said he asked for possible areas to be cut in anticipation that some reductions might be necessary. "I don't want to come out with a figure here that's just going to be copped down (by the House)," Leahy said.