Wondering whether to pack a poncho or sunhat for carols? Locking in plans for a picnic or pool party on 25 December?
Some media outlets are already offering a Christmas Day forecast – nearly three weeks out from the festivities – but is it too early to rely on the festive weather forecast?
Christmas Day is weeks away, can we predict the weather yet?
Any weather forecast more than 10 days ahead is basically “rubbish”, says Dr Martin Jucker, an atmospheric scientist and senior lecturer at the University of New South Wales Climate Change Research Centre.
The Monash University meteorologist Dr Michael Barnes agrees. “A day ahead, we’re pretty sure. Seven days ahead we’re not so sure. Beyond seven to 10 days, we have no idea.”
Barnes, who is based at the ARC Centre of Excellence for 21st Century Weather, says future weather conditions are sensitive to the current state of the atmosphere, so predictions become less accurate the further out in time you go.
When can I start making weather-dependent plans?
“Everyone’s hanging out for the Christmas forecast,” says Jonathan How, a senior meteorologist at the Bureau of Meteorology.
It’s still too early, he says, advising people to check in on 18 December.
In the meantime, the BoM’s long-range forecast gives a sense of the overall conditions for December and summer. “What it’s showing is that most of Australia can expect a warmer and wetter December. It doesn’t mean that Christmas Day is more likely to be warm and wetter for everyone.”
Where you live can also affect the reliability of the festive forecast, How says. In the west and north it’s pretty consistent; in the south and east things can change quite quickly.
“We do know that summer in Perth is pretty consistently warm and sunny,” he says. “The same goes for Darwin, you can be pretty sure that you will get some chance of showers and storms at Christmas.”
How do meteorologists actually make predictions about the weather?
“It’s really funny how everybody relies on this, and very few people know how it works,” Jucker says.
Weather forecasts rely on detailed observations of conditions – everything from clouds, wind, pressure, humidity, temperature and rainfall – collected by satellites, radar, weather balloons and at weather stations around the country.
The bureau’s forecasts are based on “millions and millions” of real-time observations and data points, How says. That data feeds into weather prediction models, which estimate future conditions based on equations that govern the atmosphere. In Australia, expert meteorologists provide additional analysis based on their experience and local knowledge.
Jucker says weather is “chaotic”, which means that tiny differences – in observations, or how the model computes – can grow and amplify the further out you go.
“When you go too far into the future, those tiny little changes can become enormous, and they can easily become the difference between blue skies and deluge and any one given point.”
What is the difference between weekly and seasonal forecasts?
The BoM provides a seven-day forecast for cities and towns. How says this short-term outlook is “deterministic”, providing “a number of the temperature and a rainfall amount”.
Whereas the long-term forecast is “probabilistic” based on the percentage chance of how conditions might differ from normal.
But I checked the forecast, and the weather turned out differently?
In some parts of the country – such as Adelaide or Melbourne – summer conditions can change quickly, How says. “You get 40 degrees at lunchtime, the cool change comes through at 4pm, and it drops back to 20 degrees.”
Barnes says some types of weather – like thunderstorms – are inherently less predictable at a local scale. Thunderstorm activity might be forecast over an area, he says, “you might not know if it’s going to rain over your house or in the next suburb”.
Ghosts of weather past – from heatwaves to white Christmas
Just because it’s summer, doesn’t mean Christmas will be warm, How says.
Last year Orange and Grenfell, in central west New South Wales, experienced a white Christmas of sorts after a giant hailstorm blanketed lawns and streets. And in 2006 Melbourne recorded its coldest Christmas, with snow in parts of Victoria.
At the other extreme, Perth sweltered through a heatwave in 2021 reaching 43C on 25 December – it’s hottest Christmas Day on record.
The day can throw up some surprises, How says. “Being in summer, that’s when we can see everything from cyclones to heatwaves to large storms. Anything is possible.”
It’s a reminder that the Australian summer and Christmastime can be high risk or the peak of the severe weather season, he says.
“That’s why we always do tell people to keep across the forecast and warnings, especially in summer … conditions can change and become more dangerous very quickly.”
