Tropical Depression Usman

Tropical Depression Usman was a weak, but deadly system that impacted the southern Philippines in December 2018. The tropical depression was first noticed as a low-pressure area to the east of Palau on December 23. Slowly intensifying, the system became a tropical depression by the next day. The system maintained its intensity while nearing the eastern portion of the Philippine islands, however due to unfavorable conditions, the depression weakened into a remnant low on December 29, while making landfall over Eastern Samar.

Tropical Depression 35W (Usman)
Tropical depression (JMA scale)
Tropical depression (SSHWS)
Tropical Depression Usman over the Philippines on December 29
FormedDecember 25, 2018
DissipatedDecember 29, 2018
Highest winds10-minute sustained: 55 km/h (35 mph)
1-minute sustained: 55 km/h (35 mph)
Lowest pressure1000 hPa (mbar); 29.53 inHg
Fatalities156 total, 26 missing
Damage$103 million (2018 USD)
Areas affectedCaroline Islands, Philippines
Part of the 2018 Pacific typhoon season

Despite a weak system, the tropical depression brought torrential rainfall over much of Visayas, with the hard-hits being the regions of Eastern Visayas and the Bicol Region. Travel was vastly disrupted, mainly by sea vessels, during a busy holiday season as the country experienced post-Christmas and incoming New Year's Eve celebrations. A total of 156 people have perished from the storm, while 26 people have remained missing. Damages from the storm has exceeded to as high as ₱5.41 billion (US$103 million).


Meteorological history
Map plotting the track and the intensity of the storm, according to the Saffir–Simpson scale

On December 23, the Joint Typhoon Warning Center (JTWC) began monitoring on a tropical disturbance that had developed about 657 km (409 mi) to the east of Palau.[1] Despite located in an area of moderate vertical wind shear, sea-surface temperatures were recorded at 28-30 °C (82.4-86 °F), which is warm enough to support development.[1] The disturbance slowly intensified and the JTWC issued a Tropical Cyclone Formation Alert (TCFA) on 09:00 UTC of December 24.[2] At this time, the convection around the system's low-level circulation center (LLCC) was rather sheared, however the Dvorak estimates were around a T1.5, signalling that the system had sustained winds of 25 knots (45 km/h (30 mph).[3] This prompted the JTWC to upgrade the disturbance to a tropical depression, giving the designation of 35W.[4] By 21:00 UTC of the same day, visible imagery showed shallow banding wrapping into a compact, but exposed LLCC, with poorly organised convection.[5] On 06:00 UTC of December 25, the Japan Meteorological Agency (JMA) declared the system to be a weak tropical depression.[6] Around the same time, the PAGASA had reported that 35W had entered their area of responsibility and began issuing bulletins, giving the local name Usman.[7]

The JMA began issuing advisories on Usman on 12:00 UTC of December 25, when they considered that the tropical depression had 10-minute sustained winds of 55 km/h (35 mph).[8] Usman remained disorganised as the system's LLCC became ragged, with deep convection being sheared to its northwest.[9] By 03:00 UTC of December 26, satellite imagery depicted that the system had multiple LLCCs revolving around its original center. The JTWC deemed the system as "very disorganised" with limited convection due to the enhanced southeasterly wind shear.[10] The system maintained its intensity until the next day, when satellite imagery showed a broad circulation with excellent outflow along with extensive deep convection flaring near its center. At this point, both the JMA and the JTWC predicted that the system would intensify into a tropical storm within the next 24 hours.[11][12] Thereafter, convection had increased around the system, and the system's center became much more organised.[13] On December 28, the JTWC increased the system's 1-minute sustained winds to 55 km/h (35 mph).[14] A rapidly expanding central cold cover began obscuring the system's center, however satellite imagery also depicted that the system's center became partially exposed and elongated.[15] On December 29, the system's central cold cover began to dissipate, and the JTWC had stated that the system began weakening and deteriorating.[16] Tropical Depression Usman made landfall in Borongan, Eastern Samar on 06:00 UTC of the same day, as the PAGASA had also downgraded the system to a low-pressure area.[17] The JMA stated that the system was located in an area of increasing wind shear.[18] Several hours later, both the JMA and the JTWC issued their final advisory on the system.[19][20] The remnant energy of the system was later absorbed by a nearby tropical disturbance, which would eventually lead to the formation of Tropical Storm Pabuk.[21]

Preparations and impact

On December 27, as Tropical Depression Usman inches closer to the eastern portion of the Philippines, the PAGASA raised a Tropical Cyclone Warning Signal #1 over much of the Bicol region and the Eastern Visayas region, with some even extending as north as Quezon province and as west as Cebu.[22] Local officials advised residents at that time to take precautionary measures in areas of low-lying and mountainous areas, especially those living near river channels.[23] The PAGASA also warned on sea travel, either to fishermen or those with small sea crafts, are risky in the seaboards of Southern Luzon, Central Luzon and the Surigao provinces due to rough seas combined with the northeast monsoon.[22][24] Heavy rains that could trigger landslides and flashfloods were expected in the areas under the alert signals.[22] Roughly 1,000 passengers later became stranded in Bacolod after sea travel was suspended.[25] The region of Western Visayas, which includes the island of Palawan, was raised a TCWS #1 on December 28,[26] as 851 passengers were stranded in that region alone.[27] As of 5 p.m. local time, the NDRRMC raised the "blue alert" status in much of the Western Visayas region, meaning that 50 percent of the RDRRMC personnel are on standby for emergencies and it is the second highest alert level.[27] At this point, the number of passengers stranded from the cause of suspended sea travel rose to 13,000,[26] where 6,000 of them were from the Bicol Region.[28] In the city of Tacloban, their mayor ordered to suspend work in government offices and their local officials have set up 71 evacuation centres ready for displaced families.[29]

Following the landfall and the deterioration of the tropical depression, widespread flooding and landslides were reported in many areas. Six towns in Masbate saw 61 families, or 259 individuals forced to evacuate their homes due to extreme flooding.[30] Local officials in the area also lifted the temporary ban for sea travel on the morning of December 29 to allow the remaining stranded passengers to travel.[30] This was the same in the Bicol Region were 3,900 people were evacuated. In Northern Samar, the heavy rained caused flooding to as high as 2–7 ft (24-84 inches).[31] In Bicol, 473.1 mm (18.6 in) of rain was recorded for the two days of December 28 and 29.[32] Strong winds and heavy rain from Usman caused the shutdown of some power plants, which led to power outages in some areas In Albay, Catanduanes and Eastern Visayas.[31] On December 30, a state of calamity was raised in the provinces of Camarines Sur, Camarines Norte and Sorsogon, along with the municipality of Polangui in Albay.[33][34] Eight other municipalities in Oriental Mindoro were also raised a state of calamity by local officials on the next day.[33]

Throughout the country, a total of 238,127 families were affected by Usman. About 36,574 houses were either partially or totally damaged.[35] A total of 156 people have died from the storm, with 105 people being injured. 26 people have remained missing. An estimated cost of damages were toppled to ₱5.41 billion (US$103 million), where ₱1.95 billion (US$37.1 million) were to be part of agricultural damages and ₱3.46 billion (US$65.9 million) were to be part of infrastructure damages.[35]

Aftermath and retirement

On January 2019, Mahar Lagmay, a Disaster scientist, claimed that the high death toll by the system was caused by the PAGASA issuing incorrect and misleading forecasts for Tropical Depression Usman.[36][37] The PAGASA then responded to the scientist's statement regarding this claim, and stated that Lagmay had used the "hour rainfall intensity" classification instead of the "24-hour accumulated rainfall" classification, where it has been used commonly in their tropical cyclone bulletins.[36] Local officials also criticised the PAGASA due to the fact how their residents became confused and complacent when Usman was downgraded to a low-pressure area, which also meant the PAGASA lifting any storm signals.[36] The PAGASA did confirm this, however they explained that the storm signals are distinct from the amount of rainfall accumulated, as the signals described more of the expected winds in the area.[36] They also added that they did release a weather advisory on the low-pressure area (that was once Usman) on December 29 regarding that "moderate to heavy rains will continue over Bicol Region in the next 24 hours."[36]

Because the system caused damages of over ₱1 billion, the PAGASA retired the name Usman, despite it being used for the first time in their naming lists. In March 2019, the PAGASA revised their lists and replaced the name with Umberto for the 2022 season.[38]

See also

References
  1. "ABPW10 PGTW 231630". Joint Typhoon Warning Center. December 23, 2018.
  2. "WTPN21 PGTW 240900". Joint Typhoon Warning Center. December 24, 2018.
  3. "Prognostic Reasoning for Tropical Depression 35W (Thirty-five) Warning Nr 01". Joint Typhoon Warning Center. December 24, 2018.
  4. "Tropical Depression 35W (Thirty-five) Warning Nr 001". Joint Typhoon Warning Center. December 24, 2018.
  5. "Prognostic Reasoning for Tropical Depression 35W (Thirty-five) Warning Nr 02". Joint Typhoon Warning Center. December 24, 2018.
  6. https://www.webcitation.org/74xDS2bfu
  7. "Tropical depression Usman enters PAR; landfall seen on Friday". ABS-CBN News. ABS CBN News. December 25, 2018.
  8. "RSMC Tropical Cyclone Advisory TD". Japan Meteorological Agency. December 25, 2018.
  9. "Prognostic Reasoning for Tropical Depression 35W (Thirty-five) Warning Nr 05". Joint Typhoon Warning Center. December 25, 2018.
  10. "Prognostic Reasoning for Tropical Depression 35W (Thirty-five) Warning Nr 07". Joint Typhoon Warning Center. December 26, 2018.
  11. "Prognostic Reasoning for Tropical Depression 35W (Thirty-five) Warning Nr 11". Joint Typhoon Warning Center. December 27, 2018.
  12. https://www.webcitation.org/74yXiFPWk
  13. "Prognostic Reasoning for Tropical Depression 35W (Thirty-five) Warning Nr 13". Joint Typhoon Warning Center. December 27, 2018.
  14. "Tropical Depression 35W (Thirty-five) Warning Nr 015". Joint Typhoon Warning Center. December 28, 2018.
  15. "Prognostic Reasoning for Tropical Depression 35W (Thirty-five) Warning Nr 15". Joint Typhoon Warning Center. December 28, 2018.
  16. "Prognostic Reasoning for Tropical Depression 35W (Thirty-five) Warning Nr 20". Joint Typhoon Warning Center. December 29, 2018.
  17. "Usman makes landfall in Eastern Samar, weakens into LPA". Rappler. December 29, 2018.
  18. https://www.webcitation.org/7524bdgDe
  19. "Tropical Depression 35W (Thirty-five) Warning Nr 021". Joint Typhoon Warning Center. December 29, 2018.
  20. https://www.webcitation.org/7523JSiCM
  21. "97W INVEST". United States Naval Research Laboratory. 28 December 2018. Archived from the original on 1 January 2019. Retrieved 1 January 2019.
  22. "20 areas under Signal No. 1 due to Tropical Depression Usman". Rappler. December 27, 2018.
  23. "'Usman' intensifies; Eastern Visayas, Caraga at risk". SunStar. December 26, 2018.
  24. "Signal No. 1 raised in 11 areas due to 'Usman'". Philippine Daily Inquirer. December 27, 2018.
  25. Erwin Necavera (December 27, 2018). "At least 1,000 passengers stranded in Bacolod due to TD Usman". Philippine News Agency.
  26. "Signal no. 1 up in several areas as Usman nears Eastern Samar". ABS-CBN News. ABS CBN News. December 28, 2018.
  27. Cindy Ferrer (December 28, 2018). "'Usman' grounds 851 passengers in Western Visayas". Philippine News Agency.
  28. Mar Serrano (December 28, 2018). "Nearly 6K passengers stranded in Bicol ports". Philippine News Agency.
  29. Jazmin Bonifacio (December 28, 2018). "71 evacuation centers ready for displaced families in Tacloban City". Rappler.
  30. "'Usman' death toll rises to 16 in Bicol". Philippine Daily Inquirer. December 30, 2018.
  31. "Heavy rain from Usman leaves 3 dead". The Philippine Star. December 30, 2018.
  32. "Bicol execs blame climate change for 'Usman' deaths: Iba ang lakas ng ulan!". Politikol Bicol. December 31, 2018.
  33. "1 killed, 2 missing as Oriental Mindoro places 8 areas under state of calamity". GMA News. GMA News. December 31, 2018.
  34. "3 Bicol provinces under state of calamity due to Usman". Rappler. December 31, 2018.
  35. Sitrep No.26 re Preparedness Measures and Effects of TD USMAN (pdf) (Report). NDRRMC. January 20, 2019. Retrieved January 20, 2019.
  36. "PAGASA: No wrong forecast for Tropical Depression Usman". Rappler. January 16, 2019.
  37. Mavin Conde (January 8, 2019). "Wrong forecast worsened Usman death toll in Bicol – expert". Rappler.
  38. "Pagasa 'retires' 3 typhoon names". Manila Standard. March 2, 2019.
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