High in Tasman responsible for cold blast

After all big weather events the Weather Watch Centre always sits down and runs through what we did well and what we didn't. Sometimes we involve management and journalists to see how the news coverage was and we reflect on things like any misquotes, typos, hype, lack of coverage - and of course the big one: were we accurate?

I have mixed feelings about our accuracy this weekend. Please note when I say "our" I'm speaking of the Weather Watch Centre. I think for the most part we were pretty good. Last Monday we first spoke of a bitterly cold Antarctic blast expected to arrive over Queen's Birthday Weekend. We expected conditions to be colder than the previous wintry outbreak two weeks ago. Snow was possible and it could be heavy. As last week wore on we backed off a little from the predictions of widespread "heavy" snow but were instead focused on two things, the very cold air on the way and the fact that it could produce snow in to some of the North Island main centres.

In the end, a large high out in the Tasman Sea controlled this blast of cold air. It pushed it further east ...so instead of slamming into Southland or Otago or Canterbury it really only brushed those regions...and instead slammed into Wellington, Wairarapa and Hawkes Bay (which are significantly further east).

The air was so cold and so dry that while temperatures were perfect for snow right across the North Island - there was simply very little moisture to make snow flakes. This was especially evident once the air dried out as it passed over the eastern ranges and Desert Road area...once it got north it dried out completely. We accurately predicted snow flakes in Wellington and also snow as far north as the Coromandel and Kaimai Ranges. On Saturday we realised the South Island wasn't really in for a rough ride with this one and instead put all our focus on the North Island.

I put out a press release on Saturday afternoon saying snow wouldn't be heavy in the main centres of the South Island, however it was possible that light snow would fall in some North Island centres...which was very rare. Snow almost never falls in North Island centres...it's usually confined to the ranges and places like Waiouru or Taihape or Ohakune. The "conservative" view was that snow flakes were possible near or in towns like Rotorua, Taupo, Tokoroa and Turangi. From what I gather that didn't happen.

Why? Well we had almost all the ingredients...except that the southerly didn't carry the moisture far enough north. But to prove that it DID have snow making capability just look at the remarkable news stories surrounding this event. Snow in Napier city! Snow around Hastings. Snow to sea level in Wellington - first time since the mid 1990s.

Snow in Tawa and Porirua with reports of brief but "heavy" snow falls in suburbs around Porirua. A sleet/light snow shower in Palmerston North. Heavy snow showers in Upper Hutt. You can see what this system did simply by "brushing" the lower North Island...imagine if that high had weakened just a little...we were talking a high thousands of kilometres wide weakening just a few hundred kms out and heavy snow would've been falling right across New Zealand.

I think I read somewhere that MetService had continued their heavy snow warnings due to the fact the temperatures were exceptionally low and there was the potential for serious issues for large centres. This air was super chilled.

No matter how you look at the weekend's weather event it was most definitely bitterly cold. Wind chills of -5 and -6 in Wellington and Christchurch on Sunday. Auckland fell from 12 to 5 in just over an hour late afternoon when that front came through. Napier and Hastings had rain, sleet and some snow showers with a temperature hovering around 1 and 2 degrees.

We're always keen to hear your thoughts on the coverage - and weather predictions from the various weather providers, including me and my team.

Speaking of my team - we've launched a new look website. You'll find it at www.weatherwatch.co.nz. Thanks to everyone who contributed over the weekend... I note nzherald.co.nz covered the blast moving up the country exceptionally well on Sunday. Your photos and eyewitness reports go right to the top!

 

Philip Duncan

Pictured above: Soccer players in Sunday's Wellington Senior Women's league at Hutt Park shelter are battered by a hail storm. Photo / Russ Burley