If the momentum of back-to-school business can be sustained through Christmas, the nation's merchants say this year will be busier and more profitable than last.

But they are concerned that the weather and events in Washington could slow consumer spending. That could yield so-so business in the critical final quarter of the retail year.

One important indicator of consumer confidence yesterday, the Conference Board's buying plans index, dropped 20 points to 114.2 in September. The survey of 5,000 families showed decreases in the number planning to buy cars, aappliances and houses in the next six months.

Interviews with merchants. Washington store owners and economists for national chains found most to be optimistic. The three months beginning Nov. 1 normally produce one-third of store sales and often half of annual profits.

Washington area retailers were generally more pessimistic in their predictions. Garfinckel's Hanne Merriman said the company's Christmas selling plan is "conservative," Mortimer Lebowitz of Morton's predicted sales would be flat for stores in the District.

Nationally, major retail chains based their optimistic forecasts on September sales figures, reported yesterday, which showed the retail growth rate for the month was the strongest of the year for many companies.

But weaknesses in some categories - particularly men's clothing - and uncertainties about events that will influence sales tempered the forecasts.

"It's been an iffy year so far and it's still going to be iffy," said Frank Mannarino, who heads the Washington area operations of W&J Sloane, the furniture division of City Stores.

For many merchants, the biggest "if" is, if the weather will turn cold soon enough to spur sales of coats, snow tires and the like. Last year's bitter cold came too late for many stores: they began marking down their winter clothes in December and the profit potential was gone by the time consumers needed to bundle up.

Even more uncertain are actions the government might take that would challenge consumer confidence. Uncertainties about energy legislation tax law changes and U.S. policy on Israel make merchants nervous but there remains a reservoir of hope.

"We are optimistic about volume in the important Christmas season ahead and expect good sales for the balance of 1977," said David Babcock, chairman of the board of May Department Stores, which include the Hecht Co. here.

At Montgomery Ward, which reported its best September gains since 1973, chairman Edward S. Donnell predicted "the balance of the fall season including Christmas will exceed earlier expections."

Donnell noted that Ward's credit card customers are "making their payments on time and purchasing well within the limits afforded them. This reassures us they are in good financial condition for the important Christmas season."

Credit sales tend to increase sharply as the holiday season approaches so that by Christmas Eve big chains typically do 90 per cent of their business with credit cards rather than cash.

Several big retailers plan special efforts this year to cash in on the holiday generousity.

J. C. Penny will supplement its usual Christmas "wish book" - a 528-page volume that's already in the mail - with a 72-page gift catalog, its first ever.Scheduled for mailing around the end of the month, the Penny's gift catalog will for the first time abandon the usual grouping of merchandise by category.

Instead of showing women's clothes with other women's clothes, the new gift catalog uses what Penney's calls "life style merchandising," like scenes of families having fun outdoors in which everything but the snow can be ordered form Penney's.

Such fancy catalogs are most of ten identified with Nieman-Marcus, which this year is pushing a $50 11-foot pole, for those who wouldn't touch anything shorter. Neiman opens its new Washington store the first week of November, hoping to parley the grand opening hoopla and the Christmas rush into a two-month bonanaza.

What happens on upper Wisconsin Avenue will have little meaning for the city's black merchants and minority businessmen, said Lebowitz, whose Morton's stores depend on the money of Washingtonians who have little to spend. He predicted the total Christmas sales in the city would do well to match last year's total and said many inner city stores will fall further behind.

Another Washington merchant, shoe retailer Frank Rich, said he is "optimistic and I didn't feel that way on April 15."

"I'm encouraged," he said, "we're beginning to see some life in our stores." While men's dress shoe sales remain slow, women's boots are selling well, and "the middle market is buying earlier this year."

At Garfinckels, "we've been planning conservatively and still are," said executive vice president Meriman.

Like the Penney merchandise manager, she cited new silhouettes in women's clothes as a positive influence on sales for the rest of the year, singling out cashmere sweaters sportswear and coats as top sellers.

Men's wear sales are benefiting from the return of the three piece suit, said a spokesman for Carleton's the downtown men's store. A new labor contract in the men's wear business has pushed middle market men's suits past the $200 point this year, producing some consumer resistance.

Skyrocketing prices - are also slowing sales of building materials, said Richard England, president of Hechinger's. "They think about paying double the price they paid a year ago for lumber," he said noting that retailers don't profit from that kind of inflation.

Soft sales of boards are more than offset by the demand for energy saving materials, he added, pointing out that "wood stoves that we stopped carrying 30 years ago are back and doing very well."

The Hechinger boss suggested an even less likely best seller than Neiman-Marcus's 11-foot-pole - bags of coals, meant for burning in stoves and fireplaces. They'll be in time to give Santa a warm welcome.