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.
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.

Meteorological history

On December 23, the Joint Typhoon Warning Center began monitoring on a tropical disturbance that had developed about 657 km to the east of Palau. Despite located in an area of moderate vertical wind shear, sea-surface temperatures were recorded at 28-30 °C, which is warm enough to support development. The disturbance slowly intensified and the JTWC issued a Tropical Cyclone Formation Alert on 09:00 UTC of December 24. At this time, the convection around the system's low-level circulation center was rather sheared, however the Dvorak estimates were around a T1.5, signalling that the system had sustained winds of 25 knots. This prompted the JTWC to upgrade the disturbance to a tropical depression, giving the designation of 35W. By 21:00 UTC of the same day, visible imagery showed shallow banding wrapping into a compact, but exposed LLCC, with poorly organised convection. On 06:00 UTC of December 25, the Japan Meteorological Agency declared the system to be a weak tropical depression. 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.
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. Usman remained disorganised as the system's LLCC became ragged, with deep convection being sheared to its northwest. 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. 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. Thereafter, convection had increased around the system, and the system's center became much more organised. On December 28, the JTWC increased the system's 1-minute sustained winds to 55 km/h. 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. On December 29, the system's central cold cover began to dissipate, and the JTWC had stated that the system began weakening and deteriorating. 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. The JMA stated that the system was located in an area of increasing wind shear. Several hours later, both the JMA and the JTWC issued their final advisory on the system. 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.

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. 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. 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. Heavy rains that could trigger landslides and flashfloods were expected in the areas under the alert signals. Roughly 1,000 passengers later became stranded in Bacolod after sea travel was suspended. The region of Western Visayas, which includes the island of Palawan, was raised a TCWS #1 on December 28, as 851 passengers were stranded in that region alone. 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. At this point, the number of passengers stranded from the cause of suspended sea travel rose to 13,000, where 6,000 of them were from the Bicol Region. 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.
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. 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. 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. In Bicol, 473.1 mm of rain was recorded for the two days of December 28 and 29. 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. 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. Eight other municipalities in Oriental Mindoro were also raised a state of calamity by local officials on the next day.
Throughout the country, a total of 238,127 families were affected by Usman. About 36,574 houses were either partially or totally damaged. 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, where ₱1.95 billion were to be part of agricultural damages and ₱3.46 billion were to be part of infrastructure damages.

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. 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. 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. 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. They also added that they did release a weather advisory on the low-pressure area on December 29 regarding that "moderate to heavy rains will continue over Bicol Region in the next 24 hours."
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.