He said the westerly flow and some sea breezes had contributed to a strong wind convergence, creating unstable atmospheric conditions. "The air in the lower atmosphere is more buoyant after the land has heated up over a fine day, then the air bubbles up and causes an unstable atmosphere and the wind definitely helps fuel the fire. Humidity does also help."
He said more thunderstorms were forecast to "crop up" from midday today.
MetService said yesterday in a severe thunderstorm watch, rainfall of thunderstorm intensity can cause surface and/or flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips. Large hail can cause significant damage to crops, orchards, vines, glasshouses and vehicles, as well as make driving conditions hazardous.
The watch also said people should be on the look-out for threatening weather conditions and listen for possible severe thunderstorm warnings.
The MetService criteria for a severe thunderstorm in New Zealand is rainfall of 25mm/h, or more, or large hail, 20mm in diameter, or larger and strong wind gusts of at least 110km/h.
The Ministry of Civil Defence and Emergency Management advises that as storms approach people should take shelter, preferably indoors away from windows; avoid sheltering under trees; move cars under cover or away from trees; secure any loose objects around their property; check that drains and gutters are clear; and be ready to slow down or stop, if driving.
During and after the storm, people should also beware of fallen trees and power lines and avoid streams and drains which pose the danger of flash flooding.