La Niña locked in – a warm early summer likely

 A moderate to strong La Niña is well-established in the tropical Pacific, and may strengthen further through the rest of 2010, says the NIWA National Climate Centre. La Niña conditions are likely to continue through to autumn of 2011.

Early summer (November to January) temperatures are likely or very likely to be above average for this time of year across the whole country.

Rainfall is likely to be normal or below normal over the South Island, with below normal soil moisture levels and stream flows likely over much of the South Island. In the North Island, rainfall is likely to be normal or above normal, with above normal soil moisture levels and stream flows in the east, according to the Centre’s latest seasonal outlook.

The National Climate Centre’s seasonal outlook states that mean sea level pressures are likely to be above normal near New Zealand and to the south of the country, for the three months as a whole.

Tropical cyclone activity is likely to be near- or above-normal this season (November–May). The risk of an ex-tropical cyclone passing close to New Zealand is slightly above the long-term average. On average, at least one ex-tropical cyclone passes within 500km of New Zealand in 9 out of 10 cyclone seasons.

Overall Picture

Temperature:

Averaged over November-January, temperatures are likely or very likely to be above average in all districts. Sea surface temperatures are presently near normal around New Zealand, but are expected to become warmer than normal around the North Island as the season progresses.

Rainfall, soil moisture, and stream flows:

Seasonal rainfall is likely to be near normal or above normal in the north and east of the North Island, near normal over the southwest North Island and the northern South Island, and normal or below normal over the rest of the South Island. Soil moistures and stream flows are likely to be above normal in the east of the North Island, near normal in the western North Island and northern South Island, and below normal over the rest of the South Island.

Regional predictions for the next three months:

Northland, Auckland, Waikato, Bay of Plenty:

Temperatures are very likely to be in the above average range. Seasonal rainfall is equally likely to be near normal or above normal, while there is no clear guidance for soil moisture levels and stream flows.

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

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

Temperatures are very likely to be above average for early summer. Seasonal rainfall totals, soil moisture levels, and stream flows 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:

Gisborne, Hawke’s Bay, Wairarapa:

Temperatures are very likely to be in the above average range. Seasonal rainfall is equally likely to be near normal or above normal, while soil moisture levels and stream flows are likely to be above normal overall.

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

Nelson, Marlborough, Buller:

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

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

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

Temperatures are very likely to be in the above average range, for the three months as a whole. Seasonal rainfall is likely to be below normal. Soil moisture levels and stream flows are likely to be below normal or near normal.

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

Coastal Canterbury, East Otago:

Temperatures are likely to be in the above average range. Seasonal rainfall is equally likely to be near normal or below normal. Soil moisture levels and stream flows are very likely to be below normal for November-January.

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

Background

The tropical Pacific is in a moderate to strong La Niña state, which is likely to continue into early 2011, and may strengthen further through the rest of 2010.