Cyclones happen every year, but Gabrielle has specific attributes that make it bigger and badder than most.

Tropical cyclone Gabrielle had been flagged as a potentially dangerous storm since its first rumblings early last week. The Australian Bureau of Meteorology reported on February 6 that “a tropical low (14U) is forming in the northeastern Coral Sea, south of the Solomon Islands.” It was described as existing in “a favourable environment for further development”, which is a meteorologist’s way of explaining the low had a good shot at becoming a cyclone. 

So it proved, with the United States Typhoon Warning Center officially designating it Tropical Cyclone 12P on February 8, while also first signalling its potential to head for the upper North Island. “12p will shortly track poleward then southeastward toward New Zealand as it rounds the str axis.” Australia’s meteorology bureau named it Gabrielle not long after, and the upper North Island, still drying out after the catastrophic floods of late January, began to brace for more.

How it evolved

Gabrielle developed into category three intensity, and tracked across Norfolk Island, 750km north-east of Cape Reinga, bringing winds that peaked at a sustained intensity of over 160km/h. As it left the tropics, it changed designation, losing the “tropical” modifier to become just a “cyclone”. Yet as WeatherWatch’s Philip Duncan explains, “[while] it loses the tropical portion of it, the storm remains actually at the same power… In fact, while its centre is no longer driven by hot tropical air, its potential danger to New Zealand actually increased as it approached.”

This is because “some of the peak winds in the middle can die out a little bit and fade down and spread out further – but then it can re-intensify suddenly,” says Duncan. “That’s what we’re expecting on Tuesday morning, to suddenly see those winds to ramp back up again, and the air pressure might even go lower than it’s been over the past day.”

Dangerous atmospheric lows

Those atmospheric lows are a large part of what makes it so distinct and dangerous. NIWA’s Ben Noll says Gabrielle “will likely break mean sea level pressure records near and/or in New Zealand,” in forecasting “a minimum central pressure of 961 hPa [hectopascals] early Tuesday.” WeatherWatch’s Duncan forecasts the storm’s apex as occurring around 4am on Tuesday, with one model suggesting a low of 963 hectopascals, while an Australian model forecasts an even more shocking low of 957 hPa.

What makes those measurements so significant is how aberrant they are to the normal bounds of atmospheric pressure in this region. If achieved they would smash the current record low for Auckland of 970 hPa, achieved several times during 170 years of records. These tend to be associated with very large storms, including the devastating 1936 cyclone, which passed within 50km of Auckland and caused immense damage and rainfall across the North Island.

This potential for record-low pressure is why forecasters have been so concerned about Gabrielle since it first started tracking our way. “The lower the air pressure the more unstable it is,” says WeatherWatch’s Duncan, and this instability is behind the enormously elevated wind and rain forecast for the next 24 hours. The very low pressure also leads to temporary sea level rise, exacerbating the potential damage caused by storm-whipped seas and leading to very high tides. As Jamie Morton reports in an excellent explainer for the NZ Herald, it also brings with it a “sting jet”, a rare phenomenon which elevates the winds created by the super-low pressure.

The path and the future

After forming below the Solomons, Gabrielle essentially headed straight for New Zealand. While travelling relatively slowly, it also expanded outwards, with that low pressure also part of the reason for Gabrielle’s immense scale, spanning two to three times the size of New Zealand. This is also why its impact will be felt across the whole North Island in the coming days, with very strong winds impacting both Northland and Wellington simultaneously. Its epicentre is predicted to travel down the east coast of the North Island, bringing torrential rain of up to 500mm in mountainous areas, along with strong winds as far west as Taranaki. 

By Tuesday afternoon its effects will likely be felt in both Auckland and Canterbury at the same time, before it finally detaches from the North Island and heads toward the Chathams. Should it live up to predictions it could leave enormous devastation across a Northern region already heavily impacted by flooding, while also vastly expanding the area impacted to the whole North Island and the upper South Island.