ANCHORAGE - Augustine Volcano erupted early Wednesday, sending an ash plume 28,000 feet into the Alaska sky.
A pair of explosions around 5 a.m. indicated the volcano had erupted, said geologist Jennifer Adleman of the Alaska Volcano Observatory. The observatory warned that more eruptions could follow.
The ash plume advanced north but remained west of Anchorage.
A flight advisory was issued for pilots for an area 20 miles east and west of the volcano and about 50 miles north.
"Fortunately, it's not going to Anchorage this time," said Bob Hopkins, meteorologist in charge in the Anchorage National Weather Service forecast office.
The Federal Aviation Administration issued temporary flight restrictions for a 5-mile radius around the volcano and for 50,000 feet above it, said spokesman Mike Fergus. Cargo or passenger traffic from Asia would usually fly through the area to Anchorage but could be easily rerouted, he said.
"It's not posing any significant traffic problems," Fergus said.
The volcano is on an uninhabited island about 180 miles southwest of Anchorage and across Cook Inlet from the Kenai Peninsula. The volcano is about 75 miles southwest of Homer.
The observatory made an observation flight to get more information about the types of gases emitted from the volcano.
Mud flows streaming down the mountain's east side had been spotted, said Stephanie Prejean of the U.S. Geological Survey. Thermal imaging could detect whether a dome is building at the summit, she said.
"It doesn't look like it's ending, even though it's relatively quiet right now," she said.
The 4,134-foot volcano last erupted in 1986. Ash from a 7-mile-high column drifted over Anchorage and kept flights out of the skies over Cook Inlet.
If the volcano follows a pattern similar to 1976 and 1986 eruptions, the observatory said, seismic activity would increase before similar or larger explosive events. But it's possible that an explosive eruption could occur with little or no warning, the observatory said.
Anchorage received .12 inches of ash in a 1976 eruption and less than .04 inches in 1986. About .2 inches fell on Homer in both eruptions.
The explosions, with magnitude 2.6 seismic activity, occurred at 4:44 and 5:13 a.m. Alaska Standard Time.
About eight hours earlier, at 9:05 p.m., the observatory had upgraded the level of concern from yellow, or restless, to orange, meaning an eruption could occur at any time. For the previous six hours, the observatory had detected "markedly" increased earthquake activity.
The eruption changed the level of concern to red.
The observatory has a Web cam stationed at the mountain. Cloudy weather prevented clear images of the volcano.
The ash cloud appeared to have low concentrations of ash, Hopkins said.
An ash fall advisory covering the west side of Lower Cook Inlet remained in effect until noon. The few residents in the area were warned to reduce outdoor activity, keep windows and doors closed, avoid outdoor exercise. They also were warned not to burn wood in stoves or fireplaces if the wood had volcanic ash on it.
Vessel operators were advised to remain inside their boats and not approach the volcano.
Bob Painter, Homer fire chief and director of emergency services, said no ash had been detected in Homer. The Kenai Peninsula Borough office of emergency services spoke to school bus drivers and Alaska State Troopers across the borough, he said.
"None have reported any ash fall," he said.
City and borough officials were reviewing emergency plans and preparation. They set up an emergency management center at the Homer fire station.
"That's going to be staffed more or less full time unless things quiet down or there's an increase in activity, further eruptions," Painter said.
One event he hoped for was continued wind from the south, away from Homer
"We're going to keep the wind blowing in the other direction," he said.
The Alaska Volcano Observatory has been closely monitoring Augustine since last spring when it detected more earthquakes under the mountain. Rates increased from one to two per day in early May to three to four per day in October and up to 15 per day in mid-December.
Using the global positioning system, the U.S. Geological Survey detected a small uplift of the volcano.
In December, small steam explosions were spotted and recorded by seismic instruments
In response, the observatory installed additional seismometers, GPS receivers, an infrasound sensor, and time lapse cameras on the flanks of the volcano and added the Web-based camera system.
Scientists in Homer plan to study thermal imaging of Augustine on Thursday for any signs of a magma dome building at the summit. Homer is a Kenai Peninsula community about 75 miles northeast of the volcano.
If weather is good and seismicity is low, scientists may venture near Augustine's base to retrieve ash collection buckets.