The snow seems to be concentrated on the west side of the country, which suggests the lake effect, only with the sea instead of a lake; the prevailing wind from the west brings air that has picked up moisture going over the sea, and the moisture freezes and falls as snow.
But why then do places like Oregon and Ireland not get so much snow, despite being similarly on the receiving end of moist air?
Those places have an entire ocean to the west, as opposed to Japan which has a narrow sea and then a continent. Conjecture: it's the combination that does the trick. Air blowing over the continent becomes very cold, then blows over the narrow sea and picks up moisture, which then freezes out. So to get maximum snow, you need a continent and then a narrow sea or lake, followed by the land which is to receive the snow.
Is that conjecture correct? Or if not, what is the reason for the amount of snow?
爱尔兰的山比日本低。我认为日本的山脉和俄勒冈州的差不多高，但是高山离日本的海岸更近。(我对这两个地区都不太了解，所以这个说法可能是错误的)
另一件要考虑的事情是，在冬天，对流阵雨在海上很常见。更长的海上路径会有更多的阵雨，因此更多的大气混合，提高气团的温度。
冬季对流在陆地上被切断，但阵雨可以利用已有的动量穿透内陆。日本山脉靠近海岸，以及它们的高度，将给降水带来更多的]地形增强]，并可能通过迫使不稳定对流上升而使其重新活跃。爱尔兰较低的地理位置不会产生如此大的影响。< / p >
Oregon has two ranges of mountains, the lower Coastal Range (<1000m) and the higher but 150km inland Cascades (~3000m). The Coastal Range has a similar height to the Irish mountains but are longer in their extent. The orographic enhancement of the Coastal Range will be much less than in Japan. The Cascades will then be in a rain shadow as the air will be less humid downwind of the Coastal Range. While the Cascades still cause orographic enhancement, this is nowhere near as much as if the Cascades were by the coast, which in Japan they effectively are. You can see this effect on the annual average precipitation map of Oregon: 
As an aside, I had wondered if the Sea of Japan might be warmer than around Oregon and Ireland; this would provide more heat, moisture and instability to the air reaching Japan. I had a quick look at February sea surface temperatures, and they seem broadly comparable to the other areas, at least for the past three years, so this enhancement mechanism seems unlikely. February appears to be the month that Japan gets the most snow, although I'm not sure if that's talking about peak snowfall or peak accumulation.
I found a few articles about Japan's snow online: