About Forecast Rainfall

Introduction

The Water and the Land (WATL) web site provides rainfall forecast maps generated automatically by weather forecast models. Up to eight models 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, 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.

It is important that you check your local Bureau weather forecasts regularly in conjunction with the rainfall forecasts given in the WATL pages. Weather forecasters provide supplementary information about the distribution, onset, duration and intensity of the rainfall as well as the likelihood of thunderstorms and other severe weather. 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. Weather forecasters use their experience to determine the most likely outcome from a wide variety of different sources.

The forecast maps have been prepared on a grid roughly 50 km by 50 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.

The forecasts are also available from the Bureau's FTP site.

Schedule
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 the proportion of available models predicting rain at or above the given threshold, expressed as a percentage.

The advantage of using multiple models to determine rainfall is the ability to estimate the chance (otherwise termed probability, or likelihood) of receiving rain. For instance, if seven of the eight models believe at least 10 mm will fall, then the probability of receiving at least 10 mm will be listed as 7/8, or 88%. Likewise, if only one model thinks there will be 10 mm, then the chance of at least 10 mm will be 1/8, or in other words, 13%. No particular model is favoured. Sometimes there may be a "chance of rainfall" when the total expected is less than 1 mm (and is not marked on the rainfall total maps).

Not all models are available out to five days. Hence the 'chance of rainfall' information available for the later days - especially very high values - should be viewed with caution. When only two models are available, if both models agree that there will be rainfall then the chance of rain will be 2/2 or 100%. If there is disagreement the chance will be 1/2 or 50%. In this case there are no intermediate colours in the maps. See the example below. Rarely, when only one model is available, the chance of rain cannot be calculated and a map with "No data available" will be shown.

Sample image of chance of rain, showing colour use for 2 or more models

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.

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

Country or region

Computer model name

Grid size in kilometres
(approximate)

Days forecast

7:40 am EST issue

6:30 pm EST issue

Australia

ACCESS-R

12

3

12 UTC

00 UTC

Australia

ACCESS-G

40

10

12 UTC

00 UTC

United Kingdom

UKMO

55 x 83

6

12 UTC

00 UTC

United States

NCEP GFS

50

7.5

12 UTC

00 UTC

Canada

CMC

60

6

12 UTC

00 UTC

Europe

ECMWF

12.5

10

12 UTC*

12 UTC*

Japan /p>

JMA

25

9

12 UTC

12 UTC*

  • * previous days model run
  • UTC = Coordinated Universal Time

References

BNOC Operations Bulletin Number 102 - Upgrades to the Operational PME System

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Check your local forecast

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Forecast rainfall accuracy

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

About rainfall

Keeping your own rainfall records

© Commonwealth of Australia 2008, Bureau of Meteorology