Dr. Patrick Mahaney says:
Although Alaska is bitterly cold in the winter, the climate can be adequately warm and humid in the summertime to support the flea lifecycle. Temperatures between 70–90 degrees Fahrenheit are needed, along with humidity levels of 50–75 percent for fleas to hatch and thrive. If the temperature and humidity are right, fleas can grow on pets and wildlife in Alaska and other northern regions.
Fleas lay eggs approximately 20 at a time, and a few days to two weeks must pass for the eggs to hatch into larvae. One to two weeks are required for larvae to develop into pupae, and a comparable period must transpire before adult fleas emerge into the environment, altogether about a four-to-six-week period for fleas to develop from eggs into adults. Larval and pupal stages are hearty enough to survive the winter and further develop when weather becomes adequately warm and humid.