A quick summary to begin with
1 The recommended season is mid-May to mid-September, and most tourists come between July and August.
2 Those of you who intend to go on long treks, keep in mind that many mountain passes are covered in snow until about mid-June.
3 The first half of September is certainly a possible (even good) time to come to Alaska. The parks are still open, the colors of fall are at their most beautiful, tourist traffic decreases and some hotels offer discount prices.
May – June
Alaska: relatively dry weather with many sunny days, but there’s still snow in high altitudes. In mid-May the cruises start coming in, most hotels open for business and the parks start opening to the public. In areas with large amounts of snow, it’s likely that not all services will be available before the beginning of June (in Denali, for example, the road into the park is usually fully opened only on the first week of June). Relative disadvantage: still no salmon in the rivers. At this period they’re still at sea, packing their bags for the journey upstream to their spawning areas.
Canadian Rockies: the weather improves as summer approaches, but these are relatively high altitudes and there’s still a high chance of snow. Some roads will probably still be blocked by snow that hasn’t thawed, even in the end of June (mainly side roads, like Hwy 40 in the Kananaskis area or the road that goes up Mount Edith Cavell).
July – August
The peak of the tourism season, flower dot in the mountains and mostly-friendly weather. Don’t forget that there’s still a good chance of rain – usually light showers that only add to the atmosphere (English rain). In July, the average temperature in Southcentral Alaska (Anchorage area) is 50° F to 64.5° F (10° C to 18° C). Alaska’s interior (Denali, etc.) is hotter and drier, and along the coast it’s a bit colder and rainy. During July the salmon start making their way upstream, bringing the bears with them.
The Canadian Rockies are slightly warmer than Alaska (for example, a July average of 71° F or 22° C in Banff), but there will certainly be rainy days and sometimes even light snow.
In Alaska, the weather in September is less predictable, and can easily develop into weeks of rain or long periods of sunshine. In early September the amount of tourists drops, many tourism businesses (hotels, travel companies, etc.) start to shut down, and the prices in some places decrease. In most areas the season ends in mid-September.
In the Canadian Rockies the season lasts until the end of the month. September weather is usually cool, dry and convenient.
October – April
The first snow usually shows up during October or November, and between October and April most of Alaska and West Canada are covered in snow. There is some winter tourism in Alaska, but it’s not very developed and mainly focuses on specific activities such as dog sledding, snow biking and extreme sports (ice climbing and types of skiing). Winter tourism in Canada is focused on skiing.
The longest day, as in the rest of the Northern Hemisphere, is June 21st (In Alaska we’re talking roughly 18 hours of daylight, and the hours of darkness are more dusk than actual darkness). Daylight hours decrease over the summer, and in September it turns dark at around 09:00-10:00 p.m.