THE weather bureau has revealed Day Seven of its long-range forecasts is wrong most of the time.

The bloopers include a "mostly sunny" outlook one week out from the disastrous Christmas Day hail storms.

"Isolated showers" were the long-range forecast for February 4 last year - the day Melbourne was swamped by flash flooding.

The 40 per cent accuracy rate for Day Seven temperatures is less than what the Day One forecast was 50 years ago, according to data compiled for the Herald Sun.

Victoria faces soaring temperatures today, with a total fire ban declared in the Wimmera, South West and Central districts.

Melbourne is set to hit 37C, with higher temperatures expected in the west.

Weather bureau spokeswoman Andrea Peace has defended the use of seven-day forecasting, but admitted the uncertainty increased dramatically from the four-day mark.

"We use the main global models that are considered to be the best, and there can still be days where even for tomorrow they can all give conflicting results," Ms Peace said.

"The need is still there but people have to understand that it's a guide, it's an outlook and there's a strong possibility that it will change as you get closer to the day."

"Severe weather" was forecast closer to Christmas Day, but thousands of Melburnians were caught out by the storms, with hailstones and flash flooding causing tens of millions of dollars in damage.

Ms Peace said it was difficult to determine the severity of thunderstorms 24 hours out.

The figures show Day One forecasts are more accurate than ever with an 85 per cent strike rate in 2011.

And the number of forecast failures - an error margin of 5C or more - was just three last year, 10 times fewer than in 1962.

The Day One error margin has halved in 50 years to just over 1C, while the Day Seven forecast averaged a 2.5C error last year.

Ms Peace said technological advances had combined to hone forecasts over the years.

"As we get better computing power, the size of the grids is going to get smaller and smaller, so the computer models will be able to resolve smaller, more localised weather phenomena," she said.

Originally published as Forecast cloudy on long-range vision