MOUNT ST. HELENS NATIONAL MONUMENT, Wash. – Mount St. Helens exhaled a spectacular, roiling cloud of steam and ash yesterday, sprinkling grit on a small town some 25 miles away.

The volcano has been venting steam and small amounts of ash daily since Friday, but yesterday morning’s burst was the largest, producing a billowing, dark-gray cloud that rose thousands of feet above the 8,364-foot-high rim of the crater and streamed to the northeast.

For days, scientists have been warning that the volcano could blow at any moment with enough force to endanger lives and property. But geologists said yesterday a more likely scenario was weeks or months of smaller-scale venting, with the possibility lava could enlarge the dome within the mountain’s gaping crater.

“There’s not necessarily going to be a big one,” said Jake Lowenstern, a U.S. Geological Survey volcanologist.

Scientists have said all along there was little chance of a repeat of the cataclysmic 1980 eruption that killed 57 people and coated much of the Northwest with ash. The town of Randle, with a population of about 2,000, kept students with asthma inside yesterday after getting a light dusting of ash.

Officials of sparsely populated Skamania County were concerned that the ash might harm hunters in the area for elk season. Officials at the Coldwater Ridge Visitors Center, 8 1 /2 miles north of the mountain, told the several dozen people at the center’s parking lot not to drive into the ash if the plume reached them. However, the cloud trailed away to the east.

Ken Marshall drove up from Valley Springs, Calif., hoping to see an eruption.

“It’s almost like clockwork,” he said. “It blows in the morning and then there are earthquakes and rockfall all the rest of the day.”

Scientists said yesterday’s steam burst opened two small new vents in the crater’s floor, and that the floor continued to lift up, a sign that magma was still building beneath the volcano. In the last several days, the lava dome within the crater has swelled by about 150 feet.

Since September 23, thousands of tiny earthquakes have shaken the mountain and several steam eruptions have occurred, the most seismic activity at the peak since the months following the 1980 blast.

Earthquakes trailed off after yesterday’s burst, dropping in both magnitude and frequency, said Bill Steele, spokesman for the University of Washington seismology lab in Seattle.

“We don’t really know what it means at this point as far as a prognosis,” Mr. Steele said. “It could mean the plug (in the volcano’s magma channel) is real up there near the surface right now – that it’s not resisting anymore. We’re watching very closely.”