This story has been updated. Find the latest updates here.
UPDATE 6:15 a.m. Tuesday:
Volcanologists are reducing alert levels associated with the Sunday and Monday eruption of Pavlof Volcano on the Alaska Peninsula, saying satellite imagery shows it is no longer emitting a continuous ash column.
According to a statement from the Alaska Volcano Observatory on Monday evening, the volcano's aviation color code has been lowered to orange and its alert level lowered to "watch," as a decline that began at about 12:30 p.m. Monday continued.
"Seismicity and infrasound signals from Pavlof have dropped to low levels, and it appears that the robust eruptive activity that began yesterday afternoon has declined for now," AVO staff wrote. "The level of seismic tremor is still slightly above background. A drifting ash cloud extending from the southern Bering Sea into Interior Alaska is still present and may pose a hazard to air travel."
Alaska Airlines canceled 41 flights Monday, with ash from the eruption reaching an altitude of 37,000 feet and drifting hundreds of miles northeast toward Alaska's Interior.
Jarod Urban, at the operations office for Ted Stevens Anchorage International Airport, said the airport hadn't received any reports of flight diversions or cancellations Tuesday.
But some travelers were reporting on Facebook that their flights had been canceled early Tuesday. Check with your carrier before going to the airport.
Pavlof Volcano, on the remote Alaska Peninsula 600 miles southwest of Anchorage, was sending up a huge column of ash Monday that covered the tiny village of Nelson Lagoon with a blanket of black grit and canceled dozens of flights to and from communities across the state.
The Pavlof eruption began at 4 p.m. Sunday and generated seismic tremors at very high levels, lightning and a 400-mile-long ash plume to the northeast rising 37,000 feet, according to a Monday morning update on the Alaska Volcano Observatory's website.
The level of volcanic activity associated with the eruption is "quite a bit more energetic" than researchers have seen since 1996, said Dave Schneider, a geophysicist with the observatory, which is operated by the U.S. Geological Survey in Anchorage. Mariners, pilots and residents of Cold Bay -- 37 miles southwest of Pavlof -- reported lava fountaining from the crater at heights up to about a half mile, Schneider said. The heat of the lava was expected to trigger mud flows into remote river valleys.
Pavlof is considered one of the most consistently active volcanoes in the Aleutian arc, with more than 40 eruptions on record.
On Sunday, the Alaska Volcano Observatory issued a "red" warning for aviation and raised the volcano alert level from normal to warning status. The National Weather Service issued warnings for aviators to avoid the plume, which stretched over 400 miles by Monday afternoon.
With ash hovering over Alaska's Interior, air traffic was being interrupted by the eruption Monday. Alaska Airlines alone had canceled 41 flights as of 3:45 p.m., including all flights traveling to and from the Interior city of Fairbanks, due to what it described as a "massive ash cloud" traveling north at 75 mph.
"Flights are suspended to Barrow, Bethel, Fairbanks, Kotzebue, Nome and Prudhoe Bay until Alaska (Airlines) is able to assess weather reports after daylight March 29, 2016," the airline said in a blog post. "If weather conditions improve, Alaska will resume its regularly scheduled 54 flights to the affected six cities tomorrow."
Approximately 3,300 passengers were affected by the cancelations, the airline said. Passengers with reservations to any of the cities listed were asked to call 800-252-7522 to check their flight status before heading to the airport.
PenAir, which provides service to numerous Southwest Alaska communities, had also canceled most of its flights Monday, affecting several hundred passengers.
Nelson Lagoon, home to about 40 people, was in the path of the ash Sunday and Monday. Village residents are accustomed to the volcano's fairly regular eruptions but this one is worse than usual, according to Village Public Safety Officer Cpl. Barrett Taylor.
"Residents could feel the ash on their face and smell it in the air yesterday evening. Then around 1:30 this morning it looked like it was basically raining ash," Taylor said by phone Monday. "Village residents told me this morning this is the worst they've seen the ash."
A layer of black covered vehicles and health aides were giving out face masks to elder residents, he said. People were being told to stay inside until the eruption subsided.
Residents in Cold Bay took photos of the "lava show" as fiery magma erupted in fountains through the night Sunday and into Monday, said Candace Schaack, who serves as mayor of the community, which about 75 people call home year-round.
Schaack said the eruption was initially obscured by clouds, but the cover cleared Sunday afternoon, leaving the ash plume visible.
"It was a little alarming to see how massive it was," she said.
By late afternoon Monday, the Alaska Volcano Observatory noted that Pavlof had settled down, at least temporarily. The intensity of the volcanic tremor had "greatly decreased," said geophysicist Schneider. Satellite and webcams also detected reduced ash production.
Projections of the ash plume's path by the Alaska Aviation Weather Unit shows the ash arching northeast over the Alaska-Canada border. Forecasters' projections indicate the affected area will largely stay the same until at least 1 a.m. Tuesday.
The concentration of the ash is difficult to gauge as it varies throughout its path, said weather service lead forecaster Bob Clay. The farther out the ash the more dispersed it becomes, but levels as of Monday evening were still hazardous to aircraft, he said.
"We're watching it," he said. "Pavlof ... waxes and wanes. Right now it's down at a lower level."
Typically, ash remains airborne and detectable by satellites for a day or two post-eruption before dispersing, Schneider said. That's important for aviation in Alaska because the National Weather Services relies on satellite detection to determine ash-related warnings as are in effect now.
It's up to airlines to make flight determinations, he said.
Pavlof is described by the Alaska Volcano Observatory as a snow- and ice-covered stratovolcano about 4.4 miles in diameter, with active vents on the north and east sides close to the summit. Eruptions generally involve sporadic lava fountains for several months and massive ash plumes as high as 49,000 feet.
Alaska Dispatch Publishing