Meteorological History of Typhoon Son-Tinh

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## Before being a named storm

On the 27th of September, nearly five days after Tropical Depression 14W being absorbed by an extratropical cyclone, a low-pressure area was created south of Majuro. The NRL gave the disturbance number  Invest 96W, while the JMA designating it as a low-pressure system. It later emerged with an area of convection. Due to it's geographical location, which is only a few hundred kilometers from the equator, which is not enough to generate Coriolis Force, high wind shear and dry air invasion, it was almost torn off from these conditions. The system was nearly stationary for three days, before being affected by two high-pressure systems. After being pushed north, it merged with another developing low-pressure system, and the system gradually organized due to moist air from the north and low wind shear. The Joint Typhoon Warning Center gave it a low chance of development and started to track the system using the Dvorak technique. Then moist air in all directions faced towards 96W, and the JTWC gave the rating "MEDIUM". It than entered an area of favourable conditions, and on the morning of September 18, the JTWC gave the rating "HIGH" and issued a Tropical Cyclone Formation Alert as the system became even more organized.

The JMA and the TMD were the first to upgrade 96W to a tropical depression, one hour after the JTWC issued a TCFA. The China Meteorological Administration and the Central Weather Bureau upgraded 96W to a tropical depression, on 12:00 CST (03:00 UTC). The JTWC became the third-last center to upgrade 96W to a tropical depression, designating it as 19W, while the VNCHMF and the PAGASA upgraded it to a tropical depression on the 04:30 UTC. Around four hours later, JTWC's microwave image showed that 19W had 1-minute winds of 65 km/h, so it upgraded 19W to a tropical storm.

## Being named and substantial intensification

Early on September 3, 19W ran into a favourable environment, and JMA's METOP-A-ASCAT image showed that 19W had 10-minute winds of 65 km/h, so the JMA upgraded it to a tropical storm and named it *Son-Tinh*. Although Son-Tinh was invaded by dry air from the north, it was located in an area of low wind shear, warm sea-surface temperatures with patches of convection, and this prompted the JMA to upgrade it to a severe tropical storm on the 4th os September. It developed a cloud-filled eye, which is only visible on the radar image. At that time the JTWC started to monitor it's wind speed as it was struggling to strengthen to a Category 1. However, it started to run into low amounts of wind shear on the next day, so the JTWC and JMA upgraded it to a typhoon, and than the JTWC and the VNCHMF. On the same day, the PAGASA started to monitor Son-Tinh as a typhoon. It then met with another low-pressure system, Invest 97W, and underwent Fujiwhara Interaction with it. After this event happened, Son-Tinh intansified into a Category 2.

It entered an area of moderate to severe wind shear, so it began a weakening trend and it weakened to a Category 1 from a Category 2. The VNCHMF even downgraded it to a severe tropical storm. Expert meteorologists believe that would dissipate at sea due to wind shear, but on the next day, it's outlying wind shear had weakened, and Son-Tinh unexpectedly transformed into a Category 2, as the eye became visible on the satellite image.

### Forecast errors

As said above, expert meteorologists believe that would dissipate at sea due to wind shear, but Hurricane Hunters Aircrafts analysed it and found out Yagi would continue to persist until it approaches the Philippines.

## Rapid intensification and peak intensity

After Son-Tinh intensified back into a Category 2, it swept Guam and the Northern Mariana Islnds, causing several deaths and more than 100 injuries. It juat passed 15 km (9 mi) south of Guam, causing severe damage, of which a tree fell and crushed down a house, causing the death of 4 people, including a teenager and an infant. After sweeping Guam, it underwent the second-fastest intensification ever recorded in a tropical cyclone - although it's pressure did not drop significantly, only from 965 hPa to 920 hPa, but it was from 165 km/h to 250 km/h in only 24 hours - the fastest since Hurricane Wilma and Typhoon Forrest in 1983. After intensifiying, it transformed into a Category 5 - the second in the year, after Super Typhoon Haikui. It opened a well-defined eye about 30 mi (48 km) according to Hurricane Hunters Aircrafts, which investigated the system three times on September 8.

It then entered another area of wind shear while entering the PAR, and while it was threatening Yap. As it entered the PAR, it underwent an eyewall replacement cycle, and weakened below Category 5 intensity. After around 2 or 3 hours, after the cycle finished, Son-Tinh opened a 50 km well defined eye about several houndred kilometers of Samar.
After this happened, the PAGASA iussued a Signal No.1 for Northern Samar, Metro Manila and Southern Quezon. About three hours later, the places which were Signal 1 were concerted into Signal No.2, except for Metro Manila. On the next day (September 9), while Visayas and the southern part of Luzon was signalled No.1 or 2, the PAGASA announced the possibility that the Signal No.4 may be issued.

## Landfall in the Philippines, then tertiary peak

Yagi made landfall on Northern Samar with 10-minute winds of 185 km/h and 1-minute winds of 260 km/h. The Signal No.5 was expected to issue for the first time since it was added to the Philippines Typhoon Warning Signals, to Northern Samar. A wind gust was recorded at 289 km/h, the highest since Haiyan in 2013. The typhoon was so large, sustained winds of 63 km/h was recorded as far as Northern Mindanao.

After landfall in Samar, it weakened to a Category 3, as it entered the Sibuyan Sea and crossed Marinduque directly. At that time, it's maximum sustained winds stayed at 205 km/h, and it was expected to weaken to a Category 2, due to wind shear. However, it maintained strength for another two hours, before making it's final Philippines landfall at the southernmost part of Batangas with winds of 195 km/h (120 mph). After that landfall, it entered the South China Sea strengthened back to a Category 4 on the 11th of September, and then a Category 5 on the same day. That was not expected to happen, according to NOAA and Hurricane Hunters aircrafts, as wind shear was much weaker than expected. Moisture from the remnants of Tropical Depression 18W supported Son-Tinh to strengthen to a Category 5, which was, again, not expected, because wind shear was expected to be strong, but it was weak. At that time, Hurricane Hunters aircrafts analysed that it held it's peak for 14.5 hours at the South China Sea, before rapidly weakening back to a Category 4.

## Final landfall

Son-Tinh rapidly weakened to a Category 4 (as mentioned above), and that was because wind shear of 30 km/h, and it made it's final landfall at Fan Lau, Lantau Island of Hong Kong with 1-minute winds of 220 km/h (140 mph) and 10-minute winds of 165 km/h, according to the JMA, though the Hong Kong Observatory classified Yagi as a super typhoon when landfalling in Hong Kong.

## Weakening and dissipation

As soon as Son-Tinh entered Mainland China, it maintained major hurricane status for another day, due to the brown ocean effect.
Next, it weakened, as it's inner eyewall dissipated, and it's circular structure became hard to define. It fully dissipated in the Sichuan Basin.
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