Super Typhoon Choi-wan

Typhoon Choi-wan, known in the Philippines as Typhoon Ramon was the strongest tropical cyclone ever to make landfall in recorded history, as well as being one of the strongest tropical cyclones ever recorded; devastating portions of Southeast Asia, particularly the Philippines, in late-November 2015. It is the deadliest and costliest tropical cyclone to hit the Philippines in recorded history, and is the costliest tropical cyclone ever outside the United States. Choi-wan is also the strongest tropical cyclone to strike a major metropolitan area. It is also among the largest tropical cyclones in terms of gale-force wind diameter.

Choi-wan had its origins over the Western Pacific Ocean, very close to the equator, well south-southeast of Kosrae. It moved to the north-northwest and shifted westerly in track. It later continued its path westwards as it gradually intensifies, but later begins its phase of Rapid deepening and explosive intensification from a category 1 to a category 5 super typhoon in just 24 hours. It weakened to a category 4 before intensifying into a category 5 for the second time for an extended period of time. Choi-wan ravaged the southern tip of Yap as a category 5, making it the strongest to hit the island.

It continually intensified moving due westwards while approaching the Philippines. As the typhoon slowly decelerated, it had attained its peak winds of 175 knots (325 km/h; 200 mph) or 10-minute sustained of 135 knots (250 km/h; 155 mph) with a pressure reading of 876 hPa, measured by a reconnaissance flight by a hurricane hunter aircraft. On 1245 UTC, November 23rd, Choi-wan had made landfall over the town of Libjo, Dinagat Islands at peak intensity which made it the strongest ever to make landfall. It had slightly weakened due to land interaction, and made its second landfall over Liloan, Southern Leyte then made several more landfalls over the Visayan Islands.

Meteorological history
On November 15, a cluster of thunderstorms was spotted south of the Marshall Islands over a very favorable environment by the Joint Typhoon Warning Center (JTWC). It became a tropical disturbance the following day, as the Japan Meteorological Agency (JMA) had classified it as a minor tropical depression. The disturbance was located at an unusually low latitude, which had a suppressed Coriolis effect, which was stated by the JTWC as a potentially inhibiting factor for intensification and development. Defying its previous reasoning, the JTWC issued a Tropical Cyclone Formation Alert several hours later as the system had steadily consolidated. At the same time, the JMA had initiated warnings on the disturbance, 6 hours before the JTWC had followed suit. As a tropical depression, the JTWC had designated the system as 27W. On the 17th, 27W had been upgraded into a tropical storm by both the JMA and JTWC and was named Choi-wan. Choi-wan executed a general westward motion because of the storm's position which is at the extreme southern periphery of a powerful subtropical ridge, before it had began turning to the northwest. Due to some dry air and lack of upper-level divergence, Choi-wan's intensification had temporarily halted for 30 hours. Due to the storm's organization, banding and strong poleward and equatorial outflow, the JMA had upgraded Choi-wan to a severe tropical storm and directly to a typhoon by the JTWC as the storm showed a very prominent central dense overcast. A pinhole eye appeared over the storm's circulation center, surrounded by hot towers which prompted the JMA to upgrade the storm into a typhoon. Due to a mild weakening of the ridge, the typhoon tracked to the west-northwest. It had began started to intensify further; Choi-wan intensified to a category 2 after 12 hours. The typhoon's eye had become much more defined, and increased in size; the convection over its central dense overcast had drastically cooled, which made the JTWC upgrade Choi-wan to a super typhoon, then to a category 5 super typhoon in the next warning. It later had weakened into a category 4 due to an Eyewall replacement cycle. However, as the cycle concluded, the storm resumed intensifying and a more defined westerly track.

The typhoon was aided by extremely favorable atmospheric conditions, with anomalously warm sea surface temperatures of 31 °C and the presence of a Tropical Upper Tropospheric Trough to the northeast. It had intensified again to a category 5, with a much larger eye having a 50-km wide diameter and having a massive core of roughly 950 km across, typhoon-force winds were felt far south over Palau and tropical storm force winds were felt to the north over Guam. This prompted the National Weather Service to issue watches and warnings over a wide swath of islands over the Federated States of Micronesia (FSM) and Guam. The storm's eyewall had lashed the island of Yap, with 1-minute winds of 145 knots (270 km/h; 165 mph) and stronger gust. The island was the worst hit by the typhoon over the entire FSM. Choi-wan continued to steadily intensify, defying the weakening forecast as the storm underwent another eyewall replacement cycle. At this time, the storm had entered the Philippine Area of Responsibility, which made the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) named the typhoon locally as Ramon.

Choi-wan's eye warmed drastically and its central dense overcast had cooled. It had been upgraded to a "violent" typhoon by the JMA and became the strongest typhoon since Typhoon Haiyan of 2013 with 10-minute sustained winds of 120 knots (220 km/h; 140 mph) and a pressure of 885 hectopascals (mbar, 26.13 inHg). Choi-wan began to threaten the Philippines particularly Visayas and Mindanao, as typhoon-force winds began impacting the area even 48 hours before the expected landfall. During the late night hours of November 22nd, typhoon Choi-wan's outflow expanded and its eye had cleared out even more. This prompted the JTWC to increase sustained winds of 165 knots (305 km/h; 190 mph) which made Choi-wan tied for the highest official sustained winds ever recorded. Choi-wan's core had become extremely organized, and became sufficient enough for the storm to reach a Dvorak number of T8.0, which is the highest in the scale. In such a rare occurrence, a hurricane hunter aircraft departed from Guam to measure the wind speed and pressure of the typhoon. The assessment of the aircraft had Choi-wan had succeeded the previous storms' wind speeds of 165 knots, and even Haiyan's 170 knots, eventually recording winds of 175 knots (325 km/h; 200 mph) and a pressure of 876 hPa (mbar; 25.87 inHg) or 135 knots (250 km/h; 155 mph) which made it the highest certified and recorded wind speed of a tropical cyclone in history.

Just hours later, on November 23rd, the extremely powerful cyclone made its first landfall over the municipality of Libjo, Dinagat Island at peak intensity. Several major Philippine cities, despite being several hundred kilometers away from the core at the time, had began feeling the brunt of the storm, with Tacloban, Cebu City and Cagayan de Oro are already feeling typhoon-force winds with gusts of up to 200 km/h. Radar imagery by Project NOAH showed a well-defined eye and eyewall lashing Surigao City to the south and Panaon Island to the north. A weather station in Surigao City, which was directly impacted by Choi-wan's southern eyewall had recorded a 1-minute wind average of 298.43 km/h and a gust of 324.76 km/h accompanied by a pressure of 897.21 hPa (mbar; 26.5 inHg) just before it was destroyed by the typhoon. Typhoon Choi-wan had began to weaken over the southern Leyte Gulf, as it still continued to bring winds of 160 knots (295 km/h; 185 mph) due to a slight collapse of the storm's structure and degradation of its eye. Hours later, Choi-wan made a second landfall over Liloan, Southern Leyte then later making a third landfall over Macrohon, Southern Leyte. Its eye had greatly decreased in diameter, which measured 9-km had the JTWC later putting its intensity at 150 knots (280 km/h; 175 mph) as it made landfall over Ubay, Bohol. Choi-wan crossed over Bohol's northern coasts which contributed to slight organization of the storm's weakened core, and increase in size of its eye. It remained unchanged in intensity and slowed down before making its fifth landfall over the Carcar-San Fernando area in Metro Cebu, just 25 km south of Cebu City. A weather station located in Mactan Airport, which was over the storm's intense northern eyewall, had recorded a very powerful gust of 301.8 km/h and a pressure of about 902.66 hPa (mbar; 26.67 inHg) before it had shut down. During the landfall over Cebu, Choi-wan's eye continued to deteriorate, convection waned and CDO weaken; As a result, the JTWC lowered its winds to 145 knots (270 km/h; 165 mph) while the JMA remained "violent". Choi-wan made its sixth landfall over Guihulngan City, then crossed the rest of Negros Island. After exiting Negros, Choi-wan's core disintegrated and made the JTWC downgrade to category 4, although it remained as a super typhoon. Its northern eyewall was felt over southern Guimaras and Iloilo City, of which both reported winds of 227.43 km/h. Choi-wan exited the Visayan Islands thru its final landfall over the towns of Tobias Fornier and Anini-y in Antique as a strong category 4. Choi-wan continued to weaken over the Sulu Sea as its eye had started to fade; At the same time, its track shifted to the West-northwest and accelerated. Choi-wan was downgraded by the JTWC to a typhoon, and a "very strong" typhoon by the JMA; it made its final Philippine landfall over El Nido, Palawan as a category 4.

Choi-wan exited the Philippine landmass over an area of favorable atmospheric conditions, which led to some of its intensification. Choi-wan's eye became more visible and its convection over its CDO had cooled, making the JTWC upgrade Choi-wan back to a super typhoon for 12 hours. Vertical wind shear, lower ocean heat content and cooler sea surface temperatures were factors of Choi-wan's steady weakening. The eye had not anymore become apparent, and had become cloud-filled. Choi-wan encountered dry air, which caused some entrainment over the storm's northwest quadrant; the convection continued to wane and later downgraded to a category 3. At this time, Choi-wan had restricted outflow which was also attributed to the disintegration of the core; Choi-wan was downgraded to a category 2 after 18 hours. On November 26, Choi-wan made landfall over Phan Thiết, Vietnam as a low-end category 2. A dramatic decrease of vertical wind shear over the area contributed to slower-than-forecast weakening, making it retain typhoon intensity for 12 more hours. On the same day, Choi-wan made its final landfall over Ho Chi Minh City, as a weakening category 1. Afterwards, Choi-wan was downgraded to a tropical storm by both the JMA and the JTWC as the storm slowly dissipated. On November 28, the JTWC issued its final warning on tropical depression Choi-wan; the JMA had followed suit.

Micronesia and Palau
Even before the actual system had formed, tropical storm watches were issued for Kosrae, Chuuk, Pohnpei and a tropical storm warning for parts of the Marshall Islands and typhoon watch for Yap and Palau, due to excellent agreement of major dynamic models of development of a typhoonwithin the next 48 hours. When tropical storm Choi-wan began intensification, large parts of Micronesia, including Palau, where under threat, so tropical storm warnings were issued by the National Weather Service for Chuuk Lagoon, Majuro, Satawal, Fananu, Losap and Puluwat; a separate typhoon watch was issued for Ulul and typhoon warnings were issued for the entire island of Yap, Kayangel and Koror.

As Choi-wan intensified to a super typhoon, many public elementary schools and some other establishments were shut down to be used as evacuation centers for the affected areas, particularly Yap, which bore the brunt of the northern eyewall. Residents over the island of Yap had feared a repeat of Super Typhoon Sudal in 2004 which previously ravaged the place. They took all of the necessary precautions ensuring the safety especially over low lying areas and those over the coastline from flash floods and storm surges.

Farther south, residents of the islands of Kayangel and Koror took advantage of the storm's distance, not knowing that the storm was large and powerful enough to knock down damage over the island 24 hours before the closest approach. In the entire Palau, wide scale evacuations took place and nearly 20,000 have evacuated from the typhoon.

Philippines
Fearing a repeat of the devastating Typhoon Haiyan, locally known as Yolanda, preparations took place and storm warnings were issued before the storm had entered their area of responsibility. The largest evacuation in the history of the country took place, with about a million people involved due to fear and preparedness. Residents of the city of Tacloban, which was directly hit by Haiyan, took this storm seriously. The PAGASA raised Public Storm Warning Signal (PSWS) # 2 over a large area 72 hours before landfall. As Choi-wan approached, PSWS # 3 was issued for these provinces and its coverage even further expanded; when it became a super typhoon in PAGASA's scale (>220 km/h; 120 knots; 140 mph), PSWS # 4 was hoisted over a large area of the Visayas and Mindanao. By the dawn of November 23, PSWS # 4 was issued for Negros Oriental, Cebu, Bohol, Samar, Eastern Samar, Biliran, Leyte, Southern Leyte, Dinagat Islands, Surigao del Norte, Surigao del Sur and Misamis Oriental.

Cebu City, a major city which lies over the forecast track eyewall and core, had initiated massive evacuations and classes were suspended for the entire week, due to the anticipated severe damage. Similar was done for the provinces of Albay, Sorsogon, the entire Visayas and the region of CARAGA.

The Philippines was declared under a state of national emergency even before the impact, so the government may allocate funds more easily and for preparedness. Hospitals, particularly over the Visayas and CARAGA, were placed under a red alert in anticipation of the large influx of casualties or injuries expected. Nearly 2,000 of the Philippine National Police were dispatched over Leyte provinces, Eastern Samar, Surigao provinces, Cebu, Bohol and Negros due to the anticipated severe and large-scale impact and aftermath.

Ahead of the storm, a large number of areas and provinces were under threat of storm surges, prompting the PAGASA to issue watches, warnings and alerts over coastal towns in the expected areas to be affected by the extremely powerful super typhoon.

Residents over the southern portions of Luzon, the entire Western, Central & Eastern Visayas and in the regions of Northern Mindanao, CARAGA over Mindanao were advised to remain indoors, anticipating for the worst. Over the mentioned regions, any form of sea and air travel was suspended amidst the onslaught of the typhoon.

Vietnam
As Typhoon Choi-wan was heading towards southern Vietnam, officials became wary of its potential effects to Vietnam's largest city, Ho Chi Minh City and nearby areas over the Mekong River due to the likelihood of flash floods. Like in the Philippines, wide-scale evacuations took place over the Southern region of the country, particularly over Southeast (Vietnam) and Mekong Delta. All vessels at sea over the southern regions were sent alerts due to anticipated rough weather and storm surge. Mandatory evacuations took place over the coastlines as preventive measures.