Severe storms have unleashed tornadoes and hail across parts of NSW and dumped more than a month’s worth of rain in one day on Sydney in a deluge that trapped commuters and sparked flood rescues in the city.

The massive rainfall event during what is usually NSW’s driest month was summoned by a low-pressure system fed by an abnormally warm Tasman Sea, which has stewed at temperatures 2 degrees above average for months.

The polar vortex spiralling over Antarctica is also rapidly weakening, an atmospheric factor which can send more rain-bearing low-pressure systems and cold fronts charging over Australia, according to forecaster Weatherzone.

The intense low-pressure system off NSW sent 90km/h winds barrelling for the coast on Wednesday and into the early hours of Thursday, causing flash fl

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