1999 Sydney Hailstorm - Conditions and Climatology

Conditions and Climatology

The conditions around Sydney on 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 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 damages caused by a single natural disaster, in addition to the 1999 storm. Hailstorms have caused more than 30% of all insured damages 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.