Summer is well and truly here as Sydney braces itself for a scorching week - but not before the chance of a thunderstorm or two this afternoon.

Temperatures are forecast to hit 40 degrees in Sydney's west in the coming week, reaching a peak on Thursday.

Temperatures in greater Sydney are expected to hit 40 degrees next week. Credit:James Alcock

Penrith is forecast to reach 37 degrees on Wednesday and 40 degrees on Thursday, while Parramatta can expect 35 degrees on Wednesday and 36 on Thursday.

Sydney's CBD will reach tops of 32.