About Forecast Rainfall

Contents

• Official rainfall forecasts
• Automated computer model rainfall forecasts
• Forecast rainfall
• Chance of rainfall
• Total forecast rainfall
• Combining models
• Forecast rainfall accuracy

Official rainfall forecasts

How do meteorologists create the official Bureau forecasts?

The whole of Australia is divided up into squares 6km long and 6km wide (in Victoria and Tasmania, squares are 3km by 3km) and every square across the nation gets a 7-day forecast! In creating these official forecasts, Bureau meteorologists utilise many computer models, weighing up which ones are 'on track' before using their output, then editing the computer model output accordingly as their experience dictates.

How can I find the official 7-day forecast for my location?

The Bureau's official 7-day forecast for any location can be found using MetEye (for larger screens) or the BOM Weather app (for smaller devices). MetEye allows you to select the location(s) you are interested in by typing in either the name of your location, or the postcode, or the latitude/longitude, or clicking at the right point on a map. For example, see this forecast for the small town of Shelford. Choosing the 'Detailed 3-Hourly Forecast' option shows even more information available in the Bureau's Shelford forecast.

I'd like some more help using MetEye. Where should I look?

As well as the 'Help' and 'FAQ' resources available in MetEye, the following resources will help you unlock all of its useful features.

• MetEye Video
• MetEye Simple User Guide
• Ten tips for a detailed weather forecast

The official rainfall forecast for my location has three numbers in it. What do they mean?

An explanation of the rainfall probability and the rainfall amounts in official forecasts can be found here.

Automated computer model rainfall forecasts

The Water and the Land (WATL) web site provides rainfall forecast maps generated automatically by weather forecast models. Models run by the Bureau of Meteorology and by five international centres are combined to produce the rainfall forecasts using the method described below in Combining Models. Testing has shown that it easily outperforms a random guess or climatology, and also the forecast from any individual weather model, to at least five days ahead. This does not mean it is perfect - no forecast system ever will be - but it does mean that people can be confident that on average, the forecast will supply them information to help shape their decisions.

The forecast maps have been prepared on a grid roughly 25 km by 25 km. Because rainfall is seldom uniform over such an area (particularly when the rain is falling as showers or thunderstorms, or when local topography is influencing the location of the heavier falls), you should treat the totals as a guide only. Sometimes rainfall patterns can be missed by the combined computer model weather forecasts, for example when showers are just on the coast. At other times the models may overestimate rainfall. If using these automated rainfall forecasts, it is important that you cross check them with the Bureau's official forecasts.

The rainfall forecast maps offer daily totals, and chance of rain, five days ahead. Maps of 4-day totals for the next 1-4 days, and 5-8 days are also available. The 24-hour rainfall forecasts are updated twice a day at approximately 8 am and 8 pm EST. The 4-day total maps are updated at midnight.

The 24 hour rainfall forecast period ends at 12 UTC (ie.10 pm EST, 9:30 pm CST or 8 pm WST). For example, Tuesday's rainfall will be the amount for rain expected for the period from 10 pm EST Monday evening to 10 pm EST Tuesday evening.

Forecast rainfall

The amount of rain forecast for each day is presented on a national colour coded map, with options to zoom in to state and then district level. Rainfall totals less than 1mm are not shown. The colours represent ranges of forecast rainfall, for example the lightest orange represents a forecast range from 1mm to 5mm

Chance of rainfall

The chance of rain is calculated using the proportion of available weather forecast models predicting rain at or above the given threshold, and the average rainfall amount predicted by the models. Empirical equations have been developed that relate the chance of rain to these quantities, and the equations adjusted so that they are well calibrated when compared to recent observations. This means that, over the past few months when the forecast probability has been 30%, for example, rain above the relevant threshold has been recorded on 30% of occasions.

Very rarely, when too few models are available, the chance of rain cannot be calculated reliably and a map with "No data available" will be shown.

Total forecast rainfall

The maps show the total amount of rain expected for the week ahead in two time blocks, days 1- 4 and days 5 - 8. The forecasts for the first four days are expected to be more accurate than days 5-8. Remember that, as the total forecast rainfall maps are updated at midnight, between 8 pm and midnight there might sometimes be slight differences between the sum of the daily maps and the total rainfall maps.

If a rainfall forecast for any one of the four days is unavailable, then a map with "No data available" will be shown.

Combining models

Numerical Weather Prediction (NWP) or computer models included in the calculation of the rainfall totals and the chance of rain are from the:

• Australian Bureau of Meteorology
• US National Oceanographic and Atmospheric Administration
• UK Meteorological Office
• Japanese Meteorological Agency
• European Centre for Medium Range Weather Forecasting
• Meteorological Service of Canada

The models vary in the length of time of the forecast and also the grid size, or the distance between points, inside them. Some computer models only provide forecasts out to 3 days ahead while others extend to 10 days ahead.

Most of the models produce a single prediction of the rainfall for each point and time. Two models from the European Centre for Medium Range Weather Forecasting are used, however. One of these also produces just one prediction per point and time while the other, the Ensemble Prediction System (EPS), produces a set of 51 forecasts for each point and time, allowing it to represent the range of probable outcomes. The EPS forecasts are particularly useful for probability forecasting.

The models and base times, in UTC, used in our rainfall prediction maps are as follows:

• * previous day's model run
• UTC = Coordinated Universal Time

Forecast rainfall accuracy

Rainfall forecasts are updated over the forecast period becoming more accurate as the date approaches.

View example of forecast change over 5 days.

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