NSW flood threat this weekend as Sydney's weather endures coldest July day in decades

Australia Broadcast Center

4 days ago

A cold and wet week across New South Wales will culminate this weekend in heavy rain, strong winds and powerful surf along the coast.
The stormy winter weather will result from a multi-centred coastal low developing in response to a broad polar air mass currently tracking slowly over eastern Australia.
The sub-Antarctic air has already produced the coldest July day in decades across parts of Sydney this week and could produce rare snow tomorrow on the ranges of northern NSW and far southern Queensland.
July produced up to 300 millimetres of rain along the NSW coast, as much as four times the average, and the soggy conditions will linger into the start of August.
Rain will continue on Friday across much of the state's north and east, with further moderate falls from the Hunter to the south coast, including around Sydney's east where up to 40mm could fall by midnight.
However, rain is likely to intensify further on Saturday as a low pressure system forms off the Northern Rivers coast then tracks south-west and makes landfall on the Mid North Coast during the evening.
This westward trajectory is likely to generate heavy falls close to 100mm across parts of the Mid North Coast and possibly the Hunter on Saturday, triggering a flood watch for more than a dozen catchments from the Orara River to Newcastle.
The path inland should also spread heavy rain to the Northern Tablelands — a scenario which could also cause flooding along the Peel and Namoi rivers.
Further south, moderate rain is likely on Saturday, including falls up to about 50mm around Sydney and the Illawarra.
Rain is likely to continue through Sunday as a new low arrives from the Coral Sea and drives moist onshore winds onto the coast, however its centre should remain far enough offshore to prevent heavy falls.
All up during the next 72 hours, totals from 50 to 100mm are likely from around Coffs Harbour to Batemans Bay, extending inland to the northern ranges.
Depending on the low's exact path and strength, parts of the Mid North Coast could see closer to 150 or even 200mm, although even at this higher end of the range, both the totals and resulting flooding would be well below the severity of the devastating May event.
Similar to the early July event, the coastal low will also drive strong winds and large waves onto the coast, although impacts will not be as extreme.
Thankfully, the low won't have time to intensify into an East Coast Low before weakening post-landfall, however maximum gusts may still briefly reach the warning threshold for damage of 90 kilometres per hour near the system's centre on Saturday, and again later Sunday from the second low.
Offshore winds should comfortably reach gale force this weekend, raising surf heights rapidly through Saturday to peak at about 5 metres on Sunday along the northern half of the NSW coast.
This should lead to maximum waves near 10m, and may trigger a warning for coastal erosion and inundation.
While the heaviest rain is firmly along the NSW coast, the coldest air on Thursday lay west over northern SA — although without much moisture the prospects for snow were limited.
However, as the polar air moves east this weekend it will encounter abundant moisture off the Tasman Sea, creating the optimal pattern for low-level snow on the Great Dividing Range.
The intersection of cold air and moisture is most likely on the Northern Tablelands, although a few flurries may also dust the higher southern Darling Downs.
Model forecast for snow shows the possibility of several hours of moderate falls on Saturday on the NSW northern ranges.
Provided this cold moisture crossover, snow may fall to around 1,000m on the NSW side of the border, possibly bringing flakes to Glen Innes, Walcha and Armidale, along with the more reliable Guyra — one of the highest towns in Australia.
When the polar air mass first arrived on Wednesday it combined with cloud and rain across Sydney to drop temperatures as much as seven below average.
Since coastal winter temperatures typically lack significant variation, the anomalous maximums became the coldest in July for decades in some suburbs.
Bankstown was the statistical stand-out — a high of just 11.2 degrees Celsius was the suburb's coldest July day in 35 years.
Nearby Canterbury observed its coldest July day on record, although the site opened relatively recently in 1996.
Terrey Hills, which is cooler than other Sydney weather stations due to its elevation, only managed a high of 10.2C, also a record, but with only 20 years of data.
Observatory Hill's top of 12.5C was a three-year low for July.