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Through Natural disaster, Atmospheric pressure and State of emergency

Pam formed on 6 March, east of the Solomon Islands and tracked slowly in a generally southward direction, slowly intensifying as it did so. Two days later, the disturbance reached tropical cyclone intensity and, over subsequent days, Pam gradually strengthened before reaching Category 5 cyclone status on both the Australian and Saffir–Simpson scales on 12 March. The next day, Pam's sustained winds peaked at 250 km/h (155 mph) as the storm moved through Vanuatu, passing near several constituent islands and making direct hits on others. On 14 March, Pam's winds began to slowly weaken, but its pressure dropped further to a minimum of 896 mbar (hPa; 26.46 inHg) before rising shortly afterwards. Over the next few days, the cyclone's weakening accelerated as it moved poleward. On 15 March, Pam passed northeast of New Zealand before transitioning into an extratropical cyclone that same day.

Early in Pam's history, a damaging storm surge impacted Tuvalu, prompting a state of emergency declaration after 45 percent of the nation's residents were displaced. Torrential rainfall occurred in the southeastern Solomon Islands, particularly in the Santa Cruz Islands. In Vanuatu, all emergency centres were activated and relief personnel were put on standby with Pam assessed as having the potential to be one of the nation's worst tropical cyclones. Catastrophic damage occurred as the storm moved through the archipelago, particularly in Efate, the location of the Ni-Vanuatu capital of Port Vila; and the Tafea islands of Erromango and Tanna. The cyclone crippled Vanuatu's infrastructure: an estimated 90 percent of the nation's buildings were impacted by the storm's effects, telecommunications were paralysed, and water shortages continue to plague the small nation. Pam later brought heavy winds and rough surf to New Zealand's North Island during its weakening stages.

Meteorological history

During the opening days of March 2015, a major westerly wind burst occurred, which subsequently contributed to the development of the 2014–16 El Niño event and the development of two tropical cyclones near Kiribati on either side of the equator. The first tropical cyclone became Tropical Storm Bavi and impacted the Marshall Islands, the Mariana Islands and Philippines. The second system was first noted as Tropical Disturbance 11F by the Fiji Meteorological Service (FMS) on 6 March, while it was located about 750 kilometres (465 mi) to the north-east of Honiara in the Solomon Islands. The system was located underneath an upper-level ridge of high pressure and within an area favourable for further development with low-moderate vertical windshear. As a result, weather forecast models anticipated the development of a significant tropical cyclone over the coming days. Initially, the disturbance floundered east of the Solomon Islands and slowly strengthened, reaching tropical depression intensity on 8 March. The storm's appearance and areal coverage of showers remained stationary until the following day, when the formation of rainbands wrapping around the centre of the system prompted the FMS to upgrade the storm's classification to a category 1 tropical cyclone on the Australian tropical cyclone scale, assigning it the name Pam. Atmospheric conditions at the time were slightly favourable for continued development as the storm continued to slowly track along the southern periphery of a high-pressure area to its north.

Following the storm's naming, Pam began to curve southwards around midday on 9 March. Computer models continued to point towards the possibility of rapid intensification occurring as the cyclone approached Vanuatu. Significant development in Pam's organisation took place throughout the remainder of the day into 10 March. The cyclone's circulation centre quickly tightened, with the central dense overcast atop it persisting in strength. At 18:00 UTC on 10 March, the FMS upgraded the system to category 3 strength, making it a severe tropical cyclone. Shortly after, microwave imagery revealed a primordial eye-feature developing within Pam. This became apparent on visible light images on 11 March. That day, Pam became quasi-stationary east of the Santa Cruz Islands before resuming its prior southwesterly motion towards the end of 11 March. The storm's eye continued to warm as its cloud tops cooled such that at 12:00 UTC, the FMS assessed Pam to have reached Category 5 intensity on the Australian cyclone scale. Six hours later, the JTWC estimated that the storm reached Category 5-equivalent intensity on the Saffir–Simpson hurricane wind scale as Pam was east of Penama.

Early on 13 March, the JTWC determined Pam reached its peak one-minute sustained winds of 270 km/h (170 mph) as it neared Vanuatu; this was increased to 280 km/h (175 mph) in post-season reanalysis. Several hours later, the cyclone began to curve towards the south-southeast, allowing Pam to pass just east of Efate. At that time, the FMS estimated Pam as having record-breaking 250 km/h (155 mph) ten-minute sustained winds. The storm's winds gradually slowed afterwards as Pam tracked west of Tafea. However, the FMS indicated that the cyclone's pressure dropped further to a minimum of 896 mbar (hPa; 26.46 inHg) on 14 March, making Pam the second most intense tropical cyclone in the South Pacific basin after Cyclone Zoe in 2002. This intensity was short-lived, however, as Pam's central pressure began rising shortly thereafter as the storm accelerated southeastward. After 12:00 UTC that day, Pam left the area of responsibility of the FMS and entered the monitoring region of New Zealand's MetService, who estimated that Pam weakened to Category 4 intensity on 15 March after maintaining Category 5 intensity for 36 hours. Shortly after, the storm's eye faded away and Pam's low level circulation became displaced from its associated thunderstorms, signalling a rapid weakening phase. Later on 15 March, both agencies discontinued issuing advisories as Pam entered a phase of extratropical transition while affecting northeastern New Zealand. The system moved eastwards, and eventually dissipated over the waters of the South Pacific on 20 March.

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