Frosts giving way to rain

If you live in Auckland and have a garden you're no doubt getting sick of the frosts as they wipe out anything remotely tropical. Auckland has been under an icy grip for a few weeks now with a massive jump in frosts compared to last winter. Temperatures right across the city have been dropping lower at night than South Island centres like Dunedin, Invercargill and even Christchurch. Day time highs have been very low, hovering around 11 and 12 as the high each day.

The Waikato region has really been hit by the frosts with the light southerly flow over the country making perfect frost conditions for the region. The air is drier and colder and clear of cloud... it tends to create less fog and therefore the temperatures plummet. We've had - 5 in Hamilton and - 7 in Tokoroa. Further south and those around the volcanic plateau have been waking to - 8 lately.

In the South Island the Blenheim region has also been hit by the big frosts. Police on Monday said ice on the roads was the worst they had seen in 20 years. Some in the Waikato are saying the frosts are the biggest they've seen in 20 years too. What about where you live? Post a comment below and let us know how severe the frosts are in your area.

Central Otago has had some big frosts with temperatures also falling to around - 8. And remember this is the air temperature which is measured a metre of so above ground. At ground level it's even colder by a few degrees.

But the frosty weather appears to only have a couple of days left (at least for the short term!) with a potentially big rain maker on the way. In fact, this next low may create rain heavy enough to cause flooding in northern and western regions of both islands and its development is one I'll be watching very closely. It's going to form in the northern Tasman Sea sucking in sub-tropical air and deepening rapidly. Yesterday some computer models showed it may even be a weather bomb (which is very rare). We often hear 'weather bomb' in the news but actually the technical description of one means they don't happen very often. A "weather bomb" is when the air pressure falls 24hPa's in 24 hours (1 hPa per hour).

If I just lost you then let me try and explain what hPa is. hPa stands for hectopascal (they're also known as millibars) this is the unit used for measuring the air pressure - just like when you're at petrol station checking the pressure in your car tyres.

I won't bore you with boring details but basically normal air pressure is 1000hPa to around 1015hPa...anything above that is usually a big high pressure system and anything under that usually a low. It's rare to see the air pressure dropping rapidly - such as 24hPa in one day (what we describe as 'weather bomb'). When we do see this rapid drop in air pressure it's just like getting a sudden flat tyre - it's unpredictable, fast, and can have major consequences.

Some of the computer models we use were showing this rapid deepening but until we're a couple days out of it I simply don't trust them. Based on history I doubt that's what we'll be getting but either way, rain, showers and northerlies are on the way for many northern regions this weekend and until the start of July. Are we now saying goodbye to blue skies and frosts and hello to higher temperatures but more rain?

Philip Duncan

 

Photo / Sarah Ivey