Polar Blast - June 21-22

[Paul Mallinson]

The other angle to snow forecasting is what the temperature regime aloft and where the vertical motion and moisture is. For instance the best snow production occurs where strong vertical motion and moisture are located in the Dendritic Ice Crystal zone between -12C and -18C, but as the cloud tops warm, snow production becomes less efficient and denser until you get down to a lower cloud top with temps around -5C or less where no snow is likely and probably drizzle or freezing drizzle occurs. Showers made up totally of graupel (small hail) seem to occur when it is a convective situation and where cloud top temps are not really in the dendritic zone and there is chunky low level buoyancy.
Paul

[Razor]

I mentioned during the previous storm a couple of days ago that snow levels vary with the intensity of the precipitation. The situation like we have over CHCH this morning and this same feature feeding heavy showers over the lower North Island can be tricky. The environmental conditions may suggest generally snow to some level - we thought virtually to sea level this morning for the CHCH area (and there are flurries being reported I see), but, when you get this wind convergence feature (I call them streamers, others on this site call them chain showers) the intensity of the precip can vary a lot.
Firstly, the best meteorological parameter to use to gauge the snow level from is the height of the wet bulb temperature where it crosses the freezing mark (called the WBZ). Snow will penetrate some value below this level. For instance 1mm/hr will allow snow to penetrate about 300m below the WBZ, precip of about 3mm/hr is about 500m (from memory) and 10mm/hr is about 800m. So in these situations, you not only have to get the location of the convergence right, but also the precip rate. Then, on top of that you have to decide how much snow will be produced! So its a very tricky game.
Paul

Thanks Paul- really appreciate your contributions to the forum.

[Skywatcher]

Just had a brief flurry of very small flakes just ahead of the last shower of rain.

[JPG]

Looking out the window of my building in Sir William Pickering Drive, nothing. Not even rain. Was definitely sleeting this morning around 8:45am for 20-30 minutes, but that's it so far.

[Richard]

The broken low cloud seems to be making its way inland - I think we may lose the sunshine we have at the moment.

Same here too,was mainly sunny but now its clouded over.

[Wildland]

The temperature at Tawa, north of Wellington, has been hovering between 5C and 7C since dawn.
That's cold for us.
Bursts of hail, some sleet, and the odd sunny spell.
A "keen" southerly.

[mikestormchaser]

Hi paul. Great having your thoughts and expertise in the forum. Although I don't understand all the dynamics of it alll and in depth detail its great to have your opinions. I saw the 850mb sitting at - 8 but I see the 500mb is at -27 this morning witch in winter isn't actually dramatically cold. The THK was around 524 witch is also marginal for sea level snow.

[Karliosis]

There have been sleety showers on and off all day here in Riccarton, including one just in the last few minutes. Very fine hail, kind of like thick icy drizzle in some of the showers earlier.
Not particularly looking forward to the severe frosts over the next few days!

[Richard]

Not particularly looking forward to the severe frosts over the next few days!

Yes they are going to be hard aright

[Richard]

Hi paul. Great having your thoughts and expertise in the forum.

I agree,i'm sure it helps a few of us here to better understand the dynamics

[TonyT]

Very fine hail, kind of like thick icy drizzle in some of the showers earlier.

That must be the "unknown" precipitation!

[Paul Mallinson]

Thanks for the positive feedback.

Mike asked this a little while ago:
"Hi paul. Great having your thoughts and expertise in the forum. Although I don't understand all the dynamics of it alll and in depth detail its great to have your opinions. I saw the 850mb sitting at - 8 but I see the 500mb is at -27 this morning witch in winter isn't actually dramatically cold. The THK was around 524 witch is also marginal for sea level snow."

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:

Karliosis on Mon 22/06/2015 14:28

"There have been sleety showers on and off all day here in Riccarton, including one just in the last few minutes. Very fine hail, kind of like thick icy drizzle in some of the showers earlier."

This sounds to me like the cloud tops are now warmer and that we are now seeing the graupel/freezing drizzle/drizzle rather than snow that I mentioned earlier this morning. (just looked the NZIR image on the Metservice web site but its hard to tell what the temp is).

Hope this helps.
Paul

[Manukau heads obs]

I see its only 7C in Taupo currently and -6 in Tekapo!

[Bradley]

I see its only 7C in Taupo currently and -6 in Tekapo!

I was surprised to learn how cold Lake Tekapo can actally go, it’s all-time record is -16.7C on both the 29th and 30th September 1949. Hard to believe it was at the end of September it recorded those minmums! Both the maximums on those days were 6C.

[moylanr]

Thanks Paul.

A colleague's system shows 6.7C at the wharf at Little Akaloa on Banks Penisula (which I realize will be different from off the coast, but is the only sea temperature monitoring site I know of nearby): http://www.andi.co.nz/

See "About" tab for site photo.

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.

PS Looks like the highest power load for Christchurch/Canterbury since 17 August 2011 coming up in the next couple of hours.

[Razor]

I see its only 7C in Taupo currently and -6 in Tekapo!

Where are you seeing the tekapo temp? I have seen a couple of sites, one showing 4pm +2 the other 4:30pm -2

I see metservice have Omarama aiming for -13!

[Manukau heads obs]

the M/S weather now..
http://www.metservice.com/maps-radar/lo ... ur-weather
its a met service station too
now at -8 in Tekapo!

[Razor]

the M/S weather now..
http://www.metservice.com/maps-radar/lo ... ur-weather
its a met service station too
now at -8 in Tekapo!

Got ya cheers. Must be in that rapid freefall now the sun is down. Meanwhile snowing again on the Port Hills here in ChCh

[Tim S]

-13C at this site: http://www.wunderground.com/personal-we ... CANTERB254

Meanwhile its still 3C here and has been all day, with drizzly sleet at sea level. Plenty of snow on the hills around the Peninsula though, Not on the Northern facing slopes though, mainly on the Southern faces as you'd expect.

[Manukau heads obs]

wow
-14C already in Twizel
maybe a record is going to be set?

[Tim S]

Does anyone know where I can access the Sounding from today? Would be interesting to see what the actual 850 temps and thicknesses were as they seem to be a fair bit higher than forecast by GFS.

Uh oh -56C in Dipton http://www.wunderground.com/personal-we ... ISOUTHLA57 Definitely Dipping in Dipton

[David]

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

[NZstorm]

Does anyone know where I can access the Sounding from today? Would be interesting to see what the actual 850 temps and thicknesses were as they seem to be a fair bit higher than forecast by GFS.

Only South Island location that puts up a balloon is Invercargill. The midday release. 850mb temp -6.7C.

[Tim S]

Cheers NZstorm, so a good 2-3C warmer than forecast, hence the snow settling level only 150 meters or so at best. Decent inversion of warmer air around 800hpa too probably didn't help things. And a 700hpa temp of around -8C. Still waiting for my first Dunedin snow day...

[Flutterbye]

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

Looking at the amount of snow that fell in the region with little melting, I'm not surprised that the frosts are so
severe. They probably won't get any relief until the high moves on.


http://www.tekapotourism.co.nz/gallery/snow_2015.html