Tropical Cyclone Cook churning across Pacific
NIWA meteorologist Seth Carrier outlines the likely path of Cyclone Cook, which is gaining strength in the Pacific.
Cyclone Cook formed early Sunday morning near Vanuatu and has since rapidly intensified into a powerful category 3 storm as it nears central New Caledonia.
The storm, with sustained winds up to 157 km/h, will likely make landfall in central or southern New Caledonia tonight, leading to possible destruction of buildings, flooding due to very heavy rainfall and storm surge, and the possibility of landslides.
Weather conditions are expected to improve across New Caledonia tomorrow as the storm moves southeast.
Tuesday into Wednesday: Tropical Cyclone Cook is expected to push southeast through the open waters of the Southwest Pacific while weakening slightly.
This weakening will be caused by the storm entering an area of the southwest Pacific with cooler water temperatures, which deprives it of the “fuel” it needs to sustain itself. In addition, winds high in the atmosphere will begin to increase, otherwise known as shear, and these upper-level winds help to tear apart tropical cyclones.
Wednesday into Thursday: Still on a southeast trajectory, Tropical Cyclone Cook is expected to move in the direction of New Zealand. While the centre of the storm is still likely to be several hundred kilometres north of the country, a plume of deep, tropical moisture is forecast to extend southward and result in locally heavy rainfall across the North Island.
Around this time, Tropical Cyclone Cook is expected to transition to an ex-tropical cyclone, meaning that it will enter a phase where it transforms from a tropical low pressure to a non-tropical low pressure.
Meanwhile, a separate area of low pressure is forecast move east across the Tasman Sea. On Wednesday, this storm will act to siphon Cook’s moisture southward toward the North Island.
Model guidance monitored by NIWA, suggests that the upper and eastern North Island are most likely to experience an impactful weather event from Wednesday through to Friday morning.
More very heavy rain is likely to fall in similar areas that were heavily affected by last week’s storm. Unfortunately, with soils across much of the North Island already saturated, the risk for flooding this week will be higher as the ground will be unable to absorb additional rainfall.
At this time, storm total rainfall amounts (Wednesday-Friday) may range from 100 to 250 mm with locally higher amounts possible in the hardest hit areas, although significant variation is likely across the North Island.
Another potential impact of ex-Tropical Cyclone Cook is the threat for high swell as it approaches the North Island. Tropical cyclones create a phenomenon known as storm surge, where a combination of winds and low pressure cause sea levels to rise ahead of the storm. This will increase the risk for dangerously high waves along the northeastern and eastern coasts of the North Island, which may result in coastal inundation and erosion.
While confidence is increasing that heavy rain will impact the North Island from Wednesday into Friday, the precise areas that will see the heaviest falls remain difficult to pinpoint.
In addition, it remains unclear whether the centre of then ex-Tropical Cyclone Cook will make a direct landfall on New Zealand, but if it were to occur, the most likely time frame would be late Thursday into Friday, somewhere upon the upper or eastern North Island. If a direct landfall were to occur, it would result in a peak of rainfall intensity and a potential for damaging wind gusts.
After the storm
Improving weather conditions are possible across the North Island on Friday.
Through the long weekend, showers are expected to occur as low pressure over the Tasman Sea slowly weakens and swirls over top of the country. There will be some sunshine, especially across the North Island, although an umbrella should be kept handy. Temperatures will be seasonably cool with decreasing humidity following a mid and late week peak.