Today is Sydney's hottest July day in 19 years

Sydney residents stepped out of their front doors into unseasonably warm air today - the city's warmest July day in 19 years.

Camden and Mascot reached 24.7 and 25.5 degrees respectively, said weatherzone.com.au meteorologist Brett Dutschke.

"It's been at least 13 years since Penrith reached 26.6 degrees, Homebush 25.0 and Canterbury 24.6 in July. This is nine above average for Penrith," he said.

Bureau of Meteorology forecaster Dmitriy Danchuk said today's "amazingly warm" conditions were the combination of a broad low pressure trough heading through from the west and a cold front moving across western parts of NSW.

"Because it's been a pretty steady and deep north-westerly stream [ahead of the trough] it's brought a warmer and drier air mass to Sydney," Mr Danchuk said.

Blue Mountains residents also enjoyed an unseasonably warm morning at 10 degrees just before 7am. A week ago the temperature did not climb above 8 degrees all day.

Mr Danchuk said the cold front would hit Sydney tonight, returning the city to its regular July pattern by tomorrow.

"There's a chance of a late shower and [temperatures] will reach 17 degrees," he said.

And if you are wondering about a dip in the ocean, Sydney's water temperature is around 17.5 degrees.