On the afternoon 14 April 1999, a powerful hailstorm formed north of Nowra and moved north toward the city of Sydney in New South Wales, Australia. It was the costliest natural disaster in Australian insurance history, causing extensive damage along the east coast of New South Wales. The storm struck the city's eastern suburbs, including the central business district, later that evening.
The storm dropped an estimated 500,000 tonnes of hailstones in its path. The insured damage bill caused by the storm was over A$1.7 billion, which is equivalent to A$3.2 billion in 2022 when adjusted for inflation, with the total bill (including uninsured damage) estimated to be around A$2.3 billion, or A$4.3 billion in 2022. It was the costliest single natural disaster in Australian history in insured damage, surpassing the A$1.1 billion in insured damage caused by the 1989 Newcastle earthquake. Lightning killed one person during the storm, and the event caused approximately 50 injuries.
The storm was classified as a supercell following further analysis of its erratic nature and extreme attributes. During the event, the Bureau of Meteorology was constantly surprised by the frequent changes in direction, as well as the severity of the hail and the duration of the storm. The event was also surprising as neither the time of year, time of day, nor the general meteorological conditions in the region were seen as conducive to extreme storm cell formation.
The conditions around Sydney on Wednesday, 14 April 1999 were calm, although a slight instability in atmospheric conditions was recorded by the Bureau of Meteorology in the region. Two instability events had been identified in the greater Sydney area, but both were considered minor by the meteorological agencies. A weak cold front was moving north along the coast, and moderate precipitation was falling over the Blue Mountains, southwest of the city. The meteorological reports and figures, however, suggested that the general atmospheric conditions were "not conducive" to support the formation of a major thunderstorm in the region.
Historical records show that the formation of severe thunderstorms for the time of day and year had been rare, and it was improbable that they would maintain their intensity and cause significant damage. This long-standing belief contributed to the Bureau of Meteorology's decision not to issue warnings in the early part of the storm's development. This event was only the second time in recorded history that hail larger than 2 cm (0.8 in) had fallen in the Sydney metropolitan area in the month of April, and only the fifth hailstorm to strike Sydney during April in the 200 years of meteorological records for the city.
Hailstorms have had a history of significant damage in Australia. Since records on insured losses by the Insurance Disaster Response Organisation began in 1967, three other hailstorms – Sydney in 1986 and 1990, as well as Brisbane in 1985 – feature on the top-ten list of most insured damage caused by a single natural disaster, in addition to the 1999 storm. Hailstorms have caused more than 30% of all insured damage inflicted as a result of natural disasters in Australia during this period, and around three-quarters of all hailstorm damage has occurred in New South Wales.
The storm cell formed at 4:25 pm AEST to the north of Nowra, roughly 115 km (71 mi) south-southwest of Sydney. After forming, it initially headed towards the coast in a northeasterly direction. The cell passed just to the west of Kiama at around 5:15 pm and gained a 'severe' classification from the Bureau of Meteorology at the same time. 'Severe' is a classification used by the Bureau of Meteorology for thunderstorms which meet a specific criteria, namely producing hailstones with a diameter of 2 cm (0.8 in) or more, wind gusts of 90 km/h (56 mph) or greater and flash flooding, or tornadoes. This classification is also used by the Bureau to classify the attributes of a storm at any given time during its life.
The storm continued to move in a northeasterly direction, crossing the coast just north of Kiama at 5:25 pm. It was downgraded from a severe thunderstorm and proceeded to move further off the coast for another 15 minutes while gaining speed to around 37 km/h (23 mph). The storm then veered northward at 5:40 pm and continued parallel to the coast. Around 6:00 pm, directly east of Wollongong, the storm changed direction again, this time to north-northeast, and continued parallel to the coastline. Moderate hailstones were recorded falling in Wollongong as the western edge of the storm passed over the area, and the storm was reclassified as severe.
The storm moved parallel to the coast in a north-northeasterly direction for the next fifty minutes. It maintained a severe classification though did not impact heavily on the coastal suburbs, because it was entirely offshore. The western edge of the storm, however, recrossed the coastline just east of Helensburgh, 40 km (25 mi) south-southwest of Sydney, at about 7:00 pm. Ten minutes later the direction of the storm veered slightly more northward and the centre of the storm crossed back onto land at Bundeena at around 7:20 pm.
The Bureau of Meteorology had not issued warnings for Sydney Airport, located on the northern shore of Botany Bay, or the rest of the eastern suburbs to prepare for large hail. They were not expecting the storm to veer northward again, but rather to continue to head further out into the Tasman Sea in a consistent north-northeasterly direction.
After crossing the coast, the storm continued to move northward, crossing Botany Bay at 7:40 pm and reaching the Airport five minutes later. It travelled across the eastern suburbs between Botany Bay and Sydney Harbour between 7:45 pm and 8:05 pm, dropping massive hailstones on both houses and businesses in the eastern suburbs district and the central business district. Some of the largest hailstones ever recorded in the Sydney region fell on the eastern suburbs during this storm. There were reports of 13 cm (5.1 in) diameter hailstones in the eastern suburbs, although the largest confirmed hailstone was 9 cm (3.5 in) in diameter. It was the first time in 52 years that stones greater than 8 cm (3.1 in) had fallen in Sydney, with the last reported event being the 1947 hailstorm.
The storm continued across Sydney Harbour and changed direction slightly to be heading north. It weakened after travelling over the harbour, and was downgraded from a severe storm at 8:15 pm. The Bureau of Meteorology had concluded that the storm would weaken after heading across Sydney Harbour, believing it was dissipating and would therefore not produce any more substantial hail as it moved northward; therefore it did not issue warnings for the northern suburbs.
Northern suburbs and dissipation
The storm then continued north for twenty minutes over the North Shore suburbs of Sydney before regaining strength and veering north-northwest again, redeveloping severe thunderstorm characteristics. The storm's redevelopment again caught the Bureau of Meteorology off-guard, who had expected the storm to dissipate and move out to sea without causing further substantial damage.
It proceeded to drop large amounts of hail on the northern beach suburbs of Mona Vale and Palm Beach around 8:50 pm, and the centre of the storm again crossed the coast and back out to sea just after 9:00 pm. The storm maintained its intensity, however, and continued to move in a northwesterly direction across Broken Bay. The western edge of the storm had a minor impact on southern suburbs of the Central Coast between 9:15 pm and 9:30 pm.
The storm moved entirely off the coastline and into open water at around 9:45 pm. It then dissipated rapidly around 9:55 pm, directly east of Gosford. It was subsequently downgraded from severe status and the storm cell had faded completely by 10:00 pm.
A second, far smaller storm cell passed along a similar route to the first later in the evening of 14 April. This cell was never given the classification of 'severe' by the Bureau of Meteorology, nor did it develop into a supercell like its predecessor. Therefore, the route of the second cell was more direct and predictable than the first, following the general movement of the cold front (see conditions and climatology), and the Bureau of Meteorology issued warnings to all residents in the second cell's projected path to expect further storm activity.