Hello!
Japan Meteorological Agency
http://www.jma.go.jp/jma/indexe.html
January
Sunny and dry. Only in northern Japan and along the Sea of Japan coast, there is lots of snowfall, and conditions are good for winter sports. Winter are the relatively short days (sunset is around 5pm in Tokyo).
City Max. Temp Min. Temp. Rainy Days Rainfall Sunshine
Sapporo -1 C (30F) -8 C (18F) 18 d 111 mm 97 h
Tokyo 10 C (50F) 2 C (34F) 5 d 49 mm 181 h
Osaka 9 C (49F) 3 C (37F) 5 d 44 mm 142 h
Fukuoka 10 C (50F) 3 C (38F) 9 d 72 mm 100 h
Naha 19 C (66F) 14 C (58F) 11 d 115 mm 95 h
February
Same as January.
City Max. Temp Min. Temp. Rainy Days Rainfall Sunshine
Sapporo 0 C (32F) -7 C (19F) 16 d 96 mm 110 h
Tokyo 10 C (50F) 2 C (36F) 6 d 60 mm 161 h
Osaka 9 C (49F) 3 C (37F) 6 d 59 mm 131 h
Fukuoka 11 C (51F) 4 C (38F) 9 d 71 mm 114 h
Naha 19 C (67F) 14 C (58F) 11 d 125 mm 85 h