A northwest winter wind, upper-air temperatures around -6°C or colder at 1,500 m, and new snow on top of a deep base: how to read a Japan snow forecast.
Most Japanese powder days follow the same pattern: a winter high over Siberia and a low to the east push cold northwest wind across the Sea of Japan, and snow falls on the mountains facing that wind. To read a forecast, check three things: the wind direction, the temperature about 1,500 m up (the 850 hPa level), and how much new snow is forecast compared with the base already on the ground. A strong northwest flow with upper-air temperatures well below -6°C is the classic setup for light, deep snow.
In winter, cold air from Siberia crosses the relatively warm Sea of Japan, picks up moisture and forms bands of low snow cloud. When that air hits the mountains of Hokkaido, Tohoku, Niigata, northern Nagano and Hokuriku, it rises and drops the snow. Weather maps show this as the "west high, east low" pressure pattern, with tightly packed north-south isobars.
Sometimes the flow converges into a single band about 1,000 km long, called the JPCZ (Japan Sea Polar air mass Convergence Zone). Where it comes ashore, snowfall can be extreme for a day or two, most often between Fukui and northern Hyogo and Kyoto (JPCZ).
| Wind | Who gets the snow |
|---|---|
| Northwest | The classic pattern. West-facing Hokkaido (Niseko, Kiroro), Niigata, northern Nagano including Hakuba and Nozawa Onsen |
| North | Northern Hokkaido and Tohoku; Nagano often gets less |
| West or southwest, ahead of a low | Warmer air, heavier snow, risk of rain at the base on Honshu |
Strong wind with tight isobars tends to dump snow on the mountains (Japanese forecasters call this the mountain-snow type, yama-yuki). When the isobars loosen and bend over the Sea of Japan, snow can fall heavily on coastal towns instead (sato-yuki, the village-snow type), and the mountains get less than you would expect.
The 850 hPa level is roughly 1,500 m above sea level, close to the top of many Japanese ski areas. A widely used rule of thumb in Japanese forecasts is that when it is about -6°C or colder up there, precipitation falls as snow even at low elevations. Warmer than that, the lower slopes and the village may see wet snow or rain.
The colder the air, the lighter the snow. A forecast of -6°C at 850 hPa means snow, not necessarily dry powder. For the light snow Hokkaido is known for, you want air clearly colder than that, which is why Kutchan, with a February average of -4.9°C at the surface (JMA), skis drier than Honshu towns averaging around 0°C.
You can see 850 hPa temperatures on free model viewers such as Windy, or on Japanese weather sites that show upper-air charts.
JMA defines snowfall (降雪量) as the depth of new snow in centimetres over a period, and snow depth (積雪深) as the depth of snow lying on the ground (JMA glossary). JMA treats about 3 cm an hour as heavy snowfall.
You need both numbers. Forty centimetres of new snow on a 2 m base skis well everywhere. Forty centimetres on a thin early-season base hides rocks and stumps, and tree runs stay closed. Settlement matters too: a snow depth figure may rise by much less than the snowfall because the old snow compacts.
Also check where the numbers are measured. JMA stations sit in valleys: Kutchan at 176 m, Hakuba at 703 m (JMA). Resort reports usually quote a base and a top figure, and the top can be double the village.
For resort-specific patterns, see our guides to Niseko snow and weather and the Hakuba Happo-one forecast. Today's numbers are in the Japan snow report.
Wind direction and upper-air temperature are usually reasonable two to three days out. Exact snowfall totals at one resort are less reliable, so check again the day before.
Not if it comes with strong wind or warm air. Wind closes upper lifts and blows snow off exposed slopes, and temperatures near or above -6°C at 850 hPa can mean heavy, wet snow at lower elevations.