South Island Met Agency (SIMA)

StormCast

StormCast is SIMA's convective forecast for the South Island. It is issued when significant thunderstorm activity is expected within the next 48 hours, with outlook maps and regional bulletins describing the thunderstorm risk, timing and likely hazards such as hail, downpours and damaging gusts.

How StormCast is produced

Forecasters assess the ingredients for thunderstorms: atmospheric instability, moisture, lift from fronts or daytime heating, and wind shear. Model soundings and high-resolution guidance are compared against current observations and satellite imagery to identify where storms are likely to initiate and how organised they could become.

StormCast maps are issued for up to two days ahead. Regional bulletins give more detail for the areas at risk, including time windows and the specific threats expected.

StormCast threat levels

Each region in a StormCast bulletin is assigned one of three threat levels.

  • Slight: conditions support some convective development. Any activity is generally brief and isolated.
  • Enhanced: conditions are more favourable for convective development, with a greater chance of thunderstorms.
  • High: conditions are highly favourable for thunderstorms, and any that develop could be stronger.

Thunderstorms in the South Island

South Island thunderstorms most often form in spring and summer when daytime heating of inland basins and plains combines with an unstable airmass, or along cold fronts moving up from the south. Canterbury, inland Otago and the Mackenzie Basin see the most frequent summer storms, while the West Coast can experience vigorous frontal thunderstorms at any time of year. Hazards include large hail, localised flash flooding from intense downpours, damaging downburst winds and frequent lightning.

Staying safe

If thunderstorms are forecast, keep an eye on the live radar, move indoors when thunder is heard, avoid sheltering under isolated trees, and never drive through floodwater. For official severe thunderstorm warnings, refer to MetService.

South Island Met Agency is an independent private forecaster and does not issue official government severe weather warnings. For official New Zealand watches and warnings, visit MetService. Read our About Us & Terms and Privacy Policy.