Hokkaido is a vast region spanning different altitudes, climates and biomes. It is quite hard to say for certain what the weather will be like where you are going, so it is better instead to look at the weather in a specific region in the specific month that your tour will be visiting.
For hikers, we have a blog post the covers weather in Daisetsuzan National Park in detail. For other regions however, there are some general patterns that apply which we have outlined for you here!
Winter
While the mountains are usually covered in snow by November, the first snows usually fall on lowland areas by the start of December. By the start of January, all of Hokkaido is covered in snow. Across all of Hokkaido, temperatures are below zero and may fall as low as -20C or lower in some areas. Snowfall blankets most regions, thicker on the mountains and in central and northern regions where the air is drier. There is less heavy snow on the coastlines, but increased humidity can make these regions feel quite a bit more chilly than inland. Winter in Hokkaido is generally quite cloudy and overcast-- despite what you may have seen in promotional images, sunny days are something of a rarity.
Spring
The thaw tends to come between early March and mid-April, depending on the region. Although snow still falls, it no longer settles on the ground except for in areas of high altitude (it is possible to enjoy winter sports on the mountains even in March!) By May, most of the snow has disappeared from low-lying areas, leaves begin to bud and flowers start to bloom. The weather is mostly clear and pleasantly cool during the day, perfect for cycling and walking in lowland areas. However, snow is still piled up thickly on the mountain trails and nights can still dip below zero.
Summer
June marks the start of summer on the coast and low-lying areas, where bright green forests burst into life. Wildlife activity is at a peak from June through to July, with whales visiting the east coast and brown bears and their cubs roaming the forests. However, at high altitude, June is still considered spring! On smaller mountains, the snows recede from the trails and are gone by mid-June. At higher altitudes, notably the Daisetsuzan National Park, the snows remain on the trails right into July, finally receding from around the second half of the month. Alpine flowers begin to bloom from around late June and hit their peak in July. By August, the snows have completely gone from all of Hokkaido's mountains and the hiking season hits its peak during the first half of the month. Hokkaido is not as hot as other parts of Japan, but the central and southern areas in recent years have seen days above 30C. Coastal areas, the north and the east however tend to hover around 26C - 28C. Of course, temperatures are much colder at altitude-- below 10C in some cases, such as Mt. Asahidake. This is why we ask our clients to bring warm layers, gloves and hats for their hike, even in summer!
Autumn
At high altitudes, summer is short and by late August, there are signs of autumn appearing in the mountains. Early autumn flowers herald the start of the changing seasons. By early September, the first signs of autumn foliage can be seen on the peaks of Daisetsuzan National Park. On the other hand, at ground level, the heat of summer has abated and the mornings and evenings return to being pleasantly cool, making for great outdoor activity conditions. Late September sees the peak autumn foliage at Daisetsuzan National Park, the first autumn foliage in Japan, which then sweeps down the mountains in a grand circle, enveloping the rest of Hokkaido as it goes. By mid-October, low-lying areas are also enjoying autumn foliage and the snows begin to crest Hokkaido's mountains. By the end of November, the mountains are covered in snow and the autumn foliage has finished, leaving a landscape waiting for the magical winter. Autumn weather is pretty changeable in Hokkaido, with sudden showers all over the region. By November, the clouds characteristic of winter tend to become more and more common until, one day, the precipitation falls not as rain but as snow. And so, the cycle begins again!