Severe Tropical Cyclone Harold was a very powerful tropical cyclone which caused widespread destruction in the Solomon Islands, Vanuatu, Fiji, and Tonga during April 2020. It was the first Category 5 tropical cyclone in 2020. The seventh named storm of the 2019–20 Australian region cyclone season, eighth named storm, and fourth severe tropical cyclone of the 2019–20 South Pacific cyclone season, Harold was first noted as a developing tropical low within a trough of low pressure during April 1, while it was located to the east of Papua New Guinea. Over the next day, the system moved south-eastwards over the Solomon Sea, before it was classified as a tropical cyclone and named Harold by the Australian Bureau of Meteorology. The system moved into the Fiji Meteorological Service's area of responsibility on April 2 and began to explosively intensify by April 3, reaching Category 4 status by April 4 on both scales. The next day, it further strengthened into a Category 5 severe tropical cyclone, the highest rating on the Australian scale. It made landfall on Espiritu Santo on April 6 and the next day, strengthening to its peak intensity, attaining Category 5-equivalent intensity on the Saffir–Simpson scale before making landfall on Pentecost Island. Moving east, it weakened below Category 5 intensity on both scales over subsequent days. It regained Category 5 severe tropical cyclone status (on the Australian scale only) while passing south of Fiji, before weakening and becoming extratropical on April 9.

Harold was the first Category 5 severe tropical cyclone to occur in the South Pacific basin since Cyclone Gita in 2018 and was also the second-strongest tropical cyclone to ever affect Vanuatu, behind Cyclone Pam in 2015. In total, 27 people died on the MV Taimareho (in the Solomon Islands), two died in Vanuatu, and one died in Fiji. Total damage from the cyclone amounted to $768 million (2020 USD).

On April 1, the Australian Bureau of Meteorology (BOM) reported that Tropical Low 12U had developed along a trough of low pressure about 825 km (515 mi) to the northeast of Port Moresby in Papua New Guinea. The disturbance initially displayed a broad low-level circulation and scattered atmospheric convection, but it was located within a favourable environment for further development, with developing dual-channel outflow, low vertical wind shear, and warm sea surface temperatures of 30 °C (86 °F). Thunderstorm activity began to coalesce about the centre later that day, and a nearby upper-level low directed the system southeast toward the Solomon Islands. Given the increase in organization, the BOM upgraded the storm to a Category 1 tropical cyclone on the Australian tropical cyclone intensity scale and named it Harold, before it passed about 135 km (85 mi) to the southeast of Honiara in the Solomon Islands. The United States Joint Typhoon Warning Center (JTWC) also initiated advisories on Tropical Cyclone Harold during that day and designated it as Tropical Cyclone 25P. The system subsequently crossed 160°E, where it moved out of the Australian region and into the South Pacific basin, which prompted the BoM to pass the primary warning responsibility to the Fiji Meteorological Service (FMS).

As the storm moved away from the Solomon Islands on April 3, it began to rapidly organise and developed a pinhole eye on microwave imagery. As a result, it was classified as a Category 3 severe tropical cyclone by the FMS at 00:00 UTC on April 4. At this time, Harold was located about 360 km (225 mi) to the northwest of Luganville in Vanuatu, and its southeast movement had slowed significantly as a result of an extension of a subtropical ridge to the storm's east. Later that day, the FMS reported that Harold had become a Category 4 severe tropical cyclone and predicted it to further intensify to Category 5 status, while the system's eye became subsumed into a large mass of atmospheric convection. The JTWC, meanwhile, assessed that Harold had peaked with 1-minute sustained wind speeds of 215 km/h (134 mph), which made it equivalent to a Category 4 hurricane on the Saffir-Simpson hurricane wind scale (SSHWS). During April 5, the JTWC reported that the system had begun to weaken, based on the ragged structure of the eye and warming of the cyclone's cloud tops. However, this weakening phase was short-lived as the system regained a 30 km (20 mi) eye and a symmetric central dense overcast, while upper-level outflow remained strong. The FMS subsequently reported that Harold had become a Category 5 severe tropical cyclone with 10-minute sustained winds of 205 km/h (127 mph). At this time, the system was located about 170 km (105 mi) to the west of Luganville and began to move polewards as a subtropical ridge to its northeast became the primary steering mechanism.

At around 22:00 UTC on April 5 (10:00 FST, April 6), Harold made landfall on the island of Espiritu Santo in northern Vanuatu, with 10-minute sustained winds of 215 km/h (134 mph). Harold quickly strengthened upon reemerging over water; early the following day, the FMS reported that Harold had peaked in intensity with 10-minute sustained winds of 230 km/h (140 mph), while it was located between the islands of Espiritu Santo and Pentecost, while the JTWC estimated that its 1-minute sustained winds reached 270 km/h (170 mph), which made it equivalent to a Category 5 hurricane on the SSHWS. The system subsequently made landfall on Pentecost, before reemerging into the South Pacific Ocean and weakening. The landfall weakened Harold and the storm began to show signs of weakening: its eye became cloud-filled, cloud tops warmed, convection reduced in extent, and dry air started to wrap into the system. As a result, the FMS reported that the system had weakened into a Category 4 severe tropical cyclone during April 7, before Harold's eye and eyewall became visible on their radar in Nadi. The system subsequently passed within 115 km (70 mi) of Suva in Fiji, before it passed near or over the Kadavu Group of islands. During April 8, the system intensified slightly before the FMS reported that Harold had become a category 5 severe tropical cyclone again, while it was located about 300 km (185 mi) to the north-east of Nuku'alofa on the Tongan island of Tongatapu. As the system continued to move south-eastwards, it passed about 115 km (70 mi) to the south of Nuku'alofa, as it started to weaken and transition into an extratropical cyclone. During the following day, Harold weakened into a Category 3 severe tropical cyclone as it moved into New Zealand's MetService area of responsibility, while strong vertical wind shear caused atmospheric convection to become displaced to the south of its eye. The JTWC subsequently issued its final advisory on Harold, as it was expected to gain frontal characteristics and complete its extratropical transition within 12 hours. MetService subsequently declared Harold to be an extratropical cyclone during April 10, before the system was last noted during the following day around 1,500 km (930 mi) to the southwest of Adamstown in the Pitcairn Islands.

After Harold developed into a tropical cyclone during April 2, the Solomon Islands Meteorological Service issued a tropical cyclone warning for the whole of the island nation. They warned that the system was expected to produce gale-force winds, rough seas, moderate to heavy swells of 2–4 metres (6.6–13.1 ft) and coastal flooding over the islands. As a result, the SIMS urged sea travellers to consider taking actions to ensure their safety and advised motorists to use extreme caution when travelling. The Royal Solomon Island Police Force recommended elevated caution for commuters and mariners.

An estimated 100,000 to 150,000 people in the Solomon Islands experienced Harold's wind, rain, and storm surge. The National Emergency Operations Center indicated that 57 houses were destroyed and another 20 were damaged. Seven classrooms were also destroyed. Falling trees and branches in Honiara resulted in widespread power outages. The National Referral Hospital in Honiara was among the buildings affected by blackouts. Some fallen trees damaged buildings and blocked roads. The San Isidoro school for the deaf outside Honiara was severely damaged, losing its roof. Heavy rainfall flooded the Kukum Highway in Honiara. A stream swollen from the rains washed out a 3 m (9.8 ft) segment of a bridge connecting parts of northwestern Guadalcanal to Honiara. Dozens of families were forced out of their homes on Guadalcanal. Flooding and fallen trees inflicted damage to buildings in Rennell and Bellona Province as well.

On the night of April 2, the ferry MV Taimareho encountered rough seas and gusts of up to 80 km/h (50 mph) generated by Harold in Ironbottom Sound while en route from Taivu to the Aiarai in West Are 'are in Malaita Province. The ship had been sent to evacuate Malaita residents from Honiara as a precautionary measure, though warnings were in effect advising ships to remain in port. Twenty-seven of the 738 passengers were knocked overboard by the waves and are presumed dead; Taimareho later took refuge in Su'u Harbor in Malaita Province. Planes and ships were dispatched to search for survivors across an area larger than 1,000 km² (390 sq mi). Rescue efforts were initially hampered by the inclement conditions; one rescue helicopter could not fly as its second pilot was quarantined. Two bodies were found off the southern coat of Malaita Province, localizing the search region; five further bodies were recovered on 6 April. The Solomon Islands government launched two investigations into the maritime incident, involving a formal inquiry conducted by the Solomon Islands Maritime Safety Administration and a criminal investigation. At least two ships were washed ashore by the storm. Heavy rains from Harold washed out much of the plains in Guadalcanal, reducing the area's food security. The government of Australia donated US$60,000 in relief funds to the Solomon Islands.