The 1999 Sydney hailstorm was the costliest natural disaster in Australian insurance history, causing extensive damage along the east coast of New South Wales. The storm developed south of Sydney on the afternoon of Wednesday, 14 April 1999, and 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. Insured damage caused by the storm were over A$1.7 billion (equivalent to $ billion in ), with the total damage bill (including uninsured damage) estimated to be around $2.3 billion. It was the costliest single natural disaster in Australian history in insured damage, surpassing the $1.1 billion in insured damage caused by the 1989 Newcastle earthquake. Lightning also claimed one life 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 for extreme storm cell formation.
Conditions and climatology
The conditions around Sydney on Wednesday, 14 April 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. The 1999 event was only the second time in recorded history that hail larger than 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.
Development of the storm
Formation and southern Sydney
The storm cell formed at 4:25 pm AEST to the north of Nowra, roughly 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 or more, wind gusts of 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 . 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.