Hottest day in a year: Sydney to swelter before temperature plunge - Sydney Morning Herald

Baking ... Sydney is set to record its highest temperature in a year. Photo: Andy Zakeli

Sydney is set for a day of topsy-turvy weather.

The city is expected to swelter through its hottest day in more than a year, with the mercury forecast to hit 35 degrees by lunch-time, while parts of the west will climb even higher to 39 degrees.

But a gusty cool change is expected to sweep over the metropolitan area soon afterwards, sending the temperature plummeting by up to 15 degrees in the space of an hour and bringing the possibility of thunderstorms late in the day.

Weatherzone meteorologist Brett Dutschke said extensive cloud cover and northerly winds created very warm conditions in Sydney overnight, when the temperature dropped to a low of 22 degrees briefly at 3am.

But much of that cloud is expected to clear on Thursday morning, when north-westerly winds will make the day heat up very quickly.

"We're likely to reach 30 degrees shortly after 9am and even reach the mid-30s right across the city, including the beaches, before lunch-time," Mr Dutschke said.

"If we do get to 35 in the city, which is quite likely, it will be the hottest day in a bit over a year. It was November last year that we had a 37-degree day, and we haven't had a hotter December day in seven years."

Penrith and Richmond are expected to climb to 39 degrees by lunch-time, while in the north of the state temperatures will hit the low 40s.

But the mercury is expected to plunge as quickly as it rose.

"Lunch-time is going to be fairly interesting," Mr Dutschke said.

"A cooler change won't be too far away. It looks like reaching the southern beaches just before midday, moving through the city close to midday and getting as far north-west as Penrith and Richmond by about one or two o'clock this afternoon."

Thunderstorms are most likely in the west late in the day.

Dangerous surf conditions are expected on the NSW North Coast, where waves could reach up to three metres from this afternoon.

Surf conditions at Sydney's beaches are expected to be small on Thursday, before the swell increases tomorrow and into the weekend, particularly at north-facing beaches.

Mr Dutschke said conditions were set to return to average in Sydney on Friday, when a top of 24 degrees is forecast, before a forecast top of 26 degrees on Saturday and 28 degrees on Sunday.

Sydney is expected to hit a top of 30 degrees on Christmas Eve, while Christmas Day is expected to dawn cloudy, with a maximum of 27 degrees and isolated showers forecast.

Boxing Day is forecast to be cloudy with isolated showers and a top of 24 degrees.

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