From the 3rd to the 4th, Yamagata, Niigata and Hokuriku experienced record heavy rainfall.



Experts say that the background of the heavy rain, which greatly exceeded the Meteorological Agency's initial forecast, was that the front continued to stagnate and that a large amount of water vapor flowed into the Sea of ​​Japan side.

“When a front extends from the sea side, there is a limit to prediction accuracy”

According to the Japan Meteorological Agency, over the past few days, a front extending from the Sea of ​​Japan has been slowly moving southward in the Tohoku region, causing heavy rain in the area south of the front.

This front was expected to move southward to the vicinity of Hokuriku on the 3rd, but as a result, it stalled for nearly a whole day around Yamagata and Niigata prefectures.



For this reason, rain clouds tended to develop in similar areas such as the southern Tohoku region and Niigata Prefecture, which are located on the south side of the front.

Professor Kazuhisa Tsuboki of Nagoya University, who is familiar with meteorological mechanisms, explained why the front was more stagnant than the Japan Meteorological Agency predicted, saying, ``If a front extends from the Sea of ​​Japan side, there is no way to measure the wind speed or water vapor content on the sea, so the position of the front is There is a limit to the accuracy of prediction, such as the degree of development and the degree of development."

'Atmospheric rivers' also cause record rainfall

Furthermore, Professor Tsuboki sees the existence of an "atmospheric river" that supplied a large amount of water vapor to the stagnant front as a background to the record heavy rain.

An "atmospheric river" is a phenomenon in which a large amount of water vapor flows in a huge belt, and Professor Tsuboki believes that it has reached about 2,000 kilometers from the East China Sea to the northeastern coast.



This time, in addition to the steam that flowed around the edge of the "Pacific High" south of Japan, a large amount of water vapor was also brought in from a tropical depression that was originally Typhoon No.



As water vapor continued to flow in from two directions, the size of the "river of the atmosphere" also increased, and it is possible that rain clouds developed one after another near the front.



Professor Tsuboki said, ``In the area where the heavy rains hit this time, large-scale floods have occurred frequently, such as the heavy rains in Niigata and Fukushima and Fukui in July 2004, but is the amount of water vapor supplied the same as at that time? There is more than one possibility," he said, adding, "There is still a lot of water vapor in the Sea of ​​Japan, and it is possible that it will continue to rain for a long time. was