The statement "Deemed impossible in northern Alaska only 30 years ago, thunderstorms eventually appeared, likely due to rising temperatures" appears to be an assumption based on a few personal experiences of researchers, most of whom, I would guess, have not been there long enough to draw any conclusion based on personal observation. I've never heard anyone in the meteorological community doubt that thunderstorms have always been a reality in the Brooks Range, and occasionally drift out of the mountains and onto the plains. And the assumed reason mentioned for this assumed appearance of thunderstorms certainly is weakened by the more data-driven statement later in the article, "Using about 25 years of readings, it’s hard to detect a statistically significant air temperature trend in the area immediately around Toolik Field Station."