Heavy rain accompanied by strong wind and lightning is expected to continue in many parts of Indonesia in the next three days.
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JAKARTA, KOMPAS – A tornado hit a densely populated area in Sidoarjo regency, East Java on Wednesday, damaging 661 houses. The destructive weather that is usually marked with the emergence of cumulonimbus and followed by heavy rain is likely to continue until December.
Herizal, head of climatology at the Meteorology, Climatology and Geophysics Agency (BMKG) said in Jakarta on Thursday that the heavy rains and strong winds were caused by the emergence of tropical cyclones in the Indian Ocean and the southern part of East Java. The Cyclonic rotation has also appeared in the western waters of Sumatra, the Banda Sea and the South China Sea.
This situation triggered the formation of turn patterns and the convergence of winds in Indonesia. The active intertropical convergence zone (ITCZ) pattern that extends from South Sumatra, Karimata Strait, Java Sea, South Sulawesi, Southeast Sulawesi to the Banda Sea also triggered heavy rain in some areas. The ITCZ is a convergence area of the Passat winds of the northern hemisphere and the Passat winds of the southern hemisphere due to low pressure around the equator.
"Cumulonimbus [dark black clouds that look like mushrooms] may appear in Central Java, East Java, Bali and West Nusa Tenggara. The emergence of this cloud triggers heavy rain and tornadoes," said Ramlan, the head of weather information and forecast at BMKG.
According to BMKG analysis, heavy rain accompanied by strong wind and lightning is expected to continue in many parts of Indonesia in the next three days. Vulnerable areas include parts of Sumatra, Java, Bali, Nusa Tenggara, Sulawesi, Maluku and Papua.
In addition, waves as high as 1.25 to 2.5 meters may be seen in the waters west of the Nias Islands to the Mentawai Islands, and the Indian Ocean to southern Java.
Changing weather patterns
Siswanto, an extreme weather and climate researcher at BMKG, said turbulence in some parts of the region was common during the turn of the season. Currently, the Asian monsoon winds that bring rain have entered the Java Sea and the northern coast of Java. Meanwhile, in southern Java, there is still a flow of monsoon winds coming from Australia.
The meeting of opposite wind directions creates a wind shear. "Cumulonimbus is common at the start of a rainy season and the wind shear can potentially trigger tornadoes," he said.
Especially for Java, the warm temperature of the land can trigger the formation of cumulonimbus. "This situation will only end after the peak of the rainy season, that lasts between December and February," Siswanto said.
The head of the center for data, information and public relations at the National Disaster Mitigation Agency (BNPB), Sutopo Purwo Nugroho, said the frequency of tornadoes has intensified since 2002. "Tornadoes are the most frequent natural disaster, second only to floods," he said.
According to BNPB data, there were 663 tornadoes in 2016, rising from 404 in 2010 and 166 times in 2008. This year, until November, there have been 624 tornadoes. "The three districts most often affected by tornadoes are Cilacap, Bogor and Bojonegoro," said Sutopo.
During the last 16 years, there were 5,247 tornadoes, killing 398 people and injuring 3,098 others. The tornadoes also displaced 301,654 people and destroyed 223,927 houses.
The tornado in Sidoarjo on Wednesday damaged 661 houses, a number of school buildings, places of worship and village offices in three villages. The incident left 36 people injured and at least 100 people displaced.
The head of the disaster mitigation agency in Sidoarjo district Dwijo Prawito said of the 36 injured, four people were still being treated. Based on weather data at Juanda airport, the tornado that hit Sidoarjo had a speed of between 70 to 80 kilometers per hour with a duration of less than 5 minutes. The winds direction was from the east of the sea to the west.
In East Nusa Tenggara, tornadoes are common in almost all regions. Last month, 68 houses, eight school buildings and a public health center building were damaged by tornadoes. In Manggarai and West Manggarai, over the past month, five people have been killed by lightning.
Detection and anticipation
According to Siswanto, the weather before the emergence of a tornado cannot be monitored by BMKG equipment. However, the emergence of cumulonimbus can be detected with satellites and BMKG radar, the results of which can be used to make weather predictions.
He suggests several steps that citizens can take to anticipate the impact of the tornado. "Be aware of cumulonimbus. If there is a strong wind and temperatures tend to be cold, this indicates that there will be heavy rain. In such conditions, people should avoid trees and billboards, as they are vulnerable to collapse because of strong wind," Siswanto said.