They spent the summer here in the shadow of Mount St. Helens totaling up the damage from the volcanic eruption May 18, the one that left 31 persons dead and 33 missing. And they found, to their surprise, that the financial toll was not nearly as great as they had feared.

The economists round that reports of the death of the Pacific Northwest had been greatly exaggerated, that actual losses were nowhere near the $2.7 billion figure bandied about early on. Losses would, they determined, be even less then the $951 million Congress has set aside to repair the damage.

But it the immediate past does not look so bleak, the longterm future does not look so bright.

It may turn out that the financial security of northeast Oregon and much of Washington was damaged not so much by the May 18 disaster as by the smaller eruptions that followed on May 25, June 12, July 22, Aug. 7 -- and by the ones yet to come.

The psychological toll may prove greater than the economic one, though the more subtle form of damage will show up as dollars soon enough.

"The short-term damage has been greatly overstated," said Cleve Anschell, an economist for Rainier Bank, Washington's largest, who has given the matter intensive study. "The long-term effect is something we have to be more concerned about."

There has been no mass exodus from the region, although Doug Gehrke, 60, who drives a daily paper route of 137 miles near the volcano, reports that 7 percent of his customers have moved out since the big blast.

The only people to leave the area in substantial numbers were those whose jobs were blown away, such as loggers, or who suffer from respiratory difficulties or hear conditions and who thus are wary of ash and trauma.

But the repeated eruptions -- and the consensus among geologists that they may continue in their upredictable persistence for decades -- have planted a seed of fear and doubt in the minds of thousands here.

Local residents know that Mount St. Helens has no chance of killing them unless they live in its immediate vicinity or stray too close at the worst possible moment. They know that the volcano-related floods, which seem inevitable after this winter, can strike them only if they live in river valleys now hopelessly clogged with volcanic debris.

They know, for the most part, that an eruption means only a dusting of ash, ash that travels wherever the wind takes it, generally east or northeast. In small quantities, they know, the ash is little more than an inconvenience that gets tracked into living rooms, leaves a gray film on automobiles, soils the leaves and blurs the highways in clouds of dust whenever the wind blows.

They know that on most days in most parts of the Pacific Northwest life is perfectly normal, that Mount St. Helens is, in the words of one regional economist, "an attractive wart." In fact, the region's prime metropolitan area, Seattle-Tacoma, 100 miles northwest of the volcano, has yet to see its first gram of ash.

But the memory persists of an afternoon in May when the sky turned darker than any moonless night, and no one knew what was happening or how long it would last. That memory, combined with repeated ashfalls, will likely become intolerable to some people as the volcano keeps puffing away.

And those among them who have the financial and social freedom to leave will do so.

"We find that people who coped with it very well in May are depressed in August," said Jim Lewis of the Central Washington Comprehensive Mental Health Center in ash-exposed Yakima, 85 miles east of the mountain.

"We're starting to see a delayed stress reaction like that experienced by Vietnam veterans. It's definitely there. There are kids who don't want to leave their homes for fear they'll get stranded by an ashfall. The little eruptions don't help. We've had floods, snows, drought. But they come and go."

All over the area, people talk that way, as if the months and years ahead are experiments to be tried a day, a week at a time. And there is little doubt that another major eruption would be enough to make many of them judge the experiment a failure.

"You've got your home here so you wait a while and see what happens," said Phyllis Durwell, who works in the town clerk's office in the flood-threatened community of Castle Rock.

"I heard a bang last night and came flying out of bed," said her colleague, Donna Lampshire. "I was sure it was the volcano."

"It's changed everything," added Alice Raphaelson. "People are different, maybe more short-tempered, less patient, more edgy. I know the kids are all changed. They're not very happy-go-lucky at all."

More important still may be the reaction of people from outside the area who, out of fear of the volcano and a failure to understand how little danger it really poses to most sections of the Northwest, will simply write off this part of the country from their futures.

"People aren't talking about leaving around here," and Michael Avey, a farmer who grows wheat and corn on 580 acres near Ephrata, Wash., 150 miles northeast of the mountain. "But no one who looks at all the ash we've got would think about moving in."

"The longterm effect, whatever it is, will be on in-migration," agrees Chuck Sauvie, an economist with Portland (Ore.) General Electric. "The irony is that the Northwest has some people, more than its share who think growth is bad and have been trying to discourage people from moving here for years. It'll depend on what businesses do. People usually follow jobs." f

If that is true, the initial signs are not good. In mid-June, the National Semiconductor Corp., a huge California-based electronics firm, canceled plans to build an $80 million plant in Vancouver that would have employed up to 2,000 workers.

"We were close to breaking ground," said a company spokesman. "Then came the third eruption of June 12 [the second to dump ash on the construction site.] We thought, in the wake of that, there would be some problem attracting people to the area.

"We made a hard, cold business decision that wasn't based on anything other than the mountain. We have retained the option on the land. If things stabilize. . . . "

The company announced a few weeks ago that it would put its new plant in Arlington, Tex.

It is too early to say whether this is an isolated case or the shape of things to come, although other electronics firms are reportedly reconsidering expansion plans in the area.

But the anedotal evidence suggests the dept of the Northwest's image problem. Listen to Dwight Crandell of the U.S. Geological Survey, a Denver-based scientist whose 1978 book predicting the behavior of Mount St. Helens reads a lot like news accounts of what happened in 1980.

"People call me from Minnesota or Illinois or whatever," he said, "and say they're thinking of moving out to the Portland area. They ask me how long I expect the volcano to act up. I tell them they won't be in any danger from it, but that I expect it to continue for a long time. I usually get the feeling they've persuaded themselves not to come."