BOM app update: How to decipher the new precipitation forecasts

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The Bureau of Meteorology (BOM) has updated the way it communicates rainfall forecasts on its official weather app and the change has sparked a wave of confusion.

So why the change, what do all the new numbers mean and how does it compare to what’s used in the rest of the world?

According to BOM forecaster Jonathan How, the change happened because the previous format was often misinterpreted.

“One of the biggest misunderstandings of the previous format was that the probability of precipitation and the amount of precipitation that came with it were very often misread,” he said.

“For example, if there was a 70% chance [of any rain]and the amount of rain was 1 to 3 millimeters, which would often be misinterpreted as a 70% chance between 1 and 3 mm.

“But really, the 70% chance refers to the chance of seeing any measurable precipitation, where “all measurable rain” is at least 0.2 mm.

“Then the range was the 50% and 25% decile, which wasn’t very well communicated,” he added.

The old format

In order to make the forecast, meteorologists run a series of model runs using Australian and international weather models.

The forecaster then brings their expertise and local knowledge to weight these models appropriately.

At the end of this process, there is a range of possible outcomes.

How to communicate what this wide range of possible outcomes is most likely to mean, succinctly to the audience, is where things get even trickier.

Forecast for Melbourne on Monday, August 22, from the Bureau of Meteorology website.(meteorological office)

The old methodology used by the BOM, and still in use on its website at the time of publication, communicates a precipitation range where the first digit represents a 50% chance of at least this amount of rain that occurs.

The second number represents a 25% chance of at least this amount of rain that occurs.

A good way to understand what these percentages mean is to imagine the whole range of possible precipitation results lined up from lowest to highest.

So in this example there is a 50% chance of rolling 15mm or more and a 25% chance of rolling 20mm or more.

Using this reporting method, even if the forecasts turn out as the models and forecasters expect, the reported range only covers 25% of the possible outcomes.

There is always a 50% chance of precipitation below 15mm and a 25% chance of more than 20mm.

To reiterate, when looking at the website forecast, half the time there will probably be less rain than the lowest number listed as possible precipitation.

What has changed in the app?

BOM application opening screen
The BOM app opening screen shows the new precipitation range, Monday August 22, 2022.(meteorological office)

The new format expands the range of possible precipitation results reported to the public and eliminates the “risk of any rain”.

This time, the first number represents a 75% chance that at least that amount of rain will occur.

This is different from the old format still used on the website, where the first number has at least a 50% chance.

While the second number represents a 25% chance of at least that amount of rain occurring, the same maximum value as in the old format.

Again, taking the example of the Monday forecast for Melbourne on August 22:

This new range, perhaps more intuitively, covers 50% of the possible results.

So in this example the forecast suggests there is a good chance of around 9-20mm of precipitation in Melbourne on Monday.

BOM app screenshot
The precipitation range in the app is the 75% chance of at least and the 25% chance of at least the values ​​shown below. The 50% chance of having at least one value should be somewhere in between. Monday August 22.(meteorological office)

Further down, the app now also expresses the 75, 50, and 25% chance of at least values ​​directly as three separate numbers.

“We had this feedback that people are missing the chance of rain. But the idea is that all that chance of rain was often misinterpreted,” Mr How said.

“This method hopefully gives people a better picture of what to expect.”

App and website do not match

According to How, the BOM also plans to update the website to use the new format, but the timeline is uncertain.

In the meantime, there’s no getting around the fact that the website and app communicate different rainfall ranges.

Screen Clipping BOM Application: Rain, 75% (high) chance of no rain, 50% (medium) chance of no rain, 25% (low) chance of less than 1mm
Rainfall forecasts from the new app can be difficult to decipher when there is very little rain on the forecast. In this example, only about 25% of the possible results suggest there will be less than 1mm. Or colloquially, he might spit a little.(meteorological office)

Mr. How admits that these discrepancies could be confusing.

“We did radio crossovers and articles like this just to help people understand why we made the changes and why for some time before the website update there will be this discrepancy,” said he declared.

“So if people are aware of what they’re watching, that should be fine. On the website, it’s still a convoluted way to get the explanation, but that will change over time once the new site Web will be integrated.”

What are they doing in the rest of the world?

Precipitation is one of the hardest things to predict.

Not only are there many different factors that can impact whether it rains, for how long, and how much, but rainfall can also be incredibly random geographically.

Coupled with the fact that it’s the easiest thing for the public to notice and measure, it’s a minefield for weather agencies.

Many agencies around the world do not report daily forecast rainfall totals as part of their seven-day online forecast equivalents.

Opting instead to stick to the chance of rain and forecast words and symbols to indicate the intensity.

In the UK, the Met Office sticks to a word forecast to indicate whether showers are likely, with a daily breakdown of the change in rainfall per hour. No quantified total of rainfall forecasts is advanced.

Screenshot of the Met UK website showing their forecast table for the week ahead.
The UK Met Office’s seven-day forecast may have an hourly probability of precipitation, but it does not take into account how much rain could fall.(UK Met Office)

In the United States, the National Weather Service lists the probability of precipitation for each day and night alongside a word forecast, sometimes indicating possible precipitation amounts in detailed forecasts.

In Japan, they show the probability of precipitation over six-hour periods for the next two days and as daily totals for the rest of the week.

No probable precipitation totals are given, but they indicate normal precipitation totals for that time of year.

In New Zealand they have an indication of how much rain is likely to occur, but they approach it quite differently from what we do in Australia.

In the next 72 hours, they have a more detailed forecast table that shows the expected rainfall in mm/hour for each hourly period.

Screenshot of the New Zealand Meteorological Services forecast grid
The NZ Met Service provides forecast rain totals by the hour for the next 72 hours.(New Zealand Weather Service)

“The difficulty with this is that it provides a very precise response both in terms of weather and intensity, but the weather is not always so predictable,” according to Lewis Ferris, communications meteorologist at the NZ Met Service.

“Another problem with this type of forecast is that when rainfall is patchy/showery, the graph may not show anything, but our text forecast will show showers. This creates an inconsistency between our products.”

For forecasts four days ahead, the online forecast gives a “probability of precipitation exceedance”.

Screenshot of the New Zealand Meteorological Services forecast grid
From four days in advance, the NZ Met Service advises people of the likelihood of more than 1mm and 10mm of rain.(New Zealand Weather Service)

This is the chance of getting more than the set values ​​of 1mm and 10mm using the model set data.

For the record, many agencies have more information hidden in the depths of their websites.

The Australian Office also has more detailed forecasts available online and on the app.

Detailed forecast information table
In the publicly accessible MetEye section of the BOM website, you’ll find a wealth of more detailed forecast information.(meteorological office)

In the MetEye section of the BOM website, there are three-hourly breakdowns of the 10, 25 and 50 “percent chance greater than” rainfall forecasts, in mm.

In the app, scrolling down also shows this three-hour breakdown where “percentage chance of at least” can be selected.

Ultimately, the uncertainty around precipitation forecasts is high and communicating the range of possibilities is difficult. Forecasts are never more than a guide.


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