Understanding rainfall forecasts as southern NSW cops a drenching

Australia Broadcast Center

21 hours ago

As wild weather hit the New South Wales coast on Tuesday, Sydney was forecast to receive between 45 and120 millimetres of rain, with even higher totals expected further south.
So, how close did forecasts land to what has eventuated?
Well, so far, they have been pretty spot on.
Strong winds were forecast for large areas of the coast. Heavy rain was expected from Sydney and further south, particularly south of Wollongong.
There was also a warning for damaging surf and potential coastal erosion.
Observations from the Bureau of Meteorology up to 9am Wednesday show the heaviest rainfall has indeed extended across the southern districts.
Sydney copped a drenching, with falls of 70mm — smack bang in the middle of their forecast range.
Bellambi, a suburb of Wollongong, recorded 90mm of rainfall up to 9am Wednesday morning.
But it was areas south of Wollongong that got the most, with broad falls between 100 and 200mm for the coastal strip.
Ulladulla had 224mm of rain up to 9am on Wednesday — its heaviest July rainfall on record for the airport, with data back to 1994.
Nowra also had its heaviest July rain in at least 25 years, recording 157mm — the majority falling in the six hours to midnight
NSW SES crews have responded to thousands of calls since the start of this event, including floodwater rescues around the South Coast.
The heavy rainfall south of Sydney was also met with significant wind gusts in excess of 100 kilometres per hour across the region, and large waves up to 13 metres at Port Kembla.
While the BOM's predictions were accurate, the forecasts in the lead-up did offer a broad range of possibilities.
Wollongong, for example, was forecast to have anywhere between 60 and 150mm on Tuesday.
It is something that speaks to the type of system that was producing the rain, and just how sensitive to change its outcome can be.
Bureau of Meteorology senior meteorologist Dean Narramore said while some systems were large and fairly consistent with the amount of rain they dropped, others were more patchy or erratic.
"A smaller range will be in a cold front, which has a large band of rain moving across southern parts of the country and it's consistent," he said.
"The large rainfall ranges are particularly when we get these east coast lows, or these [wind] sheer zones, or coastal trough.
This means just a small shift can change the outcome dramatically.
"You just have to look north and south of Sydney [yesterday]. North of Sydney there's only 20mm. South of Sydney there's almost 250 around Ulladulla," he said.
The rainfall forecast is something that is often misinterpreted though, so let's unpack what the forecast actually means, using Sydney's Tuesday forecast as an example.
If you looked at the forecast, it told you the CBD was expected to get between 45 and 125mm of rain.
What this range is showing is the most likely outcome for the suburb. But it's not telling you all the possible outcomes.
Monash University meteorologist Michael Barnes said the forecast range was based on weather models from the BOM and other agencies, which essentially look at a whole lot of plausible scenarios of what could happen, and break it down into probability.
The lower part of the range means there is a 75 per cent chance of getting at least that much rainfall — so, pretty good odds.
And the upper end of the range means there is a 25 per cent chance of getting more than that figure — so it is less likely.
In other words, of all the possible scenarios weather models had for Sydney, three quarters of them had at least 45mm of rain falling and one quarter had at least 125mm falling.
"The range they are showing is the middle bunch. It's where it's all clustering," Mr Barnes said.
"That doesn't mean there aren't [scenarios] that have something much higher or lower, and it doesn't mean those scenarios can't happen."
The reason rainfall is presented as a range, while temperature is just a straight shot, is because rainfall can be particularly tricky to forecast and can change quite easily.
Mr Barnes said this is especially true for severe weather situations.
"With temperature, the range of possibility, especially on the short-term … is generally more consistent. And that's just because of what causes heat and cold," he said.
"But with rainfall, things happen on very small spatial scales.
"Severe weather … is really tied to location, intensity, placement, timing and speed of these weather systems. And they're quite tough to get right. There often can be fairly significant spread in the small detail of these weather systems.
He said even when it is raining in your area, things can look very different from block to block.
"So everyone will know it's possible when they're driving from the shops that it's raining there, but by the time they get home its not raining. That can happen over just a few kilometres," he said.
"So to be able to capture exactly where the pockets of rainfall are going to fall at specific times are not really what these systems are built for.
"What they're capturing is the large scale estimate of what the rainfall is."
The system this week was a good example of that, with totals varying wildly from suburb to suburb around Sydney.
Of course, the rainfall range is just one part of the overall forecast.
For this storm, for example, some of the BOM's strongest warnings have been for wind speeds and dangerous swell — something that is still on the cards.
Mr Narramore said, in these events, they try to convey some of the nuance in their forecast description, severe weather warnings, and how they communicate to the media.