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MetService is advising Stewart Island residents to batten down the hatches in preparation for a storm, bringing winds over 120kmh this afternoon.
A cold front, preceded by a very strong northwest flow, is expected to move northwards over Stewart Island early this afternoon with a period of rain and
A MetService spokesman said between 11am and 2pm today, a few of these thunderstorms may be severe with damaging northwesterly wind gusts of 120-140kmh.
Wind gusts of this strength can cause some structural damage, including to trees and power lines, and may make driving hazardous.
Winds will turn southwest and ease quickly behind the front by mid afternoon.
"People in these areas should be on the lookout for threatening weather conditions and monitor for possible Severe Thunderstorm Warnings.
"For information on preparing for and keeping safe during a storm, see the Civil Defence Get Ready Get Thru website."
Warning amounts of rain are forecast for Fiordland today, and severe gales gusting to 110kmh are expected in exposed parts of Otago and Southland.
A MetService spokesman said a cold front would move on to the South Island today from the southwest, bringing warning amounts of rain in Fiordland and Westland south of the glaciers.
"Expect 70mm to 100mm of rain to accumulate, with peak rates of 15mm to 25mm per hour and possible thunderstorms.''
He said an "unsettled west to southwest'' flow would follow the front, and would be strong in the far south.
"About coastal parts of Southland, Clutha and the Otago Peninsula, there is high confidence of a period of severe westerly gales on Wednesday and Thursday.''
There was also a risk of severe gales in exposed parts of inland Otago and Southland tomorrow, he said.
Severe gales have a mean wind speed reaching 90kmh, and may have frequent gusts exceeding 100kmh.
Residents are advised to secure anything that may be hazardous if blown away by the wind.