TECHNICAL
SPRING HAS SPRUNG: TWO RED WARNINGS AND DROUGHT
Words by Georgina Griffiths : MetService meteorologist
Winter of 2021 has had an ‘extreme’ feel to it – with flooding rains and drought featuring and a highly damaging June tornado thrown in for South Auckland. MetService is New Zealand’s only authorised provider of Severe Weather Watches and Warnings. Since introducing a new colour-coded system of Warnings in May 2019, only four Red Warnings have been issued. MetService reserves a Red Warning for only the most extreme weather events, where significant impact and disruption is expected. This year, three red severe weather warnings were issued in relatively quick succession. At the end of May, MetService issued its second Red Heavy Rain Warning for the Canterbury region. Dangerous river conditions and flooding were expected. In mid-July, MetService issued its third Red Heavy Rain Warning, forecasting an extended and extreme heavy rainfall event for Buller and Westland north of Fox Glacier, with significant rainfall also expected for Nelson and Blenheim. Both of these Red Heavy Rain Warnings were on the money, with each event being highly damaging. A Red Severe Wind Warning was also issued for the Canterbury High Country on 12 to 13 September,
with extreme gusts and significant damage observed in elevated areas. In contrast, Auckland, Coromandel, Bay of Plenty, Gisborne and Hawke’s Bay remained unusually dry through until June, and these were areas that had experienced back-toback droughts through 2019 and 2020. Soil moisture in these regions remained in deficit considerably longer than usual (Figure 1, showing Napier as an example). At the time of writing, the year-to-date rainfall for Napier Airport was sitting at 55% of year-to-date normal (Figure 2), while Te Puke had recorded 82% of year-to-date normal rainfall. In contrast, year-to-date rainfall for Blenheim and Christchurch was 117% and 109% of year-to-date normal, respectively (Figure 3). Nationally, New Zealand experienced its warmest winter on record in 2021 – breaking the previous record set in 2020.
Spring has sprung The El Niño–Southern Oscillation (ENSO) remains neutral at the present time. However, there is recent chatter about a resurgent La Niña for summer – in other words, a secondtime-around La Niña. If La Niña does return by the end of the year, current modelling suggests it will be on the weaker side. What’s likely to be a more important player in New Zealand for spring (September to November), is a sharp change in weather maps. The latest long-range predictions from MetService, based on running a large group (‘ensemble’) of weather models, signal a quick transition from the active westerlies that have characterised August and early September, towards more Highs than usual lying over the South Island. Given the time of year, what this will likely mean, in practice, is a mixture of some good old fashioned spring westerlies and intense but intermittent Highs over the South Island. Keep up to date with the MetService long-range forecast at: http://metservice.com/rural/monthlyoutlook
62 NZGROWER : OCTOBER 2021
