The state typically cops about 109 severe thunderstorms in any year, with about 90 per cent of them striking during the period from now until March, according to Julie Evans, a senior meteorologist with the Bureau of Meteorology.

The suburbs of Sydney most likely to be hit by such events include an arc of suburbs in the south including Menai, Bangor and Woronora Heights and western regions such as St Clair, Castle Hill and Blacktown, according to insurance claims data from NRMA released during StormSafe Week.

Storms can form as moist air rises over mountains or as westerly winds hit sea-breeze fronts, Ms Evans said, adding that the dynamic conditions make it difficult to predict when one of the state's 1000-odd storms per year becomes severe.

"The lead time for severe thunderstorms is generally less than an hour," Ms Evans said, adding that the coming season is likely to see an average number extreme events.

Storm watchers examine 3D radars for evidence of how high a storm's load is rising, while another telltale signal is if one of the storms in a group starts to deviate from others. "Often it's slightly to the left," Ms Evans said. "That's the one you should be watching more closely."

The presence of El Nino-like conditions in the Pacific also means that NSW – and much of south-eastern Australia – faces the prospect of a more active bushfire season than usual for the coming summer.

The Bureau of Meteorology on Tuesday updated its El Nino outlook, retaining its estimate of "at least a 50 per cent chance" of such an event in 2014. El Ninos years often see below-average rainfall over inland regions of southern and eastern Australia.

That forecast comes even though much of Sydney and the state's coast have been relatively wet over the past month following a dry start to the year.

"It will be a bad fire season because the rain we've had recently just adds to the undergrowth and the fuel load of the fire," Mark Morrow, Acting Commissioner of the State Emergency Service, said.

There is also the possibility of lightning strikes during dry thunderstorms, which can spark bushfires.

Driest month

September is Sydney's driest month in a typical year and one of its sunniest. Temperatures should reach 20 degrees or warmer on all days except Friday over the coming week, with little rain forecast by the bureau.

Across the state, the most storm-affected towns are along the Mid-North Coast, such as Port Macquarie, Boambee East, Toormina and Sawtell, along with Tamworth in the north, according to the NRMA.

The SES "is expecting a lot of bushfires", Mr Morrow said, adding that any precautionary steps households take against fires will also serve them well in severe thunderstorms.

These include clearing trees over properties and clearing out gutters of material that can ignite in a blaze but also leading to water build-up and flooding of homes in a heavy downpour, he said.

Storm damage accounted for almost half of all home insurance claims in NSW during the last financial year – with 77 per cent of those made during the October-March period, according to the NRMA.