It's shaping up to be the coldest start to summer in more than 50 years.

If forecasts prove accurate - and Sydney stays below 23 degrees until Wednesday - it will be the coldest first week of summer since 1960. It's already the coldest in 44 years, Josh Fisher, a senior meteorologist at Weatherzone, said.

In the summer of 1960, each of the first 10 days was cooler than 22 degrees.

Meteorologists blame cold winds, sweeping up from near Tasmania, for the unseasonable weather.

Today's forecast temperature of 18 degrees is seven below the average for this time of year. The additional chill brought by projected 30-40km/h winds will make the city feel like 11 degrees, Mr Fisher said.

Sydney should warm to the mid-20s on Friday and next weekend, the eight, ninth and 10th days of the month, Weatherzone meteorologist Brett Dutschke said.

But it won't be a sunny reprieve. The Bureau of Meteorology is forecasting rain and cloud.

The city's long-term average maximum for this time of year is 25 degrees, Mr Dutschke said.

Mr Fisher also said summer would be cooler than average because of the influence of the La Nina weather cycle, which brings with it greater chance of clouds, rain and humidity.

"Looking further ahead, the summer as a whole is likely to be close to or cooler than average, regarding maximum temperatures. We will still get our hot days but La Nina will increase the chances of extra cloud, humidity and rainfall, hence cooler daytime temperatures," Mr Dutschke said.

Thunderstorms rolled across the city late yesterday morning and early afternoon, cooling most suburbs below 17 degrees, well below average for this time of year. The storms also brought brief rain and hail to some western and northern suburbs. Picnickers and beachgoers were sent scurrying.

Weatherzone.com.au is owned by Fairfax Media, publisher of this website.

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