Temperatures are expected to reach 40C inland on Wednesday, prompting the meteorological department to issue a yellow weather warning for extreme heat.

The warning is in force from 1pm until 3.30pm, with maximum temperatures forecast to peak at around 40C in inland areas.

A seasonal low-pressure system continues to affect the region, while a more unstable air mass is expected to move in from Friday, bringing an increased chance of showers and isolated thunderstorms.

Wednesday will be mostly clear, although periods of increased cloud are expected during the day. Isolated afternoon showers cannot be ruled out, mainly over mountainous areas.

Winds will initially be light and variable at around force 3 Beaufort before gradually turning southwesterly to northwesterly at force 3 to 4, reaching force 5 at times along the south coast. The sea will be slight.

Temperatures will reach 40C inland, 31C on the west coast, around 34C along the remaining coastal areas and 30C in the higher mountains.

Overnight, the weather will remain mostly clear, with some low cloud developing locally. Winds will gradually become northwesterly to northeasterly, while south-easterly winds are expected along the north coast, remaining light at force 3. The sea will be calm to slight.

Minimum temperatures will fall to around 22C inland, 24C on the coast and 20C in the higher mountains.

Thursday will remain mostly clear, with occasional increased cloud and a chance of isolated afternoon showers, mainly over the mountains.

On Friday and Saturday, conditions are expected to become more unstable. While mornings will be mostly clear, cloud is forecast to build during the afternoon, bringing isolated showers and thunderstorms, mainly over mountainous and inland areas.

Temperatures are expected to remain largely unchanged on Thursday before dropping on Friday and Saturday to levels close to the seasonal average.