Japan Sea Side area is the snowiest region in the world. Geographical distribution maps of daily snowfall and those of the occurence of excess snowfall (mean snowfall + 2 standard deviation) of the area have been drawn for 98 days during two winters of 1955-57. And four typical distribution patterns were distinguished. Relations between these patterns and their respective synoptic charts and upper-air conditions were checked and the followings were resulted;
1) Scattered Mountain-Snow Type (Fig. 2) …This pattern used to occur at the early stage of the monsoon burst and at the passing of the cold front. The amount of snowfall is not so deep and the areas of deeper than 20cm snowfalls are limited and scattered only on the mountaneous areas. The frequency of the occurence is more predominent than the other types and occupies about 30% of the snow days. The upper-air is relatively warmer (not colder than -18°C) on this occasion.
2) Mountain-Snow Type (Fig. 3) …Heavy snow falls mainly on the central backbone mountain area of Japan. This pattern often happens at the strong monsoon bursts and lasts 1-2 days. Snowfall reaches frequently over 80cm and the excess snowfall covers more extensive areas than those of No. 1 type. Judging from the distribution pattern and others, this type of snowfall is typically orographic origin and is observed when the upper-air is colder than -18°C and the wind speed is faster than 18m/sec at 700mb level.
3) Coastal-Snow Type (Fig. 4) …Heavier snow falls on the coastal plains or hilly inland areas than the mountaneous areas. The daily amounts are about 20-40cm. This pattern seems to appear accompanied by the active local front which is called as Hokuriku front when the pressure gradient over the Japan Sea is not steap and the monsoon is fairly weak. The average persistence of this pattern is 2-3 days. The typical upper-air condition is the weaker wind of less than 21m/sec.
4) Heavy Snowfall Type (Fig. 5) …All over the area reports heavy excess snowfall and locally it reaches more than 100cm often. The pattern appears when the monsoon is moderately strong and the upper-air is colder than -20°C. We may subdivide this into the mountain-snow type heavy snowfall and the coastal-snow type heavy snowfall.
The sequence of these patterns for the period are shown on Fig. 6.
The figure 7 shows the topography of the Hokuriku area and the daily amounts of snowfalls during Feb. 11-15, 1957 at 4 different cross-sections from the sea to the mountain arranging from the east to the west. On Feb. 11 the mountain type heavy snow falls, 12th shows the mountain-snow type distribution and 13-15th are the coastal-snow type distributions.
