Newstalk ZB
By RNZ Emergency operations staff in Lower Hutt have been monitoring river levels throughout the night after heavy rain battered the city on Saturday. Orange warnings for heavy rain were in place for much of the North Island including Taranaki, King Country, west of Taupō – until 9am Sunday – and Bay of Plenty east of Whakatāne, starting from the early hours of Sunday until 4pm, as well as the Tararua Range, the Kāpiti Coast, Wellington and the Hutt Valley until 1am Sunday. Wellington was expected to bear the brunt of it after two thunderstorms on Saturday morning inundated homes, closed streets and caused slips around the region, but the weather dispersed more than originally forecast in the evening. However, teams in Lower Hutt had been on standby, watching the Hutt River, the Waiwhetu Stream and the Black Creek in Wainuiomata overnight in case levels continued to rise or risked breaking their banks. Mud is still staining some of the streets around Stokes Valley – where residents were forced to clear mounds from footpaths, roads and driveways. Many were surprised by the sudden downpours and dramatic impacts in such a short space of time. Authorities said this event was a perfect example of why people should always be prepared. The Wellington Region Emergency Management Office’s regional manager Dan Neely said it knew there could be the possibility of thunderstorms but that the actual warnings came suddenly. “Severe thunderstorms are very difficult to forecast and severe thunderstorm warnings are issued by MetService at short notice. “Sometimes they pass with minimal impact, but as we saw today, when we had two back to back this morning, they can have significant impacts.” MetService meteorologist Brain Mercer said this weekend the possibility of thunderstorms is embedded within the band of rain. This means they bring heavy spells, on top of the rain already forecast. “In these thunderstorms, there could be rainfall of between 10 to 25 millimetres per hour from an individual cell.” But they are quite fickle. “We can get the correct setup for thunderstorms to happen, but every thunderstorm needs a trigger, needs something in order for it to actually fire off. “So the atmospheric conditions can be right for a thunderstorm, but you may not get that trigger.” He said each storm might last for 30 minutes to an hour, and they tend to be very fast moving. MetService has a team of thunderstorm forecasters who track them. “When they develop into severe thunderstorms, and by severe thunderstorms we mean they’ve got very, very heavy rain, so falls sort of 25 to or above millimetres per hour, then we will issue a thunderstorm warning on that individual cell. “However, each cell is quite short-lived. “So a thunderstorm warning will be in effect for an individual thunderstorm, but that thunderstorm may not last very long.” He said that’s why MetService issues a broader warning, saying there is a possibility of thunderstorms, because you can’t track or issue warnings that far in advance. – RNZ
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