Hawke's Bay's winter weather outlook normal after record May

Atea a Rangi at Awatoto near Napier. Photo/Paul Taylor

While winter is starting to close in, this week's falling temperatures are not out of the ordinary, and in fact, Hawke's Bay has been enjoying one of its warmest autumn periods ever.

Niwa's climate summary for May reveals that parts of Hawke's Bay experienced their warmest May in decades, and in the case of Wairoa - the third warmest May since the outbreak of World War II.

Wairoa was the best of the bunch with an average mean daily temperature of 13.6 degrees the third highest average temperature since records began in May 1945. It's lowest mean daily temperature of 9 degrees was also the third highest for the month since the same year.

Similarly the average mean temperature of 12.9 degrees recorded in Hastings represented the third warmest May since records began in 1965. Its average mean minimum daily temperature of 8 degrees was also the fourth highest since records began.

Central Hawke's Bay was also warm, with an average mean temperature for the month of 11.9 degrees, making last month the fourth warmest since records began.

In addition, the maximum daytime temperature of 24.5 degrees recorded in Napier on May 16 was the hottest temperature recorded in the whole of New Zealand last month.

Niwa meteorologist Seth Carrier said the main reason for the warmer than average temperatures were a westerly and south-westerly windflow.

"For Hawke's Bay that tends to lead to warmer temperatures because those westerly winds 'down slope' off the interior ranges. As they move down the eastern side of the ranges they heat up.

"During May we had La Nina weakening into a neutral state. A La Nina pattern leads to more north easterly winds in New Zealand, the fact that we had more of a westerly flow was a good indicator that La Nina was moving into more of a neutral state."

El Nina Southern Oscillation is an irregularly periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean, affecting climate of much of the tropics and subtropics. Its counterpart El Nino occurs when the oscillation changes.

International sources indicated that a neutral pattern would continue for the next couple of months, ahead of a possible change to El Nino.

For Hawke's Bay that would likely mean more westerly winds and warmer and drier conditions.

On the face of it, the warmer trend looks set to continue, with Niwa's latest Winter Climate Outlook report suggesting mid-winter temperatures are unlikely likely to be any colder than normal.

In Hawke's Bay temperatures are most likely to be near average (40 per cent chance), the report said.

Heading into winter proper it would be a changeable season as the La Nina pattern weakens into a period when neither the La Nina or El Nino effects are prevalent.

"When you're in a neutral condition, at least in New Zealand, you tend to get changeable conditions. We think especially the rest of this month, there's a chance of some north-easterly wind flows.

"Temperature wise in the winter season, we think near average temperatures are the most likely outcome - that's averaging temperatures over the span of three months - that certainly doesn't mean there will be a few days here and there that are warmer than average and a few days that are cooler than average."

June to August 2018 temperatures are forecast to be near average (40 per cent chance) or above average (35 per cent chance) in the north and west of the North Island, near average (40 per cent chance) or below average (35 per cent chance) in the east of the South Island, near average (40 per cent chance) for all other regions of the country.

Periodic northeasterly air flows during the first half of winter in particular may bring mild periods to northern areas. Westerly-quarter (NW to SW) winds during the second half of the winter season may lead to cooler conditions in southern and western areas.