Cut-Off Low to Drench NSW and ACT With Heavy Rain This Week

A slow-moving cut-off low pressure system is set to bring widespread and at times heavy rainfall to New South Wales and the Australian Capital Territory through the latter half of this week, with some northern and coastal regions forecast to receive 100 millimetres or more by the time the system finally pushes offshore on Friday.

As of Monday afternoon, the low was still developing near Adelaide in South Australia, already beginning to drag elevated moisture levels into western and eastern parts of NSW and generating overcast skies with light showers across the state. A separate coastal low responsible for last week's large surf and powerful swell has moved into the Pacific and is helping sustain an onshore southerly wind, though conditions at sea are expected to ease gradually through Monday.

A cut-off low is a low pressure system that breaks away from the southern jet stream and the belt of westerlies, effectively becoming an isolated circulation that tends to track north and east across southern and inland Australia. Meteorologists track these systems using upper-level wind data at around 5,500 metres altitude, where the detached circulation becomes clearly visible spinning away from the main westerly flow.

The system is expected to move over South Australia on Tuesday before pushing into the far northwest and northeast of NSW on Wednesday, drawing down a deep plume of subtropical moisture from the north. By Thursday the low should be positioned near the NSW–Queensland border, continuing to funnel rainfall into northeastern and eastern districts, before finally ejecting offshore on Friday. Even as it departs, a rotational wrap of cloud and showers is likely to swing back across eastern coastal areas on Friday, potentially adding further totals to the Hunter and Sydney regions.

Rainfall accumulations are expected to vary widely depending on location. The far west around Broken Hill and Brewarrina may see around five to fifteen millimetres, while Bourke and Cobar are in line for fifteen to forty millimetres. The Riverina and Southwest Slopes and Plains are forecast to receive twenty-five to fifty millimetres, with similar totals expected across the ACT, the South Coast and the Southern Tablelands.

The heaviest falls are currently forecast across the northern half of the state. The Northwest Slopes and Plains, Northern Tablelands, Hunter Valley and parts of the Central West are all forecast to receive fifty to one hundred millimetres, with the Hunter potentially topping that mark if Friday's coastal wrap delivers as modelled. The Mid North Coast and Northern Rivers, which have endured a run of wet weather in recent weeks, are also likely to see another significant rain event, though the inland benefits of such a widespread system represent welcome relief for drought-affected communities across western and central NSW.

Residents across affected areas should monitor updated Bureau of Meteorology warnings as the system develops, particularly given that cut-off lows can be difficult to track precisely and rainfall totals may shift as the week progresses.