A Severe Heatwave Warning has been issued for Northern NSW.
A Severe Heatwave Warning has been issued for Northern NSW. Contributed

Heatwave warning: mercury to hit 42 degrees

SOARING temperatures have prompted the Bureau of Meteorology to issue severe heatwave warnings for the Northern Rivers, for today and next week.

The mercury hit 40 degrees at Casino and 39 degrees at Lismore today, with surrounding regions also sitting in the high 30s.

Weather Zone meteorologist Graeme Brittain said soaring temperatures were caused by a low pressure trough moving north eastwards across the state.

"North westerly winds are moving hot air from the centre of Australia towards the coast," Mr Brittain said.

"The winds are not particularly strong, but the heat will increase the severity of the fire warnings in the next couple of days."

Thunderstorms are likely tonight with between 20 and 40 mms of rain falling until tomorrow, when a further 10 - 20 mms are expected.

"You definitely will get lightening with these storms, but hail is not looking as likely," Mr Brittain said.

"It's not possible to know how much lightening."

A brief respite in temperatures is expected on Monday, but from Tuesday temperatures are set to climb again.

A severe heatwave warning for the Northern Rivers will be re-issued from Tuesday to Friday with up to 42 degrees expected at Casino.

The heatwave a state-wide event with parts of west Sydney also expected to reach temperatures in the early 40s next week.

The Bureau defines a heatwave as a weather pattern with three or more days of unusually high maximum and minimum temperatures in any area.