Many Petersburg residents were jolted awake last night by a loud roar that by some accounts lasted more than 20 seconds. Theories swirled on social media – was it a landslide? A plane crash? A meteor like the one that fell in Duncan Canal nearly a decade ago?
Grant Smith is with the National Weather Service in Juneau. He said the sound was just thunder. Satellites from the National Oceanic and Atmospheric Administration detected two lightning strikes near the north end of Mitkof Island at around 11:05 p.m. He said the single clap of thunder was so long and loud because of the cold weather.
“The air molecules are a lot more dense because it’s colder, and so it can carry that energy farther and you just hear it for longer,” he said.
Smith said thunderstorms are uncommon in Southeast Alaska because of the topography. The storms happen when warmer, wetter air rises into cold air. As an air mass flows off the ocean into the panhandle, it hits the mountain ranges and spikes upward. That vertical motion would normally cause a thunderstorm, but as soon as the air mass moves away from the mountains, it loses energy. There’s not enough energy built up to create the thunder and lightning so the area usually just gets rain.
“Last night, and occasionally in the fall, that’s when we can get enough energy in the atmosphere that when that vertical motion happens,” he said. “You get that energy built up, and you can get lightning strikes.”
Smith said there was only enough energy built up last night to produce the one clap of thunder – or what sounded like one clap – with two lightning strikes close together he said the thunder may have merged into one long sound.
Smith said thunderstorms are more common in the fall in Southeast because that’s when cold air from Canada meets warm air from the Gulf of Alaska.
“It’s just a battle of the air masses – the warmer air mass of the Gulf versus a colder air mass of Canada,” he said. “And we’re just sandwiched in between, and so pressure differences can cause air to flow back and forth, it’s extreme during the fall season enough that we can get that that thunderstorm activity.”
Smith said there is some potential for lightning and thunder later this afternoon and into evening for parts of the panhandle. He said the low in the Gulf of Alaska that is causing rain showers is weakening, so the chance for lightning and thunder will go down starting tonight.
