New Zealand Weather

[J07]

NZWN 082200Z 14008KT 130V190 9999 FEW018TCU SCT035 15/11 Q1007 NOSIG RMK KAUKAU 19009KT

Reasonable thundery risk over the next few days with unstable southwesterlies associated with a cold upper air pool (about -30C at 500hPa). Forecasts are going for strong, cold southerlies for many places so it looks, TWS, that you will be right about the coldness of this.

We had the first change push through this morning, with some decent showers.

Attached shows this first change going slowly through the south yesterday, you can see exactly where the front is.

There is a very noticeable difference to the airmass out there. It's been a while since we had a southerly, and now the sun is so bright that I can't keep both eyes open when outside. Despite the cool air, the sun is gradually wearing me down!

The main airmass change comes through this afternoon.

[J07]

Further fronts cooling things off and sparking thunder and hail all along the change.

At 5PM it was 12C in Ashburton with a gusty 40mph southerly and 85% RH; 50 miles up the road in Christchurch it was 21C with also a gusty 40mph wind but this time from the northwest and with RH of 35%.

The coldest air never makes it far onto the North Island. The freezing levels fall to 800m in Southland, but never get below 3000m in Northland. Tomorrow looks like a cold and windy day on the South Island. The southwesterlies ease off late tomorrow and into Thursday. A high then moves in and the weekend, at this stage, is looking rather delightful.

Early next week, models currently keen on setting up our best friend- the split high with centres both to the east and the west. This is almost always a good situation and can hang around for quite a while and maintain light winds and predominantly fine weather over most of the country (except around the periphery).

The crux of this coming off appears to be the incoming trough on Monday. It never looked that strong anyway, but the latest output wants to fade it out and build a high in the Tasman (whilst the "old high" slides off to the east). Will have to keep an eye on this over the next few days. I suspect that Hamish still has some spanners to add to our works anyway.

The temperature drop in Christchurch was apparently 24C to 12C in 14 minutes.

[J07]

The post mortem of that cold snap shows snow to 400 metres in the south. One place never made it above 6C all that day (elevation 450 metres, but still....).

Mean speeds of 50 knots were recorded about the south coast, and the wind was from the southwest. That is extremely cold stuff!

Since then, that high gradually worked its way over us and then stalled to our east. This kept things very settled for a few days, and here in Wellington with a northeasterly synoptic flow, the weather is guaranteed to be stunning. Highs in the mid 20s, with no wind, and no clouds. If only we had northeasterlies more often!

The pattern seems to be for a low to track just south of the South Island, with a front crossing almost the whole country but becoming thermally weak about northern latitudes.

A high quickly fills in behind in the Tasman Sea. But after that, it's up for grabs. Will the high stay just a little to the east and allow disturbed southerlies with depressions tracking just to our east? Or will it predominantly ridge over us and keep the weather sunny and settled?

[Thundery wintry showers]

Judging by the GFS (you can get the Southern Hemisphere outputs, albeit in German and at a different projection, at www.wetterzentrale.de in the Topkarten section) their output suggests a southerly regime over NZ for most of the upcoming period. Surprising to see low pressure more or less stationary to the east- I thought lows in the Southern Hemisphere generally moved quickly from west to east because of the lack of continents in the way!

[J07]

Judging by the GFS (you can get the Southern Hemisphere outputs, albeit in German and at a different projection, at www.wetterzentrale.de in the Topkarten section) their output suggests a southerly regime over NZ for most of the upcoming period. Surprising to see low pressure more or less stationary to the east- I thought lows in the Southern Hemisphere generally moved quickly from west to east because of the lack of continents in the way!

Yes, with a high persistent to our west that keeps southerlies over us. Sometimes it can be quite pleasant, sometimes not.

Normally the depressions do track as you suggest. In this case it looks as though a high briefly sets up somewhere to our south and this disrupts their normal pattern.

I tend to go for the ECMWF for longish range stuff. The only thing it messes up is subtropical lows. Takes a while to get a handle on them.

[J07]

Weather a bit "meh" here at the moment. Nothing too exciting. Sunny, 20C, northwesterlies gusting to 50 knots in the city. All in a day's work for Wellington.

Some good examples of lenticular clouds attached. Evidence of significant disruption at mid levels. Resulting in Christchurch foehn wind, clear skies, 25C and 25% relative humidity. It then dropped 6C in an hour as the weak cold change swept through.

Also as the equinox rapidly approaches, the UV index has now fallen from "very high" into "high". Will start dropping sharply off in April. Not sure whether to be happy or sad!

Edited March 19, 2009 by J07