KEY POINTS:
The outlook is brighter for North Islanders battered by thunderstorms, hail and severe rain. The MetService is about to boost the number of radars used to predict severe thunderstorms from four to seven.
Its annual report says that over the past 12 months, severe weather warnings were accurate 89 per cent of the time.
However, severe weather warnings cover dangerous weather affecting large areas of the country, not local thunderstorms.
MetService weather ambassador Bob McDavitt said severe weather warnings predict weather that would affect an area of about 1000 sq km.
"We're talking bigger than Auckland".
But the MetService wants to get better at predicting thunderstorms that cause only local damage. These thunderstorms might affect an area only 10km wide.
"Thunderstorms are nasty beasts," said Mr McDavitt.
"The damage they do is often around a small area, for example the hail in Auckland last week, where the damage really only affected a few people."
The MetService needed the new radars before it could provide thunderstorm warnings with the same accuracy as severe weather.
"We do quite a good job on a broad scale using satellite pictures and computer models.
"Thunderstorms are on a smaller scale and they're harder, so we need a good network of radar.
"We need a radar system that covers the whole country."
Taranaki will be the first region to get a new radar, helping predict storms such as those that brought tornadoes to the region in July.
Mr McDavitt said the Taranaki radar, expected to be built by mid-next year, would spot storms coming over the Tasman Sea.
"Taranaki sticks out like a sore thumb over the Tasman Sea.
"It's more open to collecting thunderstorms, and because it's further west they do more damage."
The other two radars, which cost the Government $10 million in total, will be installed in the Bay of Plenty and between Gisborne and Hawkes Bay over the next three years.
Mr McDavitt said the MetService was looking at ways of providing local thunderstorm warnings on radio and television as radar coverage improved.
"We don't want to panic everyone just because there's hail on the North Shore.
"But we do want people on the North Shore to stay inside, or maybe put off driving to the mall."
Mr McDavitt said the MetService tried to predict severe weather in time for people to change their plans.
"We aim to warn people 24 hours before conditions become severe.
"If we warn people as the event is just starting, we call it a late warning and that counts as a failure."
The MetService defines severe weather as more than 100mm of rain in a day or 50mm in six hours, windy gusts of about 110km an hour, or heavy snow.
Mr McDavitt said warnings were quite common for Northland and the Bay of Plenty, while Auckland got hardly any. Fiordland gets about four warnings a month.
As for the average day's weather, Mr McDavitt said day-to-day forecasts for the four main centres were pretty accurate.
"Auckland is regularly about 90 per cent accurate for the month, sometimes up around 100 per cent."
Wellington and Dunedin were about 85 to 95 per cent accurate, with Christchurch lagging on about 80 to 95 per cent.
Mr McDavitt said Christchurch was difficult because of variable weather conditions.
"The difference between a cold day and a hot day is just a squeak of an isobar."
