Typhoon Rai was the strongest storm to hit the Philippines in the 2021 Pacific typhoon season and the first category 5 storm in December since 2016'sTyphoon Nock-ten due its rapid intensification of 85 miles per hour (137 kilometres per hour) in a short span of 24 hours. Emerging from a low-pressure area located south of the Caroline Islands on December 11,it was categorized as a tropical depression by the Japan Meteorological Agency (JMA) on December 12 and was upgraded into a tropical storm that same day due to a favorable environment.
The JMA upgraded Rai into a severe tropical storm on December 14. The system developed an eye and eventually continued developing the next day. A favorable environment caused rapid development to the system,attaining category 5 strength on December 16. The storm made its first landfall in Siargao on 06:00 UTC the same day before making landfall in Panaon Island,the northern portion of Bohol,central Cebu,and Negros. The storm started weakening because of these landfalls. On 09:00 UTC on December 17,the storm made landfall in the northern portion of Palawan Island,marking Rai's last landfall. That same day,the storm started to slowly recover and eventually develop. The system once again attained a category 5 intensity on December 18. The storm entered an unfavorable environment and dissipated on December 20.
Origin
Rai as a tropical storm on December 13
Typhoon Rai originated from a stationary low-pressure area (LPA) that formed south of the Caroline Islands on December 11.[1] As the storm reached a 56km/h (35mph) wind speed,the JMA distinguished it as a tropical depression.[2]PAGASA also started issuing warnings for the system,defining it as a tropical depression.[3] The storm was in a favorable environment with high sea surface temperatures and weak wind shear but remained stationary due to weak steering flow.[4] The storm's disorganized structure caused it to maintain its intensity during a westward shift.[5] The system started slowly moving east,then moving slightly south while still directed east.[6] The storm intensified to 65km/h (40mph) alongside a barometric pressure of 998 mbar (hPa;29.47inHg). This prompted the JMA to upgrade it to a tropical storm,assigning it the name Rai as it shifted northwest,south of the Yap State.[7] Over the following hours,it rapidly intensified to a speed of 83km/h (52mph).[8] The JMA cited a good outflow and a favorable environment.[9] The Joint Typhoon Warning Center (JTWC) issued their first advisory on the system,classifying it as a tropical depression on December 13.[10] Shortly after,the agency upgraded the system into a tropical storm due to favorable conditions with poleward and equatorward outflow.[11][12]
Rapid intensification and landfalls
Rai making landfall in multiple parts of the Philippines
Rai was by the JMA upgraded into a severe tropical storm on December 14 with a wind speed of 92km/h (57mph).[13] PAGASA upgraded the cyclone to a tropical storm on 17:00 PHT (09:00 UTC).[14] Six hours later,PAGASA upgraded the system into a severe tropical storm.[15] Along with favorable environment,the storm had good cloud characteristics as it moved along the southern edge of a subtropical high.[16] During that day,the storm's wind speed increased slightly,passing north of Palau while maintaining a northwest trajectory.[17][6] On 18:00 UTC on the same day,the storm started forming a band;it maintained its intensity because of a neutral environment.[18] Rai was upgraded into a typhoon on December 15 with a wind speed of 120km/h (75mph) alongside a pressure of 975 mbar (hPa;28.79inHg);PAGASA and the JTWC followed that same day.[19][20][21] The system started moving straight west to Samar Island.[6] The system,according to microwave imagery,developed an eye.[22] The storm eventually developed a central dense overcast,hiding the eye. Due to being in a neutral environment,the system maintained its intensity.[23] Six hours later,the storm continued developing due to good cloud characteristics and a favorable environment;satellite imagery showed dry air around the system.[24]
Upon arriving at the Philippine Sea,rapid development occurred to Rai on December 15 to a wind speed of 182km/h (113mph) due to a favorable environment. From 06:00 UTC to 12:00 UTC,the system had been upgraded to a category 2 typhoon. From 12:00 UTC to 18:00 UTC,the system rapidly strengthened from category 2 to 4. Eventually,at 00:00 UTC on December 16,the system attained category 5 strength,the sixth storm to reach that category in 2021.[25][26][6] The JTWC classified Rai as a super typhoon but downgraded the system back to a typhoon three hours later.[27][28] According to a National Oceanic and Atmospheric Administration (NOAA) map,the system was downgraded into a category 4 typhoon from its peak intensity.[6] That same day,the storm made landfall in the shores of Siargao on 06:00 UTC with wind speeds of 195km/h (121mph).[26][29] According to the Yale Program on Climate Change Communication,this landfall was the third category 5 landfall in the Philippines in two years;landfalls of this intensity were described as rare by the program.[30] Three hours later,the storm made landfall in the Panaon Island.[31] The storm made landfall over the northern portion of Bohol on 12:00 UTC.[32] Three hours later,the storm made landfall over the central portion of Cebu and eventually passed through Negros as well.[33][34] These landfalls had an intensity of category 3 and 4.[30] Due to these landfalls,the storm experienced intense weakening,losing its central dense overcast.[29] On 09:00 UTC on December 17,the storm made landfall in the northern portion of Palawan Island[35] with a category 2 intensity,marking the last landfall of Typhoon Rai.[36][6]
Intensification and dissipation
At 12:00 UTC on December 17,the JMA assessed Rai's environment as neutral,citing favorable sea surface temperatures,which were offset due to unfavorable land interaction from its recent landfalls;this assessment caused the storm to maintain its intensity. Satellite imagery showed the storm had an eye.[37] The next day,the storm started to slightly develop due to a warm surface and weak wind shear. It started to develop a central dense overcast due to good cloud characteristics and a rainband with curvature.[38] At 12:00 UTC on December 17,the storm had an increased wind speed of 182km/h (113mph),intensifying into a category 3 typhoon.[39][6] While curving north,the storm began intensifying,attaining category 4 at 06:00 UTC on December 18. Eventually,Rai attained a category 5 wind speed at 12:00 UTC.[40][6] Six hours later,the system was downgraded into a category 4 typhoon.[6] The storm gained a neutral assessment by the JMA on December 19 with weak vertical wind shear but was offset by lower sea surface temperatures and weaker tropical cyclone heat potential.[41] That same day,PAGASA issued their final report on Rai due to its far proximity from the Philippines.[42] At 06:00 UTC,the system was downgraded into a category 3. Six hours later,the system was downgraded again into a category 2 typhoon.[6] A few hours later,the system was in an unfavorable environment with even lower sea surface temperatures and dry air. Through this,the system weakened rapidly.[43] By 06:00 UTC on December 20,the system was downgraded by the JMA to a severe tropical storm.[44] Three hours later,the system was downgraded again into a tropical storm.[45] The JTWC followed suit at the same time.[46] On 12:00 UTC,the JMA issued its final advisory on Rai,downgrading the system to a tropical depression.[47] That same time,the JTWC also downgraded Rai into a tropical depression but later reissued the report,upgrading Rai back into a tropical storm as it curved more east.[48][49][6] The JTWC issued their last report on 21:00 UTC on December 20.[50] NOAA said that it dissipated at 12:00 UTC on December 21.[6]
Rai undergoing rapid intensification from December 15 to 16
Analysis and records
In a study by the Kyushu University,scientists replicated Rai by making three runs:a normal setup with land,a setup with flat land,and a setup without land. Due to these runs,the scientists identified three common phases:the first intensity peak on December 16,the weakening stage after landfall on December 17,the recovery phase on December 18,and another peak phase on December 19.[51] In a study by the Ateneo de Manila University,researchers found that the storm had the second highest intensity out of all landfalling tropical cyclones in the Philippines. When Rai made its first landfall,the researchers found that the system had a low chance of dissipation in contrast to its interaction with land.[52] The study also revealed that Rai was the strongest landfalling tropical cyclone in the Philippines in the month of December.[53]
Within the 24 hours before the landfall,Rai surpassed meteorologists' expectations by increasing its wind speed by 85mph (137km/h).[54]Nikos Peñaranda,a forecaster from PAGASA,stated that their models "weren't able to predict the way the storm intensified".[55] This rapid intensification was regarded as a cause of global warming.[54] Rai is regarded as the strongest storm to hit the Philippines in the 2021 Pacific typhoon season.[40] The storm is also regarded as the first category 5 storm in December since the 2016 Pacific season's Nock-ten.[56] Rai's re-intensification in the South China Sea was regarded as the first category 5 re-intensification in the sea since 1961.[57]
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