Winter (December-January-February)In winter (December-January-February), the Siberian High develops over the Eurasian Continent and the Aleutian Low develops over the northern North Pacific. Prevailing northwesterly winds cause the advection of cold air from Siberia to Tohoku and bring heavy snowfall to the Sea of Japan side of Tohoku (upstream of mountainous land) and sunny weather to the Pacific side (downstream of mountainous land). Spring (March-April-May)In spring (March-April-May), migratory cyclones and anticyclones that alternately move eastward prevail across Japan. Temperature increases (decreases) in front (back) of cyclonic systems due to warm southerly (cold northerly) flow. Temperature in Tohoku rises gradually with large short-term variations. The sunshine duration is the longest in the second half of spring due to the predominance of anticyclonic systems. Summer (June-July-August)The period from the middle of June to late July is the rainy season (called the Baiu). Its precipitation is caused by a stationary front, called the Baiu front, which forms where a warm maritime tropical air mass meets a cool polar maritime air mass. In the second half of summer, the North Pacific High extends northwestward around Japan, bringing hot and sunny conditions to Tohoku. The Okhotsk High sometimes appears over the Sea of Okhotsk. This causes cool moist easterly winds (called Yamase), which bring cloudy and rainy conditions to the Pacific side of Tohoku. Autumn (September-October-November)In autumn (September-October-November), temperatures in Tohoku fall gradually. Monthly precipitation amounts over the Pacific side of Tohoku are the largest in September due to the active autumnal rain front and tropical cyclones. In October, the frequent passage of anticyclonic systems brings sunny conditions and refreshing air to Tohoku. The frequency of cold northwesterly flows across Japan and precipitation (rainfall or snowfall) on the Sea of Japan side of Tohoku show an increasing tendency in November. |