Canterbury farmers are being told to prepare for snow down to 400 metres above sea level and four days of bitterly cold, windy and wet weather during the onset of lambing.
The intensity of the cold snap was still being examined by computer models, but Blue Skies Weather forecaster Tony Trewinnard said patterns were pointing to a depression deepening in the Tasman Sea and an easterly airflow increasing over the South Island today and tomorrow.
As the depression stalled over the North Island from Sunday, a cold southeasterly airstream would be directed over much of the country.
This was likely to remain over Canterbury until late Wednesday and coincide with another depression developing to the east on Tuesday and Wednesday, bringing cold and damp weather.
As much as 100 millimetres of rain could fall, and would be at its heaviest on Tuesday and Wednesday, with lambs exposed to wind chill from low temperatures.
Snow starting on Sunday could fall to 400-500m from Monday, with possibly half a metre of snow above 500m and 1m above 700m. Mr Trewinnard said conditions would be hazardous to newborn lambs, even if snowfall depths failed to eventuate, and could be expected to cause transport disruptions from flooding.
Only small fluctuations were needed to alter the intensity of weather systems to affect how much snow would fall and where it would land, he said. Although snow was unlikely below 400m, this could not be discounted.
"Sheep up on the hills will have a cold and damp period from Monday to Wednesday, probably with temperatures in the single figures, no sunshine and reasonably strong winds, so there will be a significant wind chill.
"It will be raining frequently throughout this period, and a stressful time for stock."
South Canterbury had surface flooding this week despite moderate rainfall. The ground was saturated, rivers were up because of earlier rain, and more flooding was likely.
Mr Trewinnard said farmers had dealt with August storms before and would be ensuring stock had enough feed and were in sheltered paddocks. "Every winter we get periods like this. Sometimes they coincide with lambing and sometimes they don't, but this one does."
Farmers were fortunate that heavy snow in June missed the lambing beat, he said. They had a fine window during the next few days to prepare for the latest cold snap.
With the exception of a warmer July, winter had been cooler, cloudier and wetter, in keeping with a declining and decaying La Nina in the Pacific, with few westerly airflows and often low-pressure systems coming from the Tasman.
Showers are expected to clear next Thursday, with sunny periods increasing and cold southerlies easing ahead of a longer spell of mostly fine weather. Fairfax NZ
- The Timaru Herald