A blast of cold air slated to spread across the South Island tomorrow will also kick off a few thunderstorms as it starts to push on shore.

Stewart Island and Fiordland run the greatest risk for seeing thunderstorms this afternoon and tonight.  Heavy rain, winds gusts up to 110 km/h and hail 10-15 mm in diameter are possible.  To put that into perspective, the diameter of marble is about 15 to 25mm.  Coastal Southland and the West Coast run a moderate risk for thunderstorms where small hail and wind gusts up to 110 km/h are possible.

The rest of Southland, as well as Otago, Nelson and the Kapiti Coast all run a small risk for thunderstorms.  Small hail and wind gusts up to 110 km/h are possible in these areas as well.

The thunderstorm threat for coastal Southland will continue into the overnight hours.  The threat for Nelson and the Kapiti Coast will increase to a moderate risk for the overnight hours as the atmosphere further destabilizes in response to the combination of the advancing blast of cold air.

Homepage image/ Ben Hughes

By WeatherWatch.co.nz Analyst Howard Joseph