The rainy season is coming

By Rick Thoman
Alaska Climate Specialist
Alaska Center for Climate Assessment and Preparedness
International Arctic Research Center/University of Alaska Fairbanks

With the calendar turning towards August, the western Alaska “rainy season” is underway.
Like most of mainland Alaska, late summer and early autumn is often the rainiest time of year both in terms of the frequency of rain and the amount of rain.
Physically this is related to the annual climate cycle in the northern hemisphere.
The Bering Sea and North Pacific Ocean temperatures reach the annual maximum in August in the north and early September south of the Pribilof Islands, and with that the maximum potential for evaporation into the atmosphere.
Additionally, in many years storm frequency starts to increase in the Bering and Chukchi Seas. With the solar heating decreasing, the highest latitudes start to cool by the end of July, followed quickly by Asian and North American Arctic lands. This temperature difference is the fuel that feeds storms that impact the Bering Strait region.
This week's Climate Watch graphic shows the daily percent of days over the past 50 years with more than 0.10 inches of precipitation at Nome airport. The 0.10 inch criteria is useful because this is enough rain to form significant sized puddles and Alaska Fire Service calls this much (or more) rain a “wetting rain” because it's enough to really dampen wildfire fuels.
Over the past 50 years, the frequency of days with a “wetting rain” triples between early June and early August. Reflecting Nome’s coastal location, the occurrence of days this wet stays elevated into early October before dropping off.
This contrasts sharply with Interior Alaska, where the rainy season starts about the same time, but ends a month earlier.
In Southcentral Alaska, the rainy season starts about a month later than in Nome, and in much of Southeast Alaska the rainiest month of the year is typically October.