A rare severe thunderstorm hit Juneau, Alaska, on Monday
Alaska’s capital city averages a single thunderstorm every two years or so. Juneau is home to about 32,000 people and is known for its winter weather - less so for any thunder or lightning. But the city found itself under a rare severe thunderstorm warning, as the storm hit with strong winds gusting up to 60 mph.
The National Weather Service in Juneau issued a severe thunderstorm for the city and bureau of Juneau at 3:26 p.m. Alaska time Monday - the first severe thunderstorm warning ever issued in Juneau. It’s also only the second ever posted by that office. The first was issued for the Misty Fjords area of Alaska and the Southern Inner Channels on June 27, 2019.
The agency warned that Monday’s thunderstorm was located “over Douglas Boat Harbor, moving northwest at 30 mph,” and cautioned of the potential for “60 mph wind gusts and penny size hail.”
At 3:10 p.m. Monday, a wind gust of 60 mph was clocked atop the Juneau Federal Building in downtown Juneau. The nearby Coast Guard station also logged a 47-mph gust.
The Weather Service defines a severe thunderstorm as one that produces winds gusting to at least 50 knots (57.5 mph). That makes Juneau’s storm a bona fide severe thunderstorm.
Trees were reportedly downed near Lemon Creek in the Mendenhall Valley area, with reports of a canopy being blown down a road in the Twin Lakes area. A rural weather station east of Hoonah on Rocky Island measured a wind gust of 47 mph.
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How Monday’s storm was detected
While virtually every Weather Service severe thunderstorm or tornado warning is issued based on radar aberrations, Monday’s was not. That’s because radar can’t “see” the skies over Juneau. The Weather Service’s Doppler radar is on Biorka Island, about 100 miles to the south-southwest. Mountains block the radar beam from probing the air over Juneau, leaving a blind spot.
Instead, the storm was satellite-indicated, meaning meteorologists had to rely on images from a weather satellite about 22,000 miles above the ground. That’s how they determined which way the thunderstorm was drifting.
“We used satellite detections of lightning and the National Lightning Detection Network,” said Brian Bezenek, lead meteorologist at the Juneau forecast office. “That and cloud top temperatures on satellite.”
Each summer features a few thunderstorms over the Alaska panhandle and British Columbia, but most are weak.
“We do get some thunderstorms, but more of lower-level threat ones,” Bezenek said. “We’ll do special marine warnings for boaters in the channels for gusts of 40 to 50 mph.”
Low pressure had been present to the southeast of Juneau over the northeast Pacific. Bezenek said storms were lifting northeast along a pocket of spin. Air may also have been forced up the mountainous terrain, enhancing upward motion.
“Everything aligned,” he said.