Intense rain on 27 January brought provisionally the wettest day ever recorded in Auckland, New Zealand, with a number of weather stations recording in excess of 200mm in 18 hours. The weather station of Auckland (Albany) recorded 260.6mm between 3am and 9pm and some parts of the city were hit with more than 150mm in the space of three hours, bringing severe flooding.
Maximum one-hour accumulations in the city reached 50 to 70mm, lasting for several hours. These totals amount to almost 300% of a normal January rainfall, and beat the previous record for the month of 206mm in 1986. This is not far off the all-time record for a month’s rainfall in central Auckland, which stands at 420mm in February 1869 (based on historical studies by Anthony Fowler in 2020). With more heavy rain expected, rainfall totals may reach or even break the monthly record by the end of the month.
In addition to the extreme rain, Westport in the South Island recorded its hottest day on record, reaching 28.7C on 28 January. Over the past few months, sea surface temperatures (SSTs) around New Zealand have been consistently above average, and more so throughout January, which contributed significantly to the high rainfall rates last week, as well as to the overall very wet month. The warmth allows more moisture to be held in the atmosphere, enabling much higher rainfall totals in a short period of time, in this case bringing almost three times the amount of rainfall in a normal January in just a few hours.
Current estimates suggest SSTs off the west coast of New Zealand have been about 2C to 3C above normal throughout January. Several other factors that contributed to the extreme heavy rain over the North Island included La Niña, a saturated atmospheric column and a low level jet funnelling into the island.
There has yet to be a named storm this 2022-23 season in the UK, despite often wet and windy conditions through the autumn and earlier in January. Since the naming system began in 2015-16, the lowest number of named storms until 1 February was three. However, from Tuesday into Wednesday this week winds will increase, particularly across Scotland, with gusts perhaps reaching 60 to 80mph in the far north, bringing frequent wintry showers. Whether this will be enough to warrant a named storm remains to be seen.
