How heavy is Hokkaido's snow in Mid-December?
I will be there during that time.
It'll very much depend where on the island of Hokkaido you find yourself.
Snowfall varies widely from as much as 11 metres (400 in) on the mountains adjacent to the Sea of Japan down to around 1.8 metres (71 in) on the Pacific coast. The island tends to see isolated snowstorms that develop long-lasting snowbanks, in contrast to the constant flurries seen in the Hokuriku region. Total precipitation varies from 1,600 millimetres (63 in) on the mountains of the Sea of Japan coast to around 800 millimetres (31 in) (the lowest in Japan) on the Sea of Okhotsk coast and interior lowlands and up to around 1,100 millimetres (43 in) on the Pacific side.
Sapporo, for instance, will average 132cm (52 inches) of snow during December.