Polar Blast - June 21-22

[Bradley]

Maybe close to -20 for Pukaki Aerodrome (near Twizel) tomorrow morning!?

-20C is very hard to get to, it has happened only a handful of times in NZ's history, you need everything to go right for it to happen

[Manukau heads obs]

already at -15C at Twizel!

[Razor]

Still a balmy 3 degrees here with passing showers of unknown precipitation

Annoying as apparently there is strong aurora activity tonight

[jamie]

The wind isn't dropping here. It's holding the temp but in sure once the wind goes we should see the temp five below zero quickly.

Impressive cold temps tonight. It's got a lot of things going for it. Incoming high on a departing southerly, snow on the ground, longest night of the year.....

[Nev]

Moot point but, technically, last night was very marginally longer than tonight...

[RWood]

Moot point but, technically, last night was very marginally longer than tonight...

Yes - because the day-length of the 21st is about 2 seconds less than that of the 23rd (in Wellington), the length of the 21-22 night is greater than that of the 22-23 night by half that, i.e. about 1 second {the lengths of the solar days figure in the equation but they change by a much smaller amount in such a short span of days}

[mikestormchaser]

Thanks for the positive feedback

This is an interesting point. I noticed that it is not as cold as it might be at 500hPa (about 18000ft) but I remember it was going to get as cold as -17C at 700hPa (about 10,000ft) which is a really cold winter outbreak temperature. So what we have is very cold temps at 850 (about 5000ft) and 700 of about -8C and -17C but not so cold at 500hPa. Given the sea surface temperatures off the Canterbury coast of - I'm guessing - about 14C you can see that convection will be pretty vigorous and buoyant into the mid teens say 13-15000ft and then less so further up. This can still generate heavy precip and snow production and therefore lower the snow level significantly.
Mike mentioned a thickness of 524. This is the thickness (5240 gpm) between 1000 and 500 hPa and so this may be a little deceptive given the cold temps below 500. Therefore a better measure may be the 1000 to 700 hPa thickness which would reflect the cold temperatures. This thickness is not so commonly use although the partial thicknesses of 1000 to 850 hPa and 850-700 hPa are used to forecast areas of freezing rain etc.

I was interested to read in recent posts that the precip in the Ch area was rather icy like. Take this post:

Paul

Thanks again paul, im not the quickest learner when it comes to taking info on paper. I really need to be showen practically so if i was to go onto the arl.noaa.gov website and instead of putting the THK between 1000mb and 500mb your saying use the measure between 1000mb and 700mb for a guidence of freezing rain " snow" ?? any tips on a guidence of looking at potential thunderstorm activity aswel?

[snowchaser01]

Still getting heavy sleet showers moving through CHCH... i wonder if a snow flurry or two is possible later with left over rogue showers moving through?

[Tim S]

-18C now at Twizel (Pukaki Aero), which looks like it's minimum so far. Does anyone know if this is a private station or a metservice one?

[03Stormchaser]

I believe its a metservice one, hit -19.1c earlier!

[Tim S]

Sweet, So it will be an 'official' temp then?! The lowest temp I can see at that site is -18C

http://www.wunderground.com/personal-we ... 623/mdaily

That seems to only show readings every 30mins

where did you see the -19.1C?

[Manukau heads obs]

showing -19.3C currently ! (if you click on the station)
http://www.metservice.com/maps-radar/lo ... ur-weather

[Paul Mallinson]

Thanks again paul, im not the quickest learner when it comes to taking info on paper. I really need to be showen practically so if i was to go onto the arl.noaa.gov website and instead of putting the THK between 1000mb and 500mb your saying use the measure between 1000mb and 700mb for a guidence of freezing rain " snow" ?? any tips on a guidence of looking at potential thunderstorm activity aswel?

You could get a 1000 to 700 hPa thickness, but you would have to have some benchmarks as to when that is a low value or not - for instance like we do with the 1000 to 500 hPa and we say snow is possible to sea level at about 522 etc.

For freezing rain you need to use the two thicknesses 850-700 and the 1000-850 together. So you need to have the upper one exceeding a certain value and the lower one at a cold value. I can't remember these off the top of my head but if you are interested I could look them up - something we don't need very much in NZ. I used to use these a lot when I was working for the Canadian weather service. These two layers are signalling a warmer layer overlaying the colder lower layer.

As far as TS forecasting, this is a big area that I can't cover here, but you would use the usual parameters such as the Lifted Index etc. But for thunderstorms you always have to have a trigger. Apart from surface moisture convergence, there is usually a short wave trough aloft providing the cooling/trigger aloft.

Paul

[mikestormchaser]

Thanks paul for your input

[Tim S]

Turnballs Bay June23.jpg

Photo showing the snow still quite low on the Port Chalmers side of Otago Pennisula, looks more like Queenstown than Dunners!

[Ice Maiden]

Nice photo Tim, the snow has really hung around, I was amazed how many cars still had snow on their bonnets and roofs today and it wasn't melting!

[Orion]

Great photo; and I noticed this morning on the Dunedin webcams the traffic moving quite carefully.

[RichyDunedin]

Got a lot of good photos yesterday but the file sizes were too big so here is one of Whare Flat in Dunedin yesterday where a lot of cars were stuck or had slid off the road after the snow froze.

[Tim S]

Great photo; and I noticed this morning on the Dunedin webcams the traffic moving quite carefully.

Yes carefully was the only way you could get anywhere this morning, was surprised to find that the Dunedin City Council only gritted a few corners on the bay road, though their ignorance shouldn't surprise me anymore...

Nice shot RichyDunedin, kinda of glad I live below the snow line right now!