New South Wales expected to be drenched by sustained rainfall, with potential flooding and 14-day rain streak on the horizon

A storm front in the Northern Tablelands, near the town of Guyra, earlier this week. (Supplied: Trudy’s Nature Shots)

A sustained rain event is intensifying across New South Wales, raising the risk of flooding around Sydney and the Illawarra this weekend.

Inland parts of the state can also expect widespread rain during the next 48 hours, starting in the west on Friday, and then shifting to the slopes and ranges on Saturday.

Following the weekend drenching, the seemingly endless coastal wet spell is likely to persist through next week and is likely to lift Sydney’s May rain total to well over 200 millimetres for the second consecutive month.

Rain to intensify this weekend

We are now into the fourth day of a blocking pattern where a stalled high-pressure system west of Tasmania has delivered a prolonged period of southerly winds and showers to the NSW coastline.

More than 100mm has already fallen this week around the Northern Beaches of Sydney and the Central Coast, nearly eclipsing the average for the whole of May, although the minimal extent and short duration of each downpour have prevented significant flooding.

While the surface weather chart will remain identical this weekend, what will change and increase rainfall is what’s happening a few kilometres above the ground — an upper low (a low-pressure system aloft) will drift east across the state.

The heaviest falls on Friday will land over the state’s west (under the low), with a broad ribbon from about Cunnamulla to Pooncarie, including Cobar, Ivanhoe, Wilcannia, Bourke and Brewarrina likely to see from 15 to 50mm.

In the meantime, showers will also continue in the east through Friday, and again the central stretch of coast from the Illawarra, through Sydney to the Central Coast is likely to see the heaviest falls.

The western rainband will then spread east on Saturday, engulfing most of the state, including widespread falls in excess of 20mm over the northern inland and around Sydney.

Rain should then intensify further on Sunday along the coast as the eastward-moving upper low interacts with the moist onshore flow off the Tasman Sea.

Sunday could see up to 100mm in 24 hours from the Illawarra to Sydney, possibly enough to trigger a few pockets of flash flooding according to Angus Hines from the Bureau of Meteorology

“If we do see triple digits … 150mm not out of the question … that is certainly enough to lead to areas of flooding, we could well see some impacts on the ground,” he said.

Mr Hines also advised that while it is too early to issue specific warnings that could change this weekend.

“Keep half an eye on severe weather warnings because, in a situation like this, we may issue heavy rain warnings during [the] coming days.”

Most of NSW can expect at least 20mm of rain during the next week, and well over 100mm should fall around Sydney’s east.

The ongoing rain this weekend will also lead to river rises and the possibility of minor flooding in some of the smaller coastal catchments, although rain totals and flooding should remain well below the extreme levels from last month.

Regardless of the lower severity, the NSW SES Acting Assistant Commissioner Allison Flaxman said volunteers and flood rescue operators were ready if required.

“We’re working closely with the Bureau of Meteorology and are monitoring conditions across the state,” she said.

“Isolated rainfall in excess of 100 millimetres in some coastal areas is not out of the question, but we are well positioned to respond to any calls for assistance.”

When will the endless rain finally clear?

The run of showery days will continue along the NSW coastline from Monday to Wednesday, before another area of rain possibly arrives from the west from around Thursday.

Some modelling is even hinting at another blocking high near Tasmania through the second half of next week, a scenario that could prolong the stream of onshore winds and showers into the middle of May.

It is not inconceivable that Sydney’s run of rainy days could exceed 14, the longest wet streak since early 2022 when 17 consecutive days had measurable rain – but still well short of Sydney’s longest-ever wet spell of 29 days back in early 1891.

However, if we consider a wet day to require at least 1mm of rain, the record of 16 days from 1943 and 2022 could be under threat.

So when can eastern NSW look forward to a full 24 hours of mostly clear skies? Modelling shows the blocking set-up should break down by the third week of May, allowing our weather systems to again become mobile and flush the rain off the coast.

Rain fills dams and eradicates drought

It was only last spring that serious concerns were raised by numerous agencies for drought, bushfires and water shortages following a record-dry August to October and the declaration of El Niño.

By November 65 per cent of NSW had entered some level of drought and Sydney’s water supply had fallen to the lowest level in three years, although it was still near 90 per cent.

After a wetter-than-average six months, drought is now confined to just 41 per cent of the state, of which only 2 per cent is not rated in the least severe drought category.

[Graph: NSW drought]

The above-average rain through the warmer months also subdued bushfire activity while boosting Sydney’s dam levels, which are now back to near capacity.

The additional rain in May will further reduce 12-month rainfall deficits and may cause another minor spill of Warragamba Dam.