As the BOM's Christmas outlook nears, here is the truth about forecast accuracy
The BOM will release its official Christmas Day forecast seven days out when accuracy becomes better. (Paul Miller: AAP)
The countdown is on until the holidays and some weather sites are starting to offer their first look at the forecast for Christmas Day and into the New Year.
They're tempting to look at. Especially when your plans hinge on the weather.
But just how far into the future is a forecast accurate?
According to meteorologists, the limit is a week to two weeks at best. But even within that range, it can be pretty touchy.
This Christmas is a prime example.
Two weeks the 'absolute outer limit'
Michael Logan is the general manager of national product services with the Bureau of Meteorology.
He said, generally, the accuracy of temperature forecasts at different time scales looked like this:
How often the temperature is within 2 degrees Celsius of what is forecast by BOM
- 1 day out: More than 90 per cent of the time
- 3 days: 85 per cent
- 5 days: 75-79 per cent
- 7 days: 65-69 per cent
- 7-10 days: Accuracy decreases further. Models are able to pick up some large-scale patterns in the weather.
In other words, it's reasonable until a week out for temperature. After that, the accuracy falls away a lot.
But Mr Logan said it was possible to get a more general forecast as far as 14 days ahead for some regions where the weather patterns were more stable and slow-moving.
"So 14 days is generally accepted as the absolute outer limit at the moment," he said.
"But as you go from seven through to 14 days, then the skill really does drop away in terms of actually being able to say what it's going to be at the daily."
Over in the UK, meteorologist Aiden McGivern from the country's main forecasting agency, the Met Office, put it in the language all of England understands.
"It's kind of like trying to predict the winner of the World Cup when we're only still in the group stages,"he said.
"[Between 8 and 14 days] we can say useful things about likelihoods, risks and extreme outliers and most likely outcomes. But we can't give specifics."
There are some types of weather, seasons and regions that are easier to predict at that further range than others, though, according to both the BOM and the UK Met Office.
"It tends to be the bigger the system is, the more accurately and further ahead you can forecast it," Mr McGivern said.
Mr McGivern said beyond two weeks, a daily forecast was no better than a guess based on the long-term average.
Some types of weather, like large, slow moving high pressure systems, are more predictable than others. (Supplied: Evie Batt)
Early forecasts for Christmas this year are a good example of how uncertain the forecast can be beyond a week.
BOM's first official forecast for Christmas will be released on Thursday, December 18.
But some early forecasts have already started to come through on other sites. In just five days, some have changed a lot.
Take Perth, for example.
On Thursday last week, two weeks out from Christmas, one site said Christmas Day would be about 25C.
The next day, a different service said it would be 32C.
By yesterday, that same site had upgraded its forecast to 37C.
Why a forecast gets murkier the further out it goes
To predict the weather, computer models simulate the current state of the weather across the globe, and then step it forward.
Melbourne weather station receives its first photos from the TIROS-8 satellite in 1964. (ABC News)
But the fact that we do not know precisely what the weather is doing across every cubic metre of the world means there are small errors in the starting information.
The further out you go, the bigger those errors grow.
This is why the BOM moves toward saying what is "likely" on the longer time frames, rather than putting a specific number on it.
Before: The BOM's long-range outlook is predicting warmer than average temperatures from January to March 2026.. . After: Rainfall outcomes between January and March are less clear.. .
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"By the time you get to [two weeks] it's pretty hard to have skill at the daily timescale and provide a meaningful daily forecast," Mr Logan said.
"That's why we don't want to go down that path.
"[Instead we] very much start to looks at trends."
How come some weather sites offer a month-long forecast?
There are some weather sites that will give you a daily forecast much further than 14 days.
US-based site AccuWeather, for example, is one of the more popular sites that does offer a daily forecast for three whole months.
Unsurprisingly, it's also a feature that brings in a lot of traffic to their website, according to AccuWeather chief meteorologist Jonathan Porter.
Mr Porter said these forecasts were based on a combination of information from weather forecast models and ensembles, historical data and meteorological expertise where possible.
While many do not think it is useful to give specific daily forecasts complete with temperature and rainfall figures because accuracy is so poor, Mr Porter disagreed.
"There are models that are made available by various governments all around the world that go out months in advance," he said.
"Our vision was, why should meteorologists be the only ones that have access to that type of data?"
Mr Porter admitted their accuracy was not as good as the shorter timescales, though he stopped short of putting a number on it.
He believed they were still useful, though.
"If you're looking at a particular day a week or two or more in advance, has the forecast been trending to be wetter? Has it been trending to be drier? Has it been trending to be cooler or warmer?" he said.
When used like this, it is not dissimilar to the rationale of the BOM with the long-range outlook.
Huge improvements to forecast accuracy
While the seven-day forecast of today may not seem like much, it is a massive improvement on what the forecast looked like just two decades ago.
Before 1999, the BOM only predicted the weather three days ahead.
It was not until 2006 that the forecast was extended to seven days.
This graph goes some way to showing how that skill has improved at different timescales.
Forecast accuracy seven days out never used to be accurate but now is, according to Our World in Data. (Supplied : Our World in Data/ECMWF)
The solid line is the Northern Hemisphere, while the dotted line is the Southern Hemisphere.
The seven-day forecast, for example, is now at a point where it has moderate accuracy. But before the 2000s, it was not even useful.
The biggest reason for that improvement has been in technology. We've now got faster computers, more observations and — crucially — satellites, which give meteorologists a view of the world they simply didn't have before.