Does this week’s rainfall count as ‘extreme weather’? Are we getting more ‘extreme weather’ in Marlborough?

 

Having returned to the rainy season this week with over 21mm (not a record) falling  Monday-through-Tuesday during thirteen hours of almost continuous rainfall, it is interesting to have an overview of very heavy daily rainfall totals in Marlborough over the last 31 years.

Are such occasions increasing? Research indicates that such totals are becoming more commonplace.

Since my records began in 1984, the trend for heavy daily rainfall totals has certainly been increasing.  Taking arbitrary rainfall levels of 10mm and 20mm for a twenty-four hour period, the trend for such occurrences has been upward from 22 to 30 occasions each year for the former figure, and from 3 to 6 occurrences for the latter figure over the past 31 years.

The wettest day I have recorded in Marlborough was on 23 December 2013 when 46.4mm fell.

Further research into extreme rainfall in Marlborough and taking 40mm and above as an arbitrary figure for ‘extreme’ rainfall, I note that such a daily total did not happen in the 1980s or 1990s.

The first occasion was in October 2000 with 40.6mm. Since then there have been five more days with daily rainfall totals in excess of 40mm.  This underlines the statement from the Meteorological Office that such extreme events are happening more regularly.

We are told that the world is warming and we know that warmer air holds more moisture, which eventually falls to earth. Therefore it is not surprising to find such considerable rainfall totals are occurring more frequently.

Personal observation over the years has revealed that in the summer months, when extreme rainfall arrives from the continent, it often moves northwards to the west or east of Marlborough.

To underline the continuing variability of our climate, 1996 was the driest year since 1984 with just 540mm.   The wettest year was 2002 with over twice that total – 1,146mm.  And Marlborough’s annual average over the period I have been keeping records is 840mm.

It is worth noting that 10mm of rainfall equates to 10 litres of liquid for every square metre.