This afternoon in the Terrace Theater, the Washington Opera will close its season with the grand finale of Offenbach's "Christopher Columbus"the whole cast and chorus singing the praises of a "new world where dreams will come true."

It's an optimistic parting note for the company's 30th season, and as of today's final curtain, the Washington Opera has ample ground for optimism. But 1985-86 may be remembered as the season when the company's dreams nearly turned to nightmares.

The Washington Opera sold 97.5 percent of its tickets for the season that began last October. That's the highest percentage in the company's history. But if the percentage is up, the absolute numbers are down sharply. In 1985-86, the Washington Opera had fewer tickets to sell than in any season since 1979-80. There were only 47 performances of five productions that have become standard in the last few years.

The Washington Opera has come a long way since Jan. 31, 1957, when it opened its first two-production season with Paul Callaway conducting Mozart's "The Abduction from the Seraglio." The company grew from that point, but in 1979-80, it was still presenting only 16 performances of four productions with rented sets and costumes. There are essentially two reasons for the difference: the opening of the Terrace Theater, where the Washington Opera began a trend in American opera production, and the appointment of general director Martin Feinstein, who masterminded the company's radical transformation.

The Washington Opera is not likely ever to fall back to the pre-Feinstein level, but this year it took a temporary step backward. The reason was financialan accumulated deficit of $953,000 made a cutback necessary.

It worked. Figures for the current season (not yet complete) indicate the company will probably break even. Meanwhile, its fundraising efforts have been so successful that it has more than doubled the goal of its three-year financal stabilization campaign. The original goal of $1.85 million was nearly reached in the first 18 months, so a new goal has been established: $4.5 million by June 1987. The purpose of the campaign is to eliminate the current deficit and to provide a cash reserve of working capital. In the past, the company had to function on short-term loans. With a reserve fund, it sould be able to eliminate the interest payments that have been a substantial part of its budget.

The season's backward step was reflected in the artistic quality as well as the quantity of productions. When the decision to cut back had to be made, it was found necessary to reduce the season in the Terrace. Artistically, it might have been better to omit one production downstairs in the Opera House, where the company has been doing the same kind of thing, on approximately the same level, as a half-dozen other American opera companies. A candidate for elimination might have been this season's "Un Ballo in Maschera," which was splendidly staged but sung often with more vigor than fiesse. On the other hand, a Washington Opera season with nothing by Verdi or Puccini would have been unthinkable.

One reason for cutting back in the Terrace rather than the Opera House may have been that the internationally known singers who tend to get the major roles in the Opera House are booked much further ahead than the predominantly young American singers who perform in the Terrace. Therefore the downstairs season gets locked in place before the one upstairs. Be that as it may, the Terrace seasons in recent years have been more reliably satisfying than those in the Opera House.

In 1980-81, the Terrace season made the Washington Opera unique in America. That is no longer true; other American companies have begun to follow the Washington Opera's example. But the Terrace season is usually still outstandinga standard-setter. It is notable for the special sense of ensemble performance it develops and for the valuable experience it gives to young American singers. These singers have often turned in more enjoyable performances than the established Europeans in the Opera Houseparticularly if one makes Francois Loup, always one of the highlights of the Terrace season, an honorary young American.

The Washington opera public recognized the difference between these two kinds of production, and the 1985-86 Terrace season was virtually sold out on subscription almost as soon as the tickets became available.