Weather Watch meteorologist Philip Duncan said the slow-moving cyclone’s direction will be determined in coming days by a high pressure system around Tasmania that can act like a barrier to its southern progress.
One possibility is the high-pressure system takes its time to cross the Tasman Sea, allowing Oma to drop south unimpeded and hit New Zealand directly this weekend.
The second outcome is a slow build of high-pressure gently nudging Oma north of NZ.
A third possibility is the high builds more rapidly than expected, stops Oma in its tracks and pushes it back north from where it came, near New Caledonia.
Oma is hovering in the Category 2-3 range of severity. Category 2 puts wind force in the 125k-160kmh range.
New Caledonia tends to be a staging post for tropical cyclones before the coriolis effect pulls them towards the South Pole and past NZ.
“If Oma does eventually come down this way it could prove a game-changer in terms of the rainfall it brings.”
It is a mixed bag of model predictions for Oma’s outcome, Duncan said.
“NZ is a complicated area for tropical storms as cyclones go through a technical change to become extra-tropical when they reach us. This adds a level of complexity than just forecasting a cyclone as you would in the tropics.”
Duncan likens Oma to Cyclone Winston, which two years ago had a similar issue as it tried to track south towards NZ, but was bounced back by a high-pressure system and changed direction a number of times before making landfall in Fiji with devastating effect.
He urges farmers starting to plan for autumn to keep Oma in mind over coming days.
“It may bring rain but we may also prove to be wrong.”
Weather models do not agree yet about Oma’s track but do predict a low of some type will deepen east of the North Island this weekend and is likely to bring in rain to that area regardless of Oma’s direction.
Duncan said a poll being run by Weather Watch had about 30% of respondents saying a drought is very likely.
“I would tend to wait over the coming week before making that decision.
“We are at a bit of a tipping point for this summer.
“If we get another week behind us with no rain and then prospects beyond that look dry then we will be more likely to be heading into a drought scenario.”
Whatever moisture Oma generates in coming days will be tantalisingly close to NZ.
Duncan said modelling indicates 300-400mm of rain is likely to fall only not far north of North Cape.
Sea temperatures in the Tasman are several degrees above average and conducive to cyclones maintaining their energy as they move south but only when dominating high-pressure systems are absent.
So far this summer has been dominated by a stream of high-pressure systems drifting over from Tasmania.
“That is why central North Island and the Nelson region have been particularly hot and dry for such a period.”
