Ex­treme rain breaks records

The Orchardist - - Contents - By Ge­orgina Griffiths, Met Ser­vice Me­te­o­rol­o­gist

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/.

Fig­ure 2a. Rain­fall ac­cu­mu­la­tion plots for key grow­ing re­gions Pukekohe (top map) and Palmer­ston North (lower map). The av­er­age an­nual rain­fall ac­cu­mu­la­tion is shown in black, while the to­tal for this year is shown in red (data from 1 Jan­uary un­til 20 Sep

Fig­ure 2b. Rain­fall ac­cu­mu­la­tion plots for key grow­ing re­gions Napier (top map) and Christchurch (lower map). The av­er­age an­nual rain­fall ac­cu­mu­la­tion is shown in black, while the to­tal for this year is shown in red (data from 1 Jan­uary un­til 20 Septem­ber

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