Tropical Storm Sonca (2017)

Tropical Storm Sonca (Vietnamese: Sơn Ca) was a weak tropical cyclone that impacted Indochina during the end of July 2017. As the 10th named storm of the 2017 Pacific typhoon season, Tropical Storm Sonca formed south of Hong Kong. After drifting westward for multiple days, the storm intensified into a tropical storm, receiving the name Sonca. The storm later affected Hainan, reaching its peak intensity. On July 25, the storm made landfall over the Quảng Trị province; the Joint Typhoon Warning Center (JTWC) issued their final advisory while the Japan Meteorological Agency (JMA) continued, downgrading the storm as a tropical depression. The storm officially dissipated on July 29.

The storm killed 37 people, with a damage of $313 million USD.^[a] In Vietnam, 1,500 houses were damaged, with an impact of $13 million USD. Fishermen pushed their boats ashore while heavy winds destroyed houses. The storm also affected Cambodia, killing three people and destroying 2,686 houses while the government and soldiers assisted evacuation efforts. Thailand was the most affected, with 23 fatalities and major flooding in numerous provinces.

On July 21, both the JMA and the JTWC reported that Tropical Depression 08W had developed approximately 582 km (361 mi) to the south of Hong Kong.^[1][2] The storm had a rapidly consolidating low-level circulation center. The storm had a favorable environment, with slight improvement of poleward outflow. The JTWC later assessed that there was a poor likelihood of it becoming a tropical cyclone, citing reasons such as poor initialization and small size.^[3] The storm's environment remained favorable the following day, with vertical windshear offset by poleward outflow; despite this, the JTWC did not change their prediction.^[4] Early the next day, the JTWC upgraded their prediction to likely based on newly uncovered satellite footage; they also designated the system as a tropical storm.^[5] After drifting westward for a couple of days, the system intensified into a tropical storm by JMA while nearing the island province of Hainan,^[6] receiving the name Sonca.^[5]

After 13 hours, the storm was pulled southward by a building direction. Just before the storm had its peak intensity, the storm rapidly intensified after reports of a consolidating low-level circulation center. The storm later accelerated north after a steering ridge reoriented the storm.^[7] By July 24, Sonca reached its maximum intensity with a minimum pressure of 994 hPa.^[8] Early on July 25, the JTWC issued its final advisory as the system made landfall over in the Quảng Trị Province, Vietnam.^[9][10] The storm was downgraded into a tropical depression that same day by the JMA. The storm traversed northwest, eventually weakening inside Thailand. The storm dissipated on July 29.^[11]

During the storm, heavy floods were recorded in Vietnam, causing catastrophic property damage, submerging 229 villages.^[12] A Telegram was sent to send firefighters and police to rescue people in the northern provinces of Vietnam.^[13] Vietnam Airlines cancelled 11 flights to and from the Phu Bai International Airport while VietJet Air cancelled 16.^[14] Fishermen from Quảng Trị moved their boats away from the shore due to strong waves and winds.^[15] The Nghệ An province banned all sea-related activities and ordered ships to return back to the shore.^[16] Numerous houses and billboards were damaged from strong winds in the districts of Gio Linh and Vĩnh Linh.^[17] Five spillways in the largest lake in Nghệ An, were opened, which caused a major flood which submerged 20,000 homes.^[18] Three national routes were flooded among four other provincial routes in Nghệ An.^[19] At least six people were killed when the storm made landfall on July 25, all in the provinces of Hà Tĩnh and Quảng Trị. Approximately 1,500 houses were damaged in these two provinces. Across the whole of Vietnam, roughly 5,777 hectares (14,280 acres) of paddy fields and 750 hectares (1,900 acres) of croplands were damaged.^[20] In Nghe An Province, the total damage caused by the storm reached 127 billion dong (US$5.6 million).^[19] Total damage in Vietnam reached 300.7 billion dong (US$13 million).^[21]

The National Committee for Disaster Management for Cambodia warned fishermen about the rough weather from the storm.^[22] Authorities issued flood warnings and "safety hills" were set up with shelter, food, and supplies. The Royal Cambodian Armed Forces deployed 350 soldiers to aid in evacuation efforts; a provincial official of the Kampong Thom province stated that the government was prepared to evacuate residents. The Steung Saen River reached a height of 13.4 metres (44 feet), close to emergency levels. Water levels at the Tamok Bridge rose 1 m (3.3 ft) above the average level. The governor of the Choam Khsant District stated that four communes in the district were flooded.^[23] Tropical Storm Sonca affected four provinces across Cambodia, at least three people died, and 2,686 houses were submerged. 53 households were also affected.^[24] Two of the deaths were caused by drowning.^[23] In the Choam Khsant District, 48 families had to be evacuated because of the storm. Heavy floods affected four provinces, with 550 affected families and at least 1000 displaced residents.^[25]

Flash floods across Thailand killed 23 people and affected 44 out of 76 provinces in Thailand. The hardest hit province was Sakon Nakhon, Northeast Thailand as the storm forced the closer of Sakon Nakhon Airport lasting for 3 days.^[24] Damages in Sakon Nakhon exceeded 100 million baht (US$3 million).^[26] The floods created by the storm were the strongest floods in Thailand for two decades.^[27] Heavy rain damaged bridges in Khon Kaen province and flooded buildings in Ubon Ratchathani province and Sisaket province, where some people were relocated to temporary accommodations. In Lopburi province, some people were rescued by boat after the roads became impassable. In Sukhothai province, the Yom River flooded, and sandbags were used to help contain the floodwaters. Trains had to be re-routed in Nong Khai province when telephone poles fell on the tracks.^[28] Villages in Chiang Rai were inundated due to the overflowing of a river on July 24. The next day, a flash flood occurred at the Den Chai district due to continuous rainfall. On July 26, rainfall caused the water level in the Phayao Lake to reach full capacity. Small cars were unable to pass in roads in Kalasin due to flooding. Roads were destroyed in Khon Kaen while a landslide blocked a train tunnel in Chaiyaphum. A broken flood wall caused major flooding in Sukhothai province. Flash flood occurred in eight provinces in Ranong while flooding also occurred in eight districts of Nakhon Ratchasima; the flooding in three were severe.^[29] In total, the damage from flooding triggered by the storm in Thailand reached 10 billion baht (US$300 million).^[30]

• Other storms named Sonca
• Weather of 2017
• Tropical cyclones in 2017
• Tropical Storm Aere (2016)
• Tropical Storm Vamco (2015)
• October 2017 Vietnam tropical depression
• Tropical Storm Haikui (2017)
• Tropical Storm Son-Tinh (2018)
• Tropical Storm Soulik (2024)

• JMA General Information of Tropical Storm Sonca (1708) from Digital Typhoon
• 08W.SONCA Archived 2017-08-05 at the Wayback Machine from the U.S. Naval Research Laboratory