Extreme rain breaks records
An active Tasman Sea has been the main culprit (Figure 1), with frequent, and deeper than usual, low pressure systems spinning up over the Tasman Sea and shifting onto New Zealand. These have produced frequent northwesterlies over the North Island, but also an unusual number of wet easterly events for eastern regions of both Islands.
A YEAR’S WORTH OF RAIN – ALREADY
Many locations in the upper North Island, and along the eastern South Island, have already received more than their usual annual quota of rainfall – at only nine months through the year! The extreme amount of rainfall seen in 2017 has, of course, resulted in significant problems with flooding, slips, pugging, and has exacerbated run-off during high intensity rainfall events.
Winter and early spring weather has continued to be ‘difficult’ for growers in most regions of New Zealand.
Table 1 lists year-to-date rainfall totals at specific locations around New Zealand. The greater Bay of Plenty region, Waikato, Gisborne, and Canterbury all stand out as the areas having received extreme rainfall amounts this year. At the time of writing, Tauranga, Te Puke, Hamilton, Gisborne, Rotorua, Taupo, Paraparaumu, Christchurch and Ashburton have all received more rain so far in 2017, than is normally received across a full year.
TEMPERATURES AREN’T PLAYING NICELY, EITHER
September temperatures ran on the cooler side for much of the inland South Island, but were generally closer to average elsewhere. However, on closer inspection, temperatures during September swung quite wildly between unusually cold, and extremely mild, for the time of year. Meaning challenging growing conditions, especially when you throw in a couple of decent ‘cut-off’
lows (lows with a closed isobar) that formed over the Tasman Sea during the month, and then took their time crossing New Zealand. The unstable atmosphere associated with these cutoffs produced widespread and copious hail during September, especially across the North Island.
Figure 1. If you take an average of all the weather maps for the period 1 July - 15 September 2017, and subtract the usual weather map at this time of year, you end up with the weather map anomaly (difference from normal), shown here. It clearly shows the abnormally low pressures observed over the Tasman Sea and New Zealand region during the last 2.5 months. Maps were created using the NOAA/ ESRL Physical Sciences Division website www.esrl. noaa.gov/psd/.