It’s the classic mix of polar air and warm sub-tropical air out in the Tasman Sea today that’s creating thunderstorms across much of the water and the low that’s causing these storms is moving our way.

The low is deepening today and while many of us across the country enjoy a sunny spring-like day it’s all set to go down hill – dramatically for some.

Head weather analyst Philip Duncan says Taranaki and Nelson are especially exposed to thunderstorms, some heavy tonight and during tomorrow.  “Where the cold air and warm air meet is where the roughest weather will be and that will be Taranaki and Nelson – with New Plymouth most exposed”.  Mr Duncan says downpours may be heavy enough to cause localised flash flooding in some streams and surface flooding across roads however he points out conditions won’t be widespread.  “We’re talking very heavy downpours here and there.  It’s the luck of the draw where these small but severe downpours will exactly take place”.

He says heavy downpours are likely across the west coast of both islands overnight tonight and during Tuesday.  “There could be some squall lines developing – that’s a line of very heavy showers than can deliver gale force winds that come out of no where. Often mistakenly called “mini tornadoes” these squalls simply create a “wall” of wind that can be localised and uplift roofs, fell trees and bring down power lines”.  Mr Duncan says coastal areas in the west are usually most exposed but usually only a handful of properties nationwide are severely affected.

However weak tornadoes – that don’t make it into the traditional measurement of twisters – are also possible.  “We measure tornadoes using the Fujita Scale…usually F1 to F5 (F5 being catastrophic damage).  The ones we typically get in these sorts of systems are often F0 – winds below hurricane speed but can still cause some damage”.

MetService has issued rain warnings for Mt Taranaki and West Coast regions – they can be found in the weather section at newstalkzb.co.nz. MetService is  also advising thunderstorms and “weak tornadoes” may affect some regions – their warnings can be found at metservice.com. 

For the latest lightning strikes please use the free Weather Watch Centre lightning radar on the right hand side of this page.