The strong storm Tuesday that left four people dead and disrupted transportation systems nationwide was triggered by a case of rare weather conditions occurring simultaneously.

“This was a condition which is referred to as a ‘May Storm,’ ” Maki Wakahara, a spokeswoman for the Japan Weather Association, told The Japan Times. “But it is extremely rare that it causes such huge damage.”

The stormy weather halted bullet trains and grounded airplanes across the country. A maximum instantaneous wind velocity hit a record 143.3 kph in Tonami, Toyama Prefecture, where gusts flipped over trucks and knocked out power for 2,800 households, according to Hokuriku Electric Power Co.

The Meteorological Agency remained on alert Wednesday for strong winds and high waves along coastal areas as the massive storm made its way north. The city of Sado, Niigata Prefecture, saw a maximum instantaneous wind velocity of 156.6 kph, and Hokkaido was expected to experience similar conditions early Thursday.

The typhoon-class storm was caused by a low-pressure front that originated over the Sea of Japan, where warm air from the Pacific collided with the cool air coming from the continent at approximately 1,500 meters above sea level.

Such conditions are not rare for this time of the year when the season changes from winter to spring, except that it usually does not take place over the Sea of Japan but rather in an area east of the archipelago.

Another factor that caused the low pressure system to go wild was the jet stream that blows eastward from the archipelago into the Pacific. The stream, known as the Prevailing Westerlies and which travels at an altitude of 5,000 meters, happened to dip a bit to the south toward Japan. It sucked up the warm air and intensified the already strong front.

According to the JWA, such volatile conditions — where the stormy weather quickly builds up — is called a “bakudan” (bomb) low-pressure front.

The radius of the area that saw wind speeds of more than 54 kph spanned a whopping 800 km — conditions similar to an extremely large and strong typhoon.

While the storm caused typhoonlike conditions across the archipelago, the two are different in nature. Whereas a low-pressure front is formed when cold and warm air collide and intensifies via the temperature difference, a typhoon is a tropical cyclone that develops in the northwest Pacific due to warm sea surface temperatures.

The JWA’s Wakahara said there aren’t any statistics on how many bakudan low-pressure fronts occur near Japan, but records show that a similar storm in May 1954 took shape east of Hokkaido and caused hundreds of casualties and left scores of people missing.