Head weather analyst Philip Duncan takes a look at why the forecast for Anzac Day is harder to pick this year…

A large high over New Zealand – or a deep and nasty looking sub-tropical low.  These are the two options for Anzac Day weather this Saturday…and it’s the reason we haven’t made a call on the forecast for this weekend yet.  It’s too 50/50…and we can hardly say “Anzac Day will be sunny with the risk of a sub-tropical storm”!  

MetService has issued a severe weather watch for northern New Zealand – with low to moderate risks for places like Northland, Auckland, Coromandel Peninsula, Bay of Plenty and East Cape receiving heavy rain.

However computer models are showing other possible outcomes.  According to long range weather maps the low could track into the central Tasman Sea and maybe cross the lower South Island on Sunday or Monday.  Weather data from Weather.com says there’s a 30 to 40% chance of rain for northern New Zealand late on Saturday and perhaps Sunday too.

The issue with a tough cookie like this forecast is a large high just to the east of New Zealand and a deep low north-west of us.  The high isn’t anchored over us – it’s centred east of the country and that makes NZ vulnerable from lows moving in from the west.  But if the high strengthens, as predicted by the weather maps, then that will push the low into the Tasman.  Think of the high as a bump in the ground and the low as water flowing towards it…the water will move around it not over it. If the high is over us then we’re protected from the severe stuff.

So there’s a fine balance at play here – if the high moves to the east (or weakens) then that flattens out the air pressure and allows the low to easily follow in behind it.  If the high strengthens then the low slides down around the western side of it – taking it close to NZ but offshore out in the Tasman Sea.

Definitely something to watch closely if you live in northern NZ.  To give credit to MetService they know our severe weather better than weather.com or the international weather charts.  Then again, it’s 2 against 1. 

For the rest of New Zealand I think most of the main centres are pretty safe at having a dry Anzac morning – possibly warm in the South Island with nor’westers, rain on the West Coast, and dry and calm everywhere else.

Anzac Forecast – check back later this morning for WeatherWatch.co.nz’s forecast for Anzac Day.