Broadway doesn’t have summer sales in the bag just yet.

Memorial Day traditionally marks the start of the influx of warm-weather tourism in Gotham, and the Main Stem, coming off a season that saw box office defy the gloom of the economic downtown, has good reason to hope receipts will remain high.

But it’s out-of-towners who fuel summer sales — and with recessionary worries and the recent outbreak of swine flu prompting predictions of a decline in tourism, it’s not yet certain that the usual B.O. boom will ring in as robustly as it has in the past.

“We have no way of predicting box office business over the next few months,” said Charlotte St. Martin, exec director of the Broadway League, the trade association of legit producers and presenters. “But we are cautiously optimistic that the momentum of the spring will continue throughout the summer.”

That momentum certainly seems unusually strong. After legiters spent the fall worrying the economic crisis might prompt a spate of dark theaters, the spring slate filled out to such an extraordinary extent that the final tally of shows to open during the 2008-09 season, 43, is the highest in 25 years.

Powerhouse tuners “Billy Elliot” and “West Side Story” prove the Rialto remains able to generate hits that sell upwards of $1 million a week, while plays such as current smash “God of Carnage” also share in the box office bounty.

Philip J. Smith, co-CEO of Rialto landlords the Shubert Organization, said he had not yet seen any reason to expect a significant decrease in summer sales. “Maybe I’m wrong, but I think we’ll get more domestic tourism than we have in the past,” he said.

It’s even possible the season-end cume, although unlikely to surpass the billion-dollar barrier, could break the $939 million record logged by the 2006-07 season.

Still, the summer forecast isn’t entirely sunny. Earlier this year, Gotham tourism org NYC & Co. predicted the turbulent economy would contribute to a 5% decline in tourism (to 44.5 million visitors) for the calendar year 2009, and it seems likely the bulk of that downturn will be felt during the traditional travel weeks of summer.

This month, the U.N.’s World Tourism Organization reported international tourism was off 8% between January and February. In the same report, the org said it had begun to see declines attributable to the swine flu outbreak and the fears it has prompted, with the potential for even more on the horizon.

In recent boom times, foreign tourists have generated a growing percentage of Broadway sales, particularly when the weak dollar made Rialto tickets a relative bargain for international travelers. That revenue stream looks poised to dry up somewhat if the tide goes out on foreign visitors.

Meanwhile, the jam-packed lineup of Broadway productions is certain to undergo its usual post-Tony attrition, as shows banking on awards attention finally throw in the towel.

This spring a number of well-reviewed plays, such as “Reasons to Be Pretty,” have struggled to attract aud interest in a crowded field. And even some of the better-selling nontuners, such as “Blithe Spirit,” will wrap up their limited runs in the coming months.

It’s also worth noting that while past summers have seen the early entry of a crowd-pleasing new musical or two in August — such as “Hairspray” in 2002 or, more recently, “Grease” in 2007 — this summer has no such tourist magnet to contribute sales to the Rialto pot and add fresh blood to the lineup. Skedded musical offerings including “Memphis,” “Finian’s Rainbow” and “Bye Bye Birdie” are all holding off their openings until the fall.

Nonetheless, those in the industry, buoyed by the healthy season, see reason to remain upbeat about summer box office.

Conventional wisdom posits the fiscal downturn may actually prod more Americans to buy Broadway tickets than in prior years, with belt-tightening vacationers forgoing, say, a trip to Europe in lieu of the relatively more affordable luxury of a night at the theater. And discount offers are proliferating on all but the top handful of sellout shows.