Lighter winds are being forecast for many parts of the country today.

WeatherWatch.co.nz says clouds are thickening over New Zealand ahead of the big Tasman Sea low moving in on Tuesday, Wednesday and Thursday.

"The cooler then normal southerly flow will continue in the east but warmer than normal weather in the west will combine over inland areas making for unstable cloud build ups and afternoon downpours through the South Island in particular.

"There is some chance of a few isolated inland thunderstorms."

By tonight, WeatherWatch says many places will be overcast as the low moves closer from the west.

"Winds will be slowly turning more northerly or north easterly in many places by tonight too."

Images: WeatherWatch.co.nz