Risk of thunder rises as the high holds on

Warm, humid and short on rain has been the story for the last seven days. High pressure has remained over New Zealand for more than two weeks now, although last weekend a low brushed the far north. With no major fronts bringing any air mass changes to New Zealand, the air has picked up moisture and increased in humidity. As well as making the warm temperatures feel less comfortable, extra humidity at the surface also increases the chance of convection.

“Convection means everything from fluffy afternoon cumulus clouds to thunderstorms”, explained meteorologist Tom Adams. “Surface moisture is one of the key ingredients for convection, which is why we have seen an increasing risk of thunderstorms this weekend”.

Yesterday afternoon thunderstorms sparked off in the central Otago and northern Southland regions, during which Manapouri received 20.8mm of rain. This was the highest weekly rainfall of any New Zealand town, and several rural sites saw brief falls of 10mm or more in an hour. In comparison, the only larger centres to receive more than a millimetre of precipitation in all of the last seven days were Napier and Gisborne, which saw 14.2mm and 4.6mm respectively at the start of the period.

The risk of thunderstorms is still highest for inland South Island today, with a chance of hail, although the central North Island also has a moderate risk. Thunderstorm outlooks can be found here. Into next week the risk of thunderstorms remains in these areas, as the high remains parked in place and little changes in our weather. Warm temperatures, light winds, quite a bit of cloud and showers popping up in the afternoon are all set to continue.

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