The Northern Beaches may have experienced an extremely rare rainfall event amongst a series of intense storms and flash flooding around Sydney earlier last month.
It was recorded that almost 330 mm of rain fell at Palm Beach Golf Club on 17 January over a 12-hour period, according to Manly Hydraulics Laboratory (MHL) rain gauge data.
When this is compared to the Bureau of Meteorology’s (BoM) data on rainfall depth rarity, it suggests a 1 in 2000 annual probability, meaning there is a 0.05 per cent chance of the event occurring in any given year.
MHL posts the rainfall intensity and duration for each of its stations, and although the readings come from a State Government-operated site, Swellnet Meteorologist Craig Brokensha says the numbers can’t be officially trusted unless they are from the World Meteorological Organisation (WMO), but that doesn’t mean they’re without credibility.
“With a cluster of very high readings from Palm Beach to Pearl Beach, it does increase the confidence. Also seeing the end result via videos of the flash flooding etc, it also adds validity,” he said.
Over that January weekend, Manly Observer was flooded with videos from locals enduring underwater streets, submerged vehicles, and braving rough seas on the Manly Ferry as storms took control of the Northern Beaches coastline.
Brokensha theorised that while a warmer climate and oceans bring increased capacity for torrential, localised rain events, there could have been one more factor at play, which caused the extreme weather.
“Down in Lorne on the Victorian Surf Coast, during the earlier stages of the trough which then formed into the East Coast low off the NSW coast, we saw a similar very localised, flash flooding event, and this was actually seeded and amplified from airborne ash that was present from the devastating bushfires seen just days before,” he shared.
“There’s a chance that the system still contained these finer ash particles, which provides a nucleus for additional raindrops to form on (i.e., enhanced rainfall), as it transitioned north into NSW, but without confirmation, it’s just speculation at this stage.”
BoM echoed Brokensha’s credibility concerns to us, citing that it cannot verify the Manly Hydraulics rainfall data. The Bureau uses Annual Exceedance Probability (AEP) to estimate the likelihood of observed and forecast rainfall events.
According to MHL data, Palm Beach Golf Course recorded a 46 per cent higher increase of rain over a 6-hour period than BoM’s Rare Design Rainfall Depth 1 in 2000 AEP, and 50 per cent higher over the 9-hour total.
