December 2017 was characterised by higher than normal sea level pressure over New Zealand and the surrounding seas. This pressure setup, consistent with La Niña conditions, resulted in long periods of dry, settled, and very warm weather across the country throughout the month.
Below normal (50-79% of normal) or well below normal (<50% of normal) rainfall was commonplace for large swaths of New Zealand in December, a product of the higher than normal sea level pressure entrenched across the country. In fact, a large number of locations observed either their lowest or near-lowest December rainfall on record.
Temperatures were well above average (>1.20°C of average) nearly everywhere in New Zealand. Isolated above average (0.51-1.20°C of average) temperatures were observed in Gisborne, inland Hawke’s Bay, and a small area of the Far North.
As of 1 January, soils were significantly drier than normal for the time of year across a large portion of the North Island, Tasman and northern West Coast, interior Canterbury, and much of Southland and interior Otago. Soil moisture was slightly below normal to near normal for much of coastal Canterbury and Otago. Meanwhile, near normal to above normal soil moisture was observed along the east coast of the North Island, coastal Marlborough, and Westland to Fiordland.
Departure from average air temperature for December 2017.
End of month water balance in the pasture root zone for an average soil type where the available water capacity is taken to be 150 mm.