By Eloise Gibson, NZ Herald

Auckland is about to have its fourth wet weekend in a row – if you believe the forecasts.

The stormy season is upon us, but just how stormy depends on whom you listen to.

Two voices are regularly heard telling people what to expect from their weekends.

Philip Duncan, the head forecaster at The Radio Network-owned www.Weatherwatch.co.nz, uses overseas data and a network of rain radar owned by the MetService to give an alternative forecast – sometimes in more colourful terms than the Government-owned forecaster.

Last night he was predicting a “weather bomb” to the east of the country which was likely to make its presence felt today in the Bay of Plenty, East Cape, Gisborne and northern Hawkes Bay.

Weather ambassador Bob McDavitt explains things from the MetService point of view – often with different results.

The Weekend Herald looked at their forecasts over the last three weekends and found Weatherwatch seemed more willing to pinpoint specific times and worse property damage from storms. That meant it was easier to tell when a forecast was wrong, as both services sometimes were.

When Auckland missed out on the gale-force winds predicted last weekend, the forecasters explained it in different ways.

Mr Duncan said forecasts were out because the tropical low shifted course at the last minute, while Mr McDavitt explained that a blanket of low air pushed winds up and over the city.

McDavitt said the MetService would have needed to buy another weather balloon in Tauranga to pick up on the air block. “It’s about how much money you spend on weather observations.”

Both services publicise severe weather warnings from the MetService.

When it comes to making other predictions, Weatherwatch buys weather data from a single source overseas, while MetService buys from several overseas sources then chooses the best.

Unlike Mr McDavitt, Mr Duncan is not a meteorologist but said Weatherwatch proved you did not need to be.

“The data comes in a form that is very easy to write a forecast from … whether you’re a meteorologist or not, gut instinct comes into it.”

He agreed Weatherwatch was ready to go further with its forecasts, for example describing an electrical storm as “potentially fatal” when the MetService probably would not.

He said Weatherwatch was more focused on providing a news story, while MetService had to meet set criteria before it would issue a severe weather warning.

“If we see what looks like a tropical storm coming, even 10 days out, we’ll talk about it.”

Mr McDavitt said MetService issued heavy rain and wind warnings if it was “pretty sure” it was coming in the next 24 hours.

Explaining why thunderstorms and tornadoes never arrived in Auckland after being forecast two weeks ago, he said 99.9 per cent of people in a large area with the right conditions for severe thunderstorms would not notice anything. “But the [potential] severity is enough that we have to warn the 0.01 per cent.”

Thunderstorms were harder to spot in advance than ordinary wind and rain, said Mr McDavitt. “The bubbly clouds which produce thunderstorms only have a lifetime of an hour or so, whereas heavy rain clouds last a day or so.”

Mr Duncan said MetService severe weather warnings were “exceptionally accurate”. But he believed his land-based forecasts gave more detail to enthusiasts.

 

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