Storms are generated from a disturbance in the atmosphere. Due to New Zealand’s geographic position between the sub-tropical and mid-latitude belts, and the structure of its landmass, a range of severe weather producing conditions can occur.
There are various types of storms and hazards associated with storms which all present a range of impacts. Some include:
A depression is a low pressure weather system. Deep depressions are recognised by concentric circles drawn on weather maps. The closer the lines are together the stronger the winds are. They usually originate in the tropics to sub-tropic regions to the north of New Zealand. Sometimes the systems can even originate as tropical cyclones. The duration, extent, and magnitude of weather hazards associated with depressions vary depending on the track, speed, size, and central pressure of the low pressure system. Major depression systems are common within the Bay of Plenty, and the frequency and intensity are influenced by La Nina/El Nino weather patterns. Major depression systems can bring high winds, storm surges (higher than normal tides influenced by the wind), and heavy rainfall.
Thunderstorms can range from short-lived individual thunderstorms lasting no more than one to two hours, or they can group together to form a line of thunderstorms which can last a number of hours. Depending on the speed and direction of movement, a group of thunderstorms can present a range of increased hazards or have similar hazards to an individual thunderstorm. Thunderstorms bring with them a range of hazards including thunder and lightning, severe winds, heavy rain, hail, and sometimes tornadoes. During the severe thunderstorms in the Bay of Plenty in May 2009 over 5000 lightning strikes were recorded in one day which caused havoc by cutting power, felling trees, damaging telephone lines, and damaging electrical equipment on boats.
An individual thunderstorm will usually only affect a small area, but the suddenness and intensity of the rainfall can be hazardous if it continues for a long time period. Heavy rain from groups of thunderstorms can produce landslides, floods, and cause streams and rivers to rapidly rise. Heavy rain can also cause overflow of storm water drains in urban areas which can lead to flooding.