Massive hail hit many parts of Sydney and the Central Coast yesterday as a number of supercell thunderstorms swept through the region, with the total damage bill from the event likely to be in the billions. Conditions were ripe for severe thunderstorm development, with temperatures pushing into the mid 30s and moisture levels similar to that of Darwin in the wet season, leading to huge amounts of available energy.
On top of this heat and humidity, there was very strong vertical wind shear available as moist onshore winds and a modified southeasterly change at the surface backed to strong westerly winds in the upper levels. This allowed for the updrafts to tilt and rotate, and combined with the massive amounts of energy available and also drier air in the upper atmosphere, produced very powerful updrafts capable of carrying massive hailstones.
After a sultry afternoon, the first cells stormed into the Sydney Basin from around 4pm, with the first of the supercells producing hail officially recorded to 8cm in the Liverpool and Casula areas at around 4:30pm, although by the time this cell reached the city hailstones were only reported as large as 2cm.
A secondary supercell quickly developed over Northern Sydney just after 4:30pm and rapidly intensified as it moved towards the Central Coast. 8cm hail in diameter was reported in the Berowra and Brooklyn areas from this storm, before it moved over the Central Coast causing hail as large as tennis balls at Woy Woy and golf balls at Gosford. Hail was large enough that it came in through the roof at Erina Fair, whilst many emerged from the area showing a spectacularly rotating storm.
A third supercell homed in on the city suburbs at around 6pm, producing the city's biggest hail since the 1999 hailstorm, with Rose Bay, Paddington and Surry Hills reporting hail 6-7cm in diameter.
At the same time, a fourth supercell was hitting the Camden to Cabramatta region, with hail reports of 6-8cm in diameter although photos indicate stones could have been even larger than this to softball size around Camden (unoffical 9-10cm).
Other multi-cellular cells then swept through Sydney, producing dangerous lightning and marble to golf ball size hail over the Blue Mountains and western Sydney including Penrith.
What was remarkable about this event was the spatial distribution of the hail across Sydney. In a typical large hail event, there would be no more than 1-2 supercells and the hail swathe would be in a relatively small area. This event saw a significant part of the Sydney Basin and Central Coast record big hail, which may mean the event could be the costliest since the 1999 hailstorm and will likely be in the billions.
The largest official hail at 8cm in diameter (although photos indicate larger hail) also makes it one of the largest hail sizes recorded in Sydney, but still smaller than a number of other events including the 1947 hailstorm, 1999 hailstorm and December 2007 hailstorm.
The run of severe thunderstorms is over in the city, with cooler southeasterly winds likely for the next few days before hot and dry conditions develop over the Christmas and New Years period.
Image 1: Sydney Radar - Severe Hailstorms - 21st December 2018