I was looking at this yesterday and learned the following. A 100 year flood is a flood that has a 1% chance of happening in a given year. A 500 year flood has a 0.2% chance of happening in a given year. I’m not sure if temperature has the same model as rainfall. Anchorage was settled in 1914 according to Wikipedia, so I’m assuming that there are no temperature records beyond that and that 100 years in this case refers to it only having been settled for about 100 years.