Hail, no? Thunderstorms... in Juneau?

A thunderstorm viewed from north Douglas Island, looking east towards Juneau on June 17, 2013.

While the Great Plain states are gearing up for prime storm chasing season, Juneau skies remain quiet. While thunderstorms aren’t completely unheard of in southeast Alaska, they aren’t a common occurrence either. A case study on thunderstorm climatology performed by the National Weather Service office in Juneau found that between 1970 and 2011, Juneau observed 16 thunderstorms (or averaging about one thunderstorm every two years).

In order to get thunderstorms, you need three key ingredients: a source of moisture, lift, and instability. Juneau is a maritime climate kept cool during the summer by the Gulf of Alaska waters and largely lacking the instability to create large updrafts that would build cumulonimbus. One important attribute of this reason is latitude. At 58 degrees north, the air is thinner than near the equator. Thunderstorms in the tropics can easily surpass 50,000 feet, but thunderstorms at 15,000-20,000 feet simply cannot be supported. In addition, the town is located next to the Juneau Icefield- the 5th largest glacier field in the world that acts as a “refrigerator” to cap thunderstorm development.

When Juneau does get thunderstorms, however, it is mainly the result of strong daytime heating during the summer months. By the summer solstice, Juneau peaks at just under 18 ½ hours of daylight.  Combined with moist air from Canada, this could be enough convection to lead to the formation of thunderstorms. Otherwise, thunderstorms may develop inland over British Columbia and the Yukon with cloud bases high enough to clear the Coast Mountain range.  

Though thunderstorms are rare in Juneau, they are much more common along the outer coast of Southeast Alaska and the Interior, north of the Alaska Range. Late autumn and winter is a typical time of year for thunderstorms in the Gulf, with the warm ocean as the main heat source interacting with the cooler land onshore. Regardless, thunderstorm wind gusts and lightning are still capable of causing hazards with regard to marine vessels, power outages, as well as the potential risk of forest fires.

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©2018 Meteorologist Sharon Sullivan