MOSCOW, AUG. 27 -- Soviet leader Mikhail Gorbachev has set aside the last week in October for a possible summit meeting with President Reagan in the United States, and left the second half of November open as an alternative date, according to diplomatic sources here.

The summit dates are tentative and not due to be set until Soviet Foreign Minister Eduard Shevardnadze and Secretary of State George P. Shultz meet in Washington Sept. 15 to 17, the diplomats said.

Soviet officials have avoided answering public questions on the summit plans and are instead concentrating on preparations for the Shevardnadze-Shultz meeting.

White House sources, however, said the Reagan administration is now operating on the "working assumption" that a summit will take place in the United States in late November. {Details on Page A20.}

The fast-moving preparations and a statement yesterday by West German Chancellor Helmut Kohl have helped clear both atmospheric and substantive obstacles to a U.S.-Soviet arms agreement and summit meeting, Soviet officials said today.

Kohl announced that Bonn is willing to scrap the 72 obsolescent Pershing IA nuclear missiles based in West Germany, whose warheads are under U.S. control. The issue for weeks has clouded negotiations for a U.S.-Soviet treaty on removing medium- and short-range nuclear missiles and the outlook for a summit this year to sign the treaty.

"Of course, after Kohl's statement, the situation has changed for the better," Soviet Foreign Ministry spokesman Gennadi Gerasimov said in a briefing here today.

But Gerasimov also criticized a speech Reagan made yesterday, saying it doused U.S.-Soviet relations with "a heavy, cold downpour" and "does not accord with the spirit of the times."

In the speech, Reagan called on the Kremlin to change its stand in negotiations for the treaty and to extend its policy of glasnost, or openness, to the area of military affairs.

"The statement was made out of the old baggage of anti-Soviet rhetoric," Gerasimov told Moscow-based journalists.

Despite the sharp exchange of words, western diplomats here said that Kohl's offer to scrap the Pershings gave the United States and the Soviet Union a way out of their dispute over the missiles.

Moscow has insisted that the Pershing IAs be included in the treaty under negotiation to scrap all U.S. and Soviet medium- and short-range missiles, but Washington objected, saying the missiles are West German property and should not be included in a superpower arms deal.

Gerasimov indicated that remaining squabbles will be on the agenda in preparatory sessions as well as the Shevardnadze-Shultz talks. Soviet officials have told diplomats here that the meeting could continue well beyond the fixed closing date of Sept. 17.

"There will be some obstacles" to an arms accord, Gerasimov said, "but the two sides have taken this into account."

Preparatory meetings between U.S. and Soviet diplomats are taking place in Moscow and further sessions are scheduled for other European capitals.

A session on the human rights issues, headed by Assistant Secretary of State Richard Schifter and Deputy Foreign Minister Anatoly Adamishin, ended here Tuesday. Another, on bilateral issues, headed by Deputy Assistant Secretary of State Thomas W. Simons and Deputy Foreign Minister Alexander Bessmertnykh, is supposed to end here Friday. Meetings on arms control topics are scheduled later for Geneva and Vienna, while talks on regional issues will be held in London and Bern.

Moscow tentatively favors late October for the summit, diplomats here said. Soviet diplomats have postponed trips to the Soviet Union by Australian and Portuguese leaders that were to take place then. Soviet envoys in New York also reportedly inquired about the possibility of a Gorbachev speech at the U.N. General Assembly in late October, but plans for that have apparently been scrapped. Gerasimov denied western press reports that Gorbachev is preparing for a U.N. trip.

With a major celebration of the 70th anniversary of the Soviet revolution scheduled here in early November, the second half of November would be the alternative time for the summit, Soviet officials have told diplomats here.

Soviet diplomats had earlier discussed the possibility of Gorbachev visits to Mexico, Argentina and one or two other stops in Latin America, but Latin American diplomats here say the Soviet Union has not approached their countries about such visits.

Asked today about Gorbachev's possible Latin American travel plans, Gerasimov said, "Comrade Gorbachev has his own ideas, but he will report about those ideas later."

Following his meetings with Shultz in Washington and other foreign ministers at the General Assembly, Shevardnadze expects to make a swing through Latin America, Soviet officials have said.

Shevardnadze has tentatively accepted invitations to Brazil, Argentina and Venezuela, but no dates have been fixed, the officials said.