Fig. 1: Latest satellite view of a rain storm across East/Southeast Australia.
Discussion: Heavy rains have caused widespread flooding across New South Wales this weekend due to a moist onshore tropical flow into a low-pressure trough made more intense by a blocking high-pressure area to the south (Fig. 1). A low-pressure area has formed along the low-pressure trough in far northwest New South Wales.
Over the next 48-60 hours the low-pressure area intensifies drifting southeastward through New South Wales. Once the developing storm is offshore in 42 hours the heavy rain bands will also shift offshore. A slow-moving storm due to the blocking high-pressure area to the south therefore more excessive rainfall is likely. ECM indicates 2-6 in. of rain the next 48 hours heaviest in northeast and southeast New South Wales (Fig. 2). More life-threatening flash-flooding is likely.
The cause of this excessive rainfall event is evolution of a blocking high-pressure system southeast of Australia forcing a gathering of low-pressure across Eastern Australia which entrains above normal low-level atmosphere moisture associated with warmer-than-normal sea surface temperature anomalies (SSTA) surrounding Australia.
After the storm a much drier regime brings relief to East/Southeast Australia in late March/early April.
Fig. 2: ECMWF 48-hour rainfall total across East/Southeast Australia.