MetService has revealed it only expects to accurately forecast each day’s maximum temperature 75 per cent of the time.

The existence of an internal performance measure comes after The Dominion Post tracked MetService’s forecast for every day last month to test the forecaster’s accuracy.

The information was compiled using forecasts and data made public in the newspaper and on the MetService website.

MetService would not discuss our findings – and threatened to charge for the release of its March forecast details.

March 7 was the hottest day of a miserable month in the capital, with 3.4 hours of sunshine, southerlies and no rain. All fairly unremarkable – except it was nothing like what had been forecast five days earlier.

Despite the hours spent poring over computer models, forecasters still struggle to get it right in a country where the weather is notoriously difficult to predict.

As rival forecaster WeatherWatch.co.nz head analyst Philip Duncan puts it: “The problem with forecasting in this country is that the shape is so narrow. So anything that comes to us from the north or south can very easily miss us and that’s where our biggest storms come from.

“It’s sort of like balancing an egg on a roof – it usually comes down one side or the other, it very rarely comes straight down over the top of us.”

MetService has 55 forecasters, many with decades of experience.

The state-owned enterprise’s annual report reveals it only aims to correctly forecast the daily maximum temperature, within 2 degrees Celsius, 75 per cent of the time.

For the year to June last year its success rate was 76 per cent.

It also has a hit rate of just 75 per cent for correctly forecasting rain, which it exceeded by 8 per cent last year.

MetService’s heavy rain warnings were correct 94 per cent of the time, severe gales 89 per cent and heavy snow 82 per cent.

But get forecasts badly wrong and people start baying for blood.

Mr Duncan said he received hate mail most weeks.

“In some ways I accept that we can always improve in our accuracy, but there are always going to be people who don’t read things through properly so they jump to conclusions.”

MetService produces two-day, five-day and 10-day outlooks and spokesman Daniel Corbett conceded that, the further out the forecast, the harder it was to predict.

Up to five days out, forecasters combined their own knowledge and experience with computer-assisted models.

The private Auckland-based WeatherWatch buys data from meteorologists and scientists, which its three weather analysts translate into layman’s terms.

It did away with 10-day forecasts, which Mr Duncan said should be taken with a grain of salt.

Photo by Zelda Wynn

By Bronwyn Torrie- The Dominion Post