Two record breaking Australian hailstorms: storm environments, damage characteristics and rarity

Australia's second largest city, Melbourne, experienced its worst hailstorm in 156 years of weather records during Saturday afternoon on March 6, 2010. Sixteen days later on March 22, the Western Australian capital city, Perth, experienced its most damaging weather event in 113 years, another severe hailstorm. Both storms produced giant hail – to 10cm diameter in Melbourne and 8cm diameter in Perth, extensive flash flooding and local wind damage. Climatologically neither of these two major Australian cities is a high risk for giant hail. However there were very unusual aspects to the environmental conditions for both storms.

Each hailstorm produced insurable damage over $1AUD billion with public infrastructure damage and community disruption doubling the total storm damage costs. Over 156,000 damage claims were received for the Perth storm and 130,000 for Melbourne's event. To illustrate the extreme nature of these storms, damage statistics from the Insurance Council of Australia since the 1960s show the largest and costliest thunderstorm related weather event for Melbourne occurred in 1982 with $AUD98 million (adjusted to 2007 dollars) damage. Historically Perth has had no major damaging hailstorms with the two largest weather events producing insurance claims (2007 dollars) of $AUD218 million (TC Alby,1978) with a 1994 winter-time low producing $AUD117 million damage.

An analysis of climate and newspaper records dating back 100 years or more have not revealed the occurrence of any other hailstorms close to the same magnitude as these two storms. Both cities have been inhabited across the areas affected by the storms for over 100 years. These records have identified major weather events, including extreme bushfires, severe winter storms, other severe thunderstorms and, for Perth, tropical cyclones, so some historical indication of storms of the size and intensity of these two severe hailstorms would be expected. It is concluded that these storms were more intense over a wide area than anything documented for the two cities. They were 5 to 10 times more damaging than the previous largest weather events experienced.

The Melbourne storm was a left moving supercell at the time it produced its largest hail. It formed in a prefrontal trough over the outer suburbs northwest of the city. Climatologically the low level moisture is insufficient for high precipitation supercells to form in these troughs. On this occasion near surface moisture from a 120 year ARI flood event over one thousand kilometers to the north was drawn southwards into this trough. A mature upper level cut-off low, uncommonly strong for March, completed the meteorological picture, producing a rarely seen storm environment for the Melbourne region. The storm rapidly moved towards the southeast and produced a large volume of small hail, closely followed by very heavy rainfall. Falls of over 40mm in 15 to 30 minutes generated extensive flash flooding. During this time the storm passed over the central city where large crowds had gathered for an annual street parade. Extensive flooding occurred to a depth of 1 metre in the worst affected areas. The exceptionally deep runoff was due to large quantities of hail stripping trees of their leaves which blocked street drainage. Heavy runoff then accumulated quickly down the paved streets leading to large water depths at the bottom of sloping roadways.

The storm changed into a left-moving supercell once it reached the inner southeastern suburbs of Melbourne. From here eastwards the hail size rapidly increased with the areas around the outer eastern suburbs of Lysterfield, Rowville and Ferntree Gulley experiencing 8 to 10cm hail. Localised severe wind damage also occurred in this region. The hail damage was greatest along several narrow, parallel swathes, 50 to 100m wide and around 3 km long with extensive hail damage to cars and major roof damage to houses.

The Perth event was a multi-cellular severe thunderstorm outbreak. The synoptic weather pattern was a surface west coast trough with a well developed middle level low centred to the west, an unusual combination of features for March. This provided the necessary instability, as indicated by the morning stability indices from the most representative radiosonde station of Geraldton - a Lifting Index of -4.1 and CAPE of 477 J/kg. Northerly steering winds, rare for Perth, were around 40 knots in the storm layer. The convection was triggered by strong daytime heating and low level convergence into the trough. The lead thunderstorm was the most intense, tracking southwards - first across a market garden region where crops were severely damaged, then across the densely populated northern suburbs. Here the storm had similar characteristics to the first stage of the Melbourne storm. Large quantities of small hail stripped trees and blocked drains. Then very heavy rain produced deep flash floods with water related damage being the major feature in this part of Perth. The storm continued southwards across the inner northwest then west of Perth. Largest hail fell over car dealerships in Osborne Park with hail to 8cm diameter, severely damaging thousands of cars. Giant hail continued to fall across western Perth and Nedlands, severely damaging one of Perth's largest hospitals and the state's oldest university. Stained glass windows over 80 years old were destroyed by hail of 6cm diameter. Extreme flash flooding produced land slides in the area with extensive damage being experienced at a university library and to a block of apartments. The strongest measured wind gusts were 120km/h but wind damage evidence points to likely maximum wind gusts near 150km/h.

A key question is whether the extreme storms were the result of natural climate variability or if there is also a climate change influence on top of this variability. Previous research work investigating severe hailstorms affecting the greater Sydney area (Leslie, Leplastrier and Buckley 2008) showed that severe thunderstorm activity is prone to large natural variability near Sydney and this may be the dominant factor at Melbourne and Perth. The research also showed a future climate change linked increase in frequency and intensity of hail storms for Sydney post 2010. The question remains open for Melbourne and Perth until detailed studies are completed.