From KGW.com (subscription only):

MOUNT ST. HELENS, Wash. -- A 25,000-foot plume of steam and presumably ash emerging from Mount St. Helens was visible for miles on Monday evening, visible from downtown Portland and elsewhere around northwest Oregon and southwest Washington.

The emission about 5:28 p.m. was the largest one to occur at the volcanic mountain in months. It followed increased earthquake acitivity that was reported about 2:30 p.m. and continued until about 5:30 p.m., when the seismology equipment stopped generating measurements. ...

The volcano reawakened last September with several eruptions and emissions occuring, although it quieted in more recent times. Earlier this month, scientists said they felt the volcano was in a "fairly stable" period of activity.

Below the fold: full text of updated KGW.com article.

MOUNT ST. HELENS, Wash. – Mt. St. Helens, relatively quiet for the last several months since awakening last fall, jettisoned a 30,000 foot plume of ash and steam that was visible in the Tuesday twilight of downtown Portland and Vancouver.

While the U.S. Geological Survey said the eruption was minor, Mt. St. Helens’ latest show awed Portland area, southwest Washington and Columbia Gorge onlookers, who saw a stunning view of the restless mountain, thanks to a clear, spring-like day.

"It was the most amazing ash plume -- by far the biggest I've seen since this activity started last September," said Stephanie Burhop, who witnessed the eruption from her home in nearby Cougar, Wash. It is the closest community to the mountain.

The grey and white plume, mostly drifting to the northeast, was expected to drop ash over some areas of southwest Washington, prompting the National Weather Service to issue an ash advisory for the reigion until 9 p.m. on Tuesday and for the east slopes of the Washington Cascades until 10 p.m.

The heaviest amounts were expected to fall in Skamania County, while Cowlitz County and Clark County could get a dusting of ash. None will fall in Portland or Vancouver, said KGW meteorologist Dave Salesky.

"There's no change currently on the alert status on the mountain," said Rob Harper of the Washington Department of Emergency Management. He said no communities had yet requested any state assistance.

Flights to and from Portland International Airport weren't affected by the eruption, said Steve Johnson, a spokesman for the Port of Portland.

Geologists with the USGS said they were caught off-guard by the eruption, which began about 5:25 p.m. Scientists noticed quake activity ramping up hours before the eruption of steam and ash but didn’t think it was unusual, said Dan Dzurisin of the USGS.

The plume was accompanied by an earthquake of about 2.0 magnitude, said the University of Washington seismology laboratory. Scientists lost radio contact with three of their instruments at the volcano, probably because of ash, Dzurisin said.

The event knocked out some of the UW’s instruments in the crater, but there's no indication that it caused damage outside the crater, the lab said.

"It didn't really look like anything large like this was going to happen until it actually occurred," said Corlyn Dreidger of the USGS.

The USGS does not believe this eruption was caused by part of the new lava dome collapsing. A new dome has been rising inside the crater beside the old dome that was built up since the 1980's.

"This eruption happened with some force," Dreidger said. "...This one's different."

Lava also isn’t flowing on to the lava dome or crater, Dzurisin said.

Although the mountain appeared to have entered a stage of "placid dome building," geologists have warned that things could change quickly.

"We said this could change any time," Dreidger said. "St. Helens has proved again she has lots of surprises."

The volcano in southwest Washington rumbled to life again last fall, pumping out lava and creating a huge new lava dome as the mountain nears the 25th anniversary of the May 18, 1980 eruption that killed 57 people.