Bureau of Meteorology explains what's behind the weather this week

The warm weather and high fire danger will take a cool turn tomorrow as a cold front sweeps through the Northern Tablelands area.

Don't put your jumpers away just yet as the maximum temperature will drop each day this week and it will only get up to 12 degrees here in Walcha on Saturday.

A spokesperson for the Bureau of Meteorology (BOM) said a trough of low pressure is moving slowly through the far northeast of New South Wales, preceded by hot, dry northwesterly winds.

"A strong cold front approaching Tasmania moved through southern parts of the state today and will bring a much cooler air mass," he said.

"A low-pressure system is expected to develop over the Tasman Sea associated with this cold front, which will then bring cooler southwest to southeasterly winds across all of New South Wales for the remainder of the week.

"As the low moves away towards New Zealand a high currently approaching the Bight will move to the southern Tasman Sea, maintaining a moister onshore flow over the state later in the week."

There will be frost tomorrow morning in Walcha, Armidale and Guyra when minimum temperatures drop below 0 while maximum temperatures will hover around 20 degrees in most places, winds will be south to southwesterly 20 to 30 km/h turning southeasterly 15 to 25 km/h during the morning and early afternoon and there will be a high fire danger.

There is a high chance of showers on Friday and thunderstorms are predicted across the region.

Saturday looks to be the coolest day across the Northern Tablelands with a very high chance of showers and more than a 10-degree drop in maximum temperatures compared to yesterday's scorcher.

Overnight temperatures will fall to between 2 and 10 degrees with daytime temperatures reaching between 11 and 18 degrees.