Springtime rain ahead for some
A wetter than normal spring is on the cards for the north and east of the North Island, while for most of the South Island the next three months could be drier than usual.
In its seasonal climate outlook, published today, Niwa said a weak short-lived El Nino weather pattern was predicted for spring and summer.
Despite that, seasonal forecast models had not yet shown any signs of the enhanced south-westerly and westerly airflow over New Zealand usually associated with such events.
Rather, westerlies were expected to be weaker than normal from September to November.
That was because higher than normal pressures were expected in the south Tasman Sea and over southern New Zealand, along with lower pressures to the north of the North Island.
Spring rainfall, soil moisture and river flows were likely to be above normal or near normal in the north and east of the North Island, Niwa said.
In the west of the North Island and in Nelson-Marlborough they were expected to be near normal, while in remaining South Island regions rainfall was expected to be near or below normal.
Air and sea temperatures were likely to be near average around the country. Frosts and spring snowfalls could not be ruled out.
Niwa said that nearly all the climate models it monitored predicted the presence of a weak to moderate El Nino during the rest of 2012, but it was predicted to decay in the first quarter of 2013.
While sea surface temperatures continued to warm in the central equatorial Pacific, and were now above the accepted El Nino threshold, other important El Nino indicators remained close to normal. Those included the strength of the trade winds and the location of convection along the equator.
During an El Nino event, trade winds weaken leading to a rise in sea surface temperature in the eastern equatorial Pacific and a reduction of upwelling off South America. There is heavy rainfall and flooding over Peru, and drought over Indonesia and Australia.