Last month was Alaska's warmest May in 91 years of recordkeeping, the National Oceanic and Atmospheric Administration said in a monthly report released Wednesday.
The average statewide temperature posted in May was 44.9 degrees, 7.1 degrees above the long-term average for the month, said the report, issued by NOAA's National Centers for Environmental Information.
Alaska's previous warmest May was in 2005, when the statewide temperature averaged 43.7 degrees, according to NOAA.
Both daily highs and daily lows ran well above normal, setting their own records, the center said. The average high temperature was 8.1 degrees above normal, and the average minimum temperature was 6.1 degrees above normal. The month was also relatively dry, the 16th driest May on record, the NOAA report said.
The warm trend held for just about entire state, from the Arctic to the rainforest of Southeast Alaska.
"The most unusual thing about May, for me, was it was the warmest of record over a huge area," said Rick Thoman, climate science and services manager for the National Weather Service in Alaska.
Alaska is so large that there are usually marked regional differences in weather patterns, but that was not the case this year, he said. Records were set around the state, from Kotzebue to Juneau, he said.
The warm May "is part of the trend that we've had over the last two years," Thoman said.
Much of the cause, he said, is the large and persistent pattern of very warm sea-surface temperatures in the North Pacific, a phenomenon that has become known as "The Blob," a name bestowed by University of Washington climate scientist Nicholas Bond.
Another factor, Thoman said, is the continued persistence in atmospheric patterns, with high-pressure systems getting stuck over western North America and low-pressure systems getting stuck over the eastern part of the continent, he said. In winter, high-pressure systems over Alaska would result in cold temperatures, but summer's extended sunlight means such systems result in warmth.
Thanks to the warm ocean, the parked atmospheric systems and other factors, the rest of Alaska's summer is expected to be warm as well, Thoman said. NOAA's long-term forecast "tilts the odds toward significant warming across the entire state," Thoman said.
But regional differences are expected.
The most dramatic warming in early summer is expected in to the southern coastal regions, which are most affected by the high sea-surface temperatures, he said. Later in the season, the lack of sea ice "will up the chances" for warming on the North Slope, he said.
Arctic sea ice extent has been running low all year, according to the National Snow and Ice Data Center. The winter maximum, hit in late February, was the lowest annual maximum since satellite records began in 1979, and the melt season started early. May's ice extent was the third lowest for any May on satellite record, the center said last week.
An important caveat: Summer weather is harder to predict than winter weather, Thoman said. The temperature difference between low and high latitudes is reduced in summer, lessening the likelihood of big storms and increasing the effects of small-scale weather systems, he said. Last year provided an example of that unpredictable variability, he said.