Niwa Outlook

SEASONAL CLIMATE OUTLOOK

Mild conditions likely to continue through winter

28 May 2010

The NIWA National Climate Centre outlook for winter 2010 says that mean temperatures are likely to be above average for the time of year across most of the country. However, short-term cold snaps and frosty periods typical of winter will still occur.

The El Niño conditions that prevailed since winter 2009 have dissipated, and the equatorial Pacific is now in a neutral state. Recent trends suggest a La Niña could develop by early spring.

Near normal seasonal rainfalls are likely in most places, but the Centre says that the southwest of the North Island (southern Taranaki, Manawatu, Horowhenua through to Wellington) is likely to experience normal or below normal rainfalls.

For winter 2010, normal or below normal soil moisture levels and stream flows are likely in the north and west of the North Island, but over the rest of New Zealand near-normal conditions are the most likely outcome.

The centre’s latest outlook states that mean sea level pressures are likely to be higher than normal over the North Island, associated with somewhat stronger than normal westerlies over the South Island, on average for June-August.

Overall Picture

Temperature:

Winter temperatures are likely to be above average over most of the country, but average or above average in the east of the South Island. Despite the likelihood of a milder than normal winter, typical winter cold spells and frosts are still expected at times. Sea surface temperatures are expected to be near average around New Zealand over the winter period.

Rainfall, soil moisture, and stream flows:

Rainfall is likely to be near normal over most of the country, but normal or below normal in the southwest of the North Island. Normal or below normal soil moisture levels and stream flows are likely in the north and west of the North Island, but over the rest of New Zealand near-normal conditions are the most likely outcome.


Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:

Temperatures are likely to be above average.  Seasonal rainfall totals are likely to be near normal, while soil moisture levels and stream flows are equally likely to be near normal or below normal.

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

Temperature Rainfall Soil moisture Stream flows
Above average 50% 20% 20% 20%
Near average 40% 50% 40% 40%
Below average 10% 30% 40% 40%

Central North Island, Taranaki, Wanganui, Manawatu and Wellington:

Above average seasonal temperatures are likely. Rainfall totals are equally likely to be near normal or below normal, while stream flows and soil moisture levels are equally likely to be in the near normal or below normal range, for the three months as a whole.

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

Temperature Rainfall Soil moisture Stream flows
Above average 50% 20% 20% 20%
Near average 40% 40% 40% 40%
Below average 10% 40% 40% 40%

Gisborne, Hawke’s Bay, Wairarapa:

Temperatures averaged over the three months are likely to be in the above average category. Seasonal rainfall totals, stream flows and soil moisture levels, are all likely to be in the  normal range.

Probabilities are assigned in three categories; above normal, near normal, and below normal. The full probability breakdown is:

Temperature Rainfall Soil moisture Stream flows
Above average 50% 20% 20% 20%
Near average 40% 50% 45% 45%
Below average 10% 30% 35% 35%

Nelson, Marlborough, Buller:

Seasonal temperatures are likely to be in the above average range. Rainfalls, stream flows and soil moisture levels are likely to be near normal overall.

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

Temperature Rainfall Soil moisture Stream flows
Above average 50% 25% 25% 25%
Near average 40% 50% 45% 45%
Below average 10% 25% 30% 30%

West Coast, Alps and Foothills, Inland Otago, Southland:

Temperatures are likely to be in the above average category. Seasonal rainfall, stream flows and soil moisture levels are likely to be near normal.

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

Temperature Rainfall Soil moisture Stream flows
Above average 50% 25% 30% 30%
Near average 40% 50% 45% 45%
Below average 10% 25% 25% 25%

Coastal Canterbury, East Otago:

Temperatures are equally likely to be in the average or the above average category, on the whole during winter. Seasonal rainfall totals, soil moisture levels and stream flows are likely to be in the normal range.

Probabilities are assigned in three categories; above average, near average, and below average. The full probability breakdown is:

Temperature Rainfall Soil moisture Stream flows
Above average 40% 25% 30% 25%
Near average 40% 50% 45% 45%
Below average 20% 25% 25% 30%