FAQ - Rainfall Forecast Terminology

Introducing new forecast terminology

We are changing the way we talk about rain on our website, to help people make sense of our rain forecasts.

Example of forecast with new terminology

Example of forecast with new terminology

  1. What is changing in the text forecast?
  2. What does the new terminology mean?
  3. Why are the text forecasts changing?
  4. When is the forecast changing?
  5. What does the percentage chance of rain mean?
  6. Why does the percentage chance of rain on the right not always match the value on the left on the town and city forecasts on our web site?
  7. What does the possible rainfall amount mean?
  8. Where can I get more detailed forecast information for my location?

What is changing in the text forecast?

We are replacing terms that people found ambiguous—such as isolated, scattered, patchy and widespread—with clearer terminology in many of our routine forecasts.

Instead, in the short forecast on the left, above, if there is a chance of rain, we now use words like possible or likely.

In the full text forecast on the right, above, we describe the chance of rain as slight, medium, high, or very high, with a percentage equivalent. And, if rain is only expected for part of the day, we’ll say when.

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What does the new terminology mean?

The new terms tell you how likely you are to receive any rainfall on that day. This scale is shown in the table below:

Chance of rain Terminology used
0%, 10% No mention of rainfall in forecast.
20%, 30% Slight (20%) chance of…
Slight (30%) chance of…
40%, 50%, 60% Medium (40%) chance of…
Medium (50%) chance of…
Medium (60%) chance of…
70%, 80% High (70%) chance of…
High (80%) chance of…
90%, 100% Very high (90%) chance of…
Very high (near 100%) chance of…

 

If necessary, the forecast text will include other useful information, such as the possibility of heavy rain, timing of any rain and other significant weather such as thunderstorms, fog, snow and winds.

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Why are the text forecasts changing?

As the Bureau’s weather information becomes more detailed and precise, we want the language of our forecasts to follow suit.

People have told us that terms such as patchy, isolated, scattered or widespread are confusing. They just want to know how likely it is to rain, and how much rain is expected if it does.

The new terminology makes our forecasts clearer and easier to understand to better complement our graphical services.

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When is the forecast changing?

Our standard routine forecasts are changing in early October 2014.

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What does the percentage chance of rain mean?

The percentage shows the likelihood of rainfall in that location during the period, which may be a full day or three hours. For example, if the chance of rain for Mildura is 30%, there is about a one in three chance of getting wet. A 30% chance of rainfall also means a 70% chance of not receiving any rainfall at all.

For more information on how to interpret the chance of rain percentage, have a look at this blog: http://media.bom.gov.au/social/blog/209/right-as-rain-how-to-interpret-the-daily-rainfall-forecast/

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Why does the percentage chance of rain on the right not always match the value on the left on the town and city forecasts on our web site?

For some locations, the detailed forecast on the right side of the page is for an area, such as major metropolitan areas. So, we provide the average chance of rain across that whole area.

On the short forecast on the left side, the percentage chance of rain is always for a single location within the larger area forecast, so it can be different. For example, the Sydney forecast is for the central business district which could be quite different to that of a suburb in western Sydney.

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What does the possible rainfall amount mean?

Possible rainfall

The rainfall range helps to show how rainfall might vary according to the type of weather. Both numbers relate directly to a distinct chance of receiving that amount if it rains.

The first number (5 mm in this example) represents a 50% chance of that amount occurring (that is, there is a 50% chance of getting 5 mm if it rains).

The second number represents a 25% chance of that amount occurring (that is, there is a 25% chance of getting 10 mm if it rains).

On days where we expect showers or thunderstorms, the possible rainfall range might be quite large (for example, 5 to 30 mm). When steady rainfall is expected over a wide area, the range might be smaller (for example, 10 to 15 mm).

This is the most likely rainfall range, though some locations in the area may still get rainfall amounts outside of the range, due to showers being very localised.

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Where can I get more detailed forecast information for my location?

For more information, visit MetEye; the weather map system that enables you to look at weather forecast maps and tables in more detail.

For local weather on your smartphone, try our mobile website (m.bom.gov.au) which gives forecasts for the next 24 hours in detail for your location, so you can check the latest information before going outside.

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