



Wasn't it supposed to be sunny?

We rely on them when choosing an outfit and to help us to decide whether or not to take an umbrella.

Probably more importantly, they help farmers, emergency services authorities and big businesses make informed decisions.

But how often do weather forecasters get it right?

Neil Bennett from the Bureau of Meteorology said improvements in technology had helped improve the accuracy of forecasts.

He said WA temperature forecasts were about 90 per cent accurate - "accurate" considered to be within three degrees above or below the forecast - and forecasts for rain were 85 to 90 per cent accurate

Mr Bennett said forecasts three or more degrees out were considered to be a small error and five or more were a really bad error.

"Plus or minus a degree or two, nobody really notices that," he said.

"The temperature accuracy is very high."

Rainfall accuracy is measured on a yes or no scale - either it rains or it does not rain.

Super computers produce much of the data on which weather forecasts are based.

The computers perform complex equations using a combination of information from around the world, including from satellites.

Forecasters then look at the computer-generated information and analyse what the data predicts.

Mr Bennett described forecasters as "value-adding" to the data created by the computers.

"They check if that information seems accurate and value add," he said.

While a lack of mountains makes forecasting slightly easier in WA than places which had to take topography into account, the state's huge size presents a challenge.

The analysis of all parts of the state is not as comprehensive as in the eastern states and there is therefore a lack of data.

Mr Bennett said seven-day forecasts were now possible but beyond that was more difficult to predict.

"People get annoyed that there is a seven-day forecast and the forecast for the seventh day may change but that's how it works, our computer models update the information," he said.

"Twenty years ago we were struggling to get to four-day forecasts, four was a stretch, now four-day forecasts are pretty accurate within three or four degrees and rainfall can be predicted to within a few hours.

"But as occurs, as we get better at forecasting, people's expectations get higher, people want more accuracy."

Mr Bennett said the bureau received a lot of emails from people complaining about weather forecasts - despite the predictions being "as good as any other state".

"We always reply, unless they are exceptionally abusive," he said.

"We try to explain why the weather was not as it was forecast, or in a lot of cases we have to tell them that the forecast they are referring to was not produced by us."

3 comments so far

  • I don't like the way that forecasts now give a percentage probability of rain from 5% to 95%. It looks like hedging bets.

    If they could man-up and say for example there will be 20mm of rain between 10am and 1pm it would be much more useful. Similarly 'showers' means nothing - just a vague term ranging from nothing to torrential downpours.

    However much worse are the ass covering climate forcasts which only give a probability of exceeding a long term mean. No matter what happens they are right - they just say it was one of the 'off' years.

    Date and time
    February 06, 2013, 12:09PM
    • You're assuming that weather forecasting is an exact science. Fact is, it's not. It is comparable to a "best guess estimate" to within a degree of certainty of what the LIKELY forecast weather will be. The chances of the actual weather encountered differing from the forecast increase exponentially the further the forecast period extends. People need to understand that forecasts issued by the Bureau of Meteorology aren't "instructions" for the weather to follow.

      G.L. Higginbottom
      Date and time
      February 06, 2013, 1:06PM
  • How many small errors can you make before you have to fix it? If i keep making small errors at work constantly I would be disciplined for it. I know its not an exact science but I do recall as a child to a teen that when the weatherman on the news said it will reach 35.3 today then it was 35.3. Now there does always seem to be that 3 degree margin and I think it makes a difference. Did we outsource somewhere or did someone retire?

    Date and time
    February 06, 2013, 8:57PM

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