Sydney weather LIVE: ‘Bombogenesis’ triggers severe weather warning as rain sets in

Good morning and welcome to The Sydney Morning Herald’s live coverage of today’s weather, as a rapidly intensifying low-pressure system off the east coast brings heavy rain and wild winds to Sydney and around the state.

Here’s what we know so far.

• The Bureau of Meteorology has issued a severe weather warning for Sydney and much of the NSW coastline, from Moruya Heads in the South to Taree in the North, as a “vigorous” coastal low offshore brings damaging, locally destructive winds and possible heavy rainfall.
• In Sydney, heavy rain could bring six-hourly totals of 70 to 90 mm, while isolated downpours could see up to 120mm of rainfall.
• Wind gusts are already reaching up to 50km/h, with speeds between 60-70km/h expected to set in on Tuesday afternoon, with peak gusts of up to 110km/h
• Destructive wind gusts in excess of 125km/h could hit exposed coastal areas from Bondi to Wollongong from later today.
• Flash flooding is possible for the coastal fringe south of Seal Rocks on the Mid North Coast and north of Ulladulla on the South Coast.
• NSW SES deputy commissioner Debbie Platz advised households to secure outdoor furniture, clear gutters, move vehicles away from large trees and download the Hazards Near Me app to stay up to date.
• The rapid intensification of the low into a fierce weather system, dubbed “bombogenesis” by meteorologists, is a rare event most often seen during winter. You can read more about the science of the phenomenon here.
• The forecast movement of the coastal low would see damaging winds and heavy rain persist into Wednesday.

The SES has pre-deployed 395 volunteers across the coast and positioned high-clearance vehicles to Kiama, Albury, Hawkesbury, Maitland and Dungog, as NSW braces for the latest wild weather event.

Emergency vehicles and helicopters were also deployed to Taree, which is still reeling from May’s flood disaster.

“This is a time for our communities across NSW to be very vigilant and very prepared. We’re looking at thunderstorms, rain, strong winds, coastal erosion and damaging surf,” SES deputy commissioner Debbie Platz said.

Platz urged people to download the Hazards Near Me app, secure outdoor furniture and trampolines, clear gutters and move vehicles away from large trees.

The rapid intensification of the low into a fierce weather system, dubbed “bombogenesis” by meteorologists, is a rare event most often seen during winter.

“Over the next 24 hours or so, we will see the low form quite deep, quickly, and the central pressure dropping significantly, somewhere in the order of 20 to 30 hectopascals,” the bureau’s Daniel Hayes said on Monday.

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That plunge in pressure easily meets the threshold for an east coast low, but another low-pressure system stewing further out to sea will interact with the coastal system, potentially siphoning off energy and reducing the potential for the wide-scale impacts that would define an east coast low.

Forecasting the interaction between the two low-pressure systems is tricky.

“The two can just dance around each other and both survive but pull energy out each other. Or they could merge, and you eventually get one system that’s stronger,” Hayes said.

The two lows could undergo the Fujiwhara effect, where two systems rotate around each other, according to Weatherwatch meteorologist Anthony Cornelius. That would further complicate forecasting and may push the bad weather into three-day event, he said.