Demystifying weather forecasts

By Meg Humphrys (with advice and assistance from Alister Hawksford and the team at Bureau of Meteorology in Canberra)

Alice Springs Pastoral Industry Advisory Committee (ASPIAC) is the centre’s ministerially appointed group to advise the department on research, development and extension issues. The group aims to meet three times a year, and additionally contributes priorities to Meat and Livestock Australia.

For the May ASPIAC meeting, the group had a presentation from Alister Hawksford, the Director of Agriculture for the Bureau of Meteorology based in Canberra.

Alister informed the group about some of the common misinterpretations made about weather forecasts and gave listeners some tools to better understand the presentation of forecasts. To ensure that more producers in the region benefit we have summarised some of the key points below.

A seasonal forecast map is only useful, when used with a skill map.

A three month seasonal rainfall forecast map is only useful in conjunction with a skill map, which shows the accuracy of past weather predictions. The left hand map in Figure 1 is a forecast map, and the right hand map is a skill map. The triangle in Figure 1 represents a location in the Channel Country is placed in an area where the accuracy of past predictions is quite low, less than 45 per cent, and therefore too low to be relied upon. It would not be wise to base management decisions on the likelihood of rain using that forecast.

Alternatively, the star is in a region where the bureau has been right more times than not, 65 per cent or more, so we can have more confidence in using the forecast for decision making.

Forecast maps are available for detailed locations

It’s not commonly known that there are forecast maps for quite high resolution locations.

The steps to find them are:

  1. Go to the BoM homepage (www.bom.gov.au)
  2. Click on ‘Climate and Past Weather’ (under ‘Our Services’)
  3. Click on ‘rainfall outlook’ (at the top right of the page)
  4. Click on ‘chance of above median’
  5. Choose a month or period of time (for example – September)
  6. Click the magnifying glass on the right-hand side and type in the location. You may need to play around to find the one closest to you.
  7. The pop up box gives past accuracy (if you click on the accuracy term – moderate or high for example – it gives an exact percentage).
  8. To see a bar graph of rainfall predictions, click the month back or forward.

This can be done for rainfall and temperature as can be see down the left-hand side of your screen when in the function.

This function will actually be upgraded around the end of August so the functions may change slightly.

The “chance of rain” when interpreting the forecast is actually lower than you think.

Interpreting Figure 3 as “there is a 60 per cent chance of receiving two to six millimetres of rain” is incorrect. The forecast is actually saying that there is 60 per cent chance of getting any rain, which is defined as receiving over 0.2mm.

There is a 50 per cent chance of getting more than two millimetres or more rainfall and a 25 per cent chance of getting more than six millimetres or more. This prediction maybe useful for painting the house but not for predicting pasture growth.

For more information on how to interpret rainfall predictions, Alister suggested reading a blog post which can be found at the Bureau of Meteorology website.

One of the most effective ways that BoM displays rainfall data is in three hourly forecasts.

BoM’s three hourly forecasts are useful for when it looks like rain and there are rain sensitive farm operations to undertake.

This can be found by following the instructions below:

  1. BoM home page (www.bom.gov.au)
  2. Click on Meteye (right down at the bottom of the page)
  3. Select location (from the search box)
  4. Click on ‘see text view for location’
  5. The detailed three hourly forecast is then shown.

Find out more about weather and climate

For more information about weather and climate predictions and how that can inform your business we have some events running in the Alice Springs region over the coming months.

Alison Kain, the “Climate Mate” for the Barkly (and nominally for Alice Springs) will be presenting at the Old Man Plain’s Field Day on Thursday 26 September with Chris Materne.

Alison will also be running Climate Mate workshops in the region. The Alice Springs workshop is on Sunday 15 September.

Advertisements for these events are noted in this Rural Review edition.

Last updated: 23 September 2019

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