The January window: timing the deepest month.

The Siberian pressure system that buries the snow country is not one weather event but a rhythm — cold air crossing warm sea, picking up moisture, and dropping it on the first high ground it meets. The rhythm strengthens through December, peaks in the third week of January, and begins to lengthen its intervals by mid-February.

On Hokkaido, the window is wide and forgiving. The island sits closer to the source, the air is colder, and the snow stays dry from mid-December through early March. Book any fortnight in that range and the statistics are on your side; the famous January depth is a matter of degree, not kind.

Honshu is sharper-edged. Nagano and Niigata take their snow in heavier, wetter pulses, and the reliable heart of the season is tighter: roughly January 10th to February 20th. Before that window a warm week can strip a lower mountain to grass; after it, the freeze-thaw cycle begins its afternoon work on south faces.

Two practical conclusions. If your dates are fixed and fall outside the window, choose altitude and northern aspect — Shiga Kogen and Kagura hold quality longest. If your dates are flexible, the third week of January on either island is, by every dataset we have assembled, the single safest bet in the northern hemisphere.