"If we had a weak El Nino, it's the opposite of what we had last year - and if we don't we are still expecting southwesterlies more than last year, when we had easterlies."
Oceanic indicators in the west and central tropical Pacific remained close to El Nino thresholds, but the atmosphere had yet to show any significant response to the warmer than normal sea surface temperatures.
Ms Griffiths put the chance of a weak El Nino developing at 50-50 - but Niwa's outlook wasn't dependent on such an effect.
"The key message is there's going to be a dry flavour to the early summer period."
In contrast, most areas of the South Island could still expect near-normal rainfall.
Soil moisture and river flows were forecast to follow the same regional patterns as rainfall, except that soil moisture levels were likely to be below normal in the north of the North Island, and both soil moisture levels and river flows were likely to be below normal in the east of the North Island.
Areas such as Gisborne and Hawkes Bay would need to be watched closely as soil moisture levels were already down, Ms Griffiths said.
Meanwhile, temperatures were likely to be average or above average in the north and east of the North Island and near average in other regions.
Sea surface temperatures were also expected to remain near average around the country.