A heat wave unprecedented in the past 73 years has toppled weather records across Alaska, particularly in Southeast Alaska.

On Friday, the National Weather Service thermometer at Juneau International Airport hit 54 degrees, tying the highest temperature ever recorded in December there.

The airport is the city’s official measuring point, and according to records kept by the Weather Service since 1936, three of the 10 warmest December days in Juneau’s history have come in the past week. The city has set four daily high-temperature records.

It was warm elsewhere in the city, too. On Thursd...