November 2021 is less than 5 mm off being Canberra’s wettest November, and if rainfall levels are at the higher end of predictions – 100 mm between Wednesday and Friday – it could become the wettest month on record.
The forecast comes as a La Niña in the Pacific Ocean was officially declared by the Bureau of Meteorology today (23 November).
BOM meteorologist Hugh McDowell said the rainfall expected in the second half of this week is the combination of the low-pressure system coming from the Great Australian Bight with the tropical moisture coming from the north.
The combination will move into NSW tomorrow and then spread across the ACT on Thursday and Friday before clearing over the weekend. Heavy, persistent rain is likely and thunderstorms are possible, particularly on Thursday and Friday.
The La Niña rainfall period was announced by the BOM this afternoon.
Typically during La Niña events, rainfall becomes focused in the western tropical Pacific, creating a wetter than normal period. Mr McDowell said that doesn’t guarantee a wetter than average summer but rather makes it more likely than usual.
“It’s hard to say definitively, but we’re looking at a wetter than average summer, so with La Niña declared it means rainfall is more likely and we’re more likely to see more periods where we see rainfall across eastern Australia, including the ACT and its surrounds,” said Mr McDowell.
“But, it doesn’t necessarily mean for sure that we will see constant rain or very, very frequent rain events. It could come in many forms. It could be heavy rainfall and then dry periods in between, or it could be more frequent, lesser rainfall periods with more cloudy conditions.”
He still assumes there will be extended dry periods this summer.
The BOM delayed declaring La Niña, as the climate warning has become harder to determine due to the impact of a warming climate.
The Bureau previously shifted to La Niña “watch” on 14 September 2021, and to La Niña “alert” on 12 October 2021. La Niña is expected to persist until at least the end of January 2022.
Original Article published by Max O’Driscoll on Riotact.