Tropical Cyclone Lusi began its life over Vanuatu in early March, as the system tracked between Fiji and New Caledonia it intensified, reaching a category 3 status (scale of 1-5) for a time with sustained winds around 150 km/h close to the centre of the storm. Lusi, like other cyclones, formed as a result of the warm "bath-like" waters in the tropics and light winds aloft to allow them to develop.
As Lusi travelled south she encountered cooler waters and the increased shear, which helped to weaken her as she moved towards New Zealand.
Although a weaker feature than she had been in the tropics, Cyclone Lusi steamed her way into the northern parts of New Zealand during the early hours of Saturday, March 15. She brought wind gusts in excess of 130km/h in the far north at Cape Reinga and at other exposed locations. The rainfall distribution associated with Lusi was rather a patchy affair, with locations exposed to the prevailing northeast wind flow receiving the most whereas other locations with more lee shelter receiving significantly less.
The ranges of the Tasman region recorded some of the highest rainfall, more than 200mm in 24 hours in some spots.
Some of the already dry pastures from Waikato to Taihape did not receive much precipitation at all, with 4.5mm at Hamilton, Paeroa to the north reported 133mm over the same period. New Plymouth recorded 2.8mm, while Mount Taranaki managed over 150mm during the same time.
Cyclone Lusi brought a prevailing wind flow that spread across the country mainly from the northeast, given the fact that winds flow clockwise around lows in the southern hemisphere. A strong northeast flow of wind and rain spread in across the north of the North Island as Lusi approached the country. This strong northeast wind flow had the most impact on locations exposed to that direction (i.e. eastern coastal locations of Northland, North Auckland and Coromandel).
There were reports in these locations of downed trees and power outages. In contrast, other areas to the west not exposed to that wind direction generally did not feel the wrath of the strong winds and rain as badly as locations along the coast. The hills and topography across many parts of New Zealand can also have a dramatic effect on how far inland rain and wind spreads.
Tropical cyclones also can be rather erratic in the way they move, a bit like bulls in a china shop. They are steered by upper-level winds in the atmosphere but given their difference in behaviour to mid-latitude weather systems, their exact tracks can be difficult to exactly pin down more than a couple of days in advance. This can make forecasting more challenging because there still are severe elements in the storms but perhaps not for all of the country. Hence the phrase, "keep up to date with the latest forecasts from us when storms are approaching".
What is to come?The neutral state in the ENSO (El Nino-Southern Oscillation) has kept much of the Pacific, including New Zealand, riding the fence with no general steer from the tropical Pacific for the shape and frequency of the weather patterns over the last year or so. In the coming months all eyes will be on the first signs of warming in the sub-surface equatorial Pacific to see if that holds the key to a developing El Nino.
The rest of the autumn will be more of the same variability where we again look to the local factors and the seasonal changes for hints to our longer-range weather patterns. We can expect the typical seasonal shift north of the anticyclones, along with a return of the Tasman Sea weather systems which in time will begin to have more meat on the bone in terms of rainfall as they pass across northern areas. The southerly changes behind the systems will also become more noticeable as we head towards winter.