Some severe storms hit St Clair during the afternoon with hail, very heavy rain and some powerful winds causing blackouts and traffic chaos. The storms developed over the Blue Mountains southwest of Sydney and moved rather quickly towards the Sydney region. A severe thunderstorm warning was released by the Bureau of Meteorology for storm winds large hailstones and heavy rainfall. Although the hail size reached the severe threshold of 2cm in some suburbs, the more significant threat were the winds. The worst damage seem to occur in the suburb of Whalan with debris and trees also reported in Glendenning and St Clair. Downburst winds known as microbursts descend rapidly from thunderstorms and can sometimes accelerate and spread out at the surface. Winds of about 100km/h resulted in trees being felled and branches stripped from trees across the suburbs.
After a parched summer, the region was still saturated from heavy and persistent rainfall in previous weeks. Severe runoff was quite widespread and some roads underwater.
The storms moved towards the coast and seemed to weaken as they progressed northeast to Gosford and Newcastle.