For many of the major population centers of Japan, the weather in January is cool and crisp, with little chance of the ground being snow covered.

Sapporo, located on the northern island of Hokkaido, is an exception to this general trend. There, winters are long and cold and snow covers the ground for much of the winter season.

Tokyo and Osaka both have daytime temperatures that hover around 10 C and sunny days abound.

The west coast of Japan tends to be much wetter in January by contrast because of the dominant winds that come from the sea of Japan during that season. Humid air travels eastwards, hits the mountain range and precipitates on the west coast. Hence, there is plenty of snow in the Japanese Alps (e.g. Nagano) during that time of year.

The southern main island of Kyushu has a climate that is similar to the one of Tokyo, despite being located hundreds of kilometer to the south of the capital.

Okinawa, the southernmost prefecture of the country, is the only area of Japan that enjoys a relatively warm weather in January with a mean daytime temperature of around 20 C.

In a nutshell, for most populated parts of Japan, the weather in January is cool with a good dose of sunshine; Hokkaido is cold; the western part is cool and wet; the mountainous parts are snowy; Okinawa is warmish.

As a reference, here is the average high temperature in January for some major cities in Japan from north to south.

Sapporo, Hokkaido: -0.6 C (30.9 F)
Sendai, northern Honshu: 5.3 C (41.5 F)
Tokyo, central Honshu: 9.6 C (49.3 F)
Osaka, southern Honshu: 9.5 C (66.2 F)
Fukuoka, Kyushu: 9.9 C (49.8 F)
Naha, Okinawa: 19.5 C (67.1 F)