Polar Blast 23-26th July

[Manukau heads obs]

I would think you would need a relatively thick layer of cold air at the surface
for the rain to turn into snow before reaching the ground
even then I do not think i would be proper snow, but some sort of ice (groupal)?

the SI/Canty weather guys would know more about this

[spwill]

I would think you would need a relatively thick layer of cold air at the surface
for the rain to turn into snow before reaching the ground
even then I do not think i would be proper snow, but some sort of ice (groupal)?

Yes, the rain wont turn back to snow but it can freeze into pellets of ice, I have see this in the Uk, not sure if it happens in NZ.
In Central Otago I have seen light rain freeze into ice on reaching chilled ground.

[NZstorm]

Sometimes you can get an isothermal layer close to 0C between the freezing level and the ground. That can give surprise snowfall when the freezing level height or thickness do not suggest low level snow. That kind of effect is not unusual in inland Otago/Canterbury.

Yes, I have observed ice pellets in the UK.

[TonyT]

I'm not sure thats correct, as the temp appears to have risen higher than that in the evening. As far as I could tell from Cliflo stats for Chch Aero, the coldest maximum for that site (back to the 1950s) was 1.7 in August 1992, and the coldest July maximum was 3.3 in 1959. We'll just have to wait and see what the maximum was for 25th July 2011 and reasess.

Yes, I was wondering about that also. The 1.9C was reported to have been Chch's highest daytime temp yesterday to 5pm, although according to MS's website, Chch Aero reached 2.1C well before then. It continued to rise to 5.0C before midnight, then apparently plummeted to -6.3C early this morn.

The 1918, July 21st, T-max of 1.2C was recorded at Chch Gardens and would've been for the 24 hrs to 9am the following day. There are also plenty of incidences where Chch has recorded 24 hr T-maxs well below 5.0C, including 1.4C at Chch Gardens in June, 1919. It's also not that unusual for eastern S.I. areas to record higher night-time temps than the preceding day.

Cliflo shows a max at the Airport of 5.0, minimum of -6.3 (and interestingly a grass minimum of -6.2). A max of 5 was last reached in 2006, and a minimum of -6.3 last reached in 2007, so Monday certainly cant be claimed as any sort of record based on the readings. Readings for the gardens site are not yet in (early next month I guess), so we dont know what they will show, but the EWS at Kyle St got to 3.6, so I would be very surprised if the record of 1.2deg was threatened.

Also interesting to see Cliflow showing a min of -5 at Rangiora on Tuesday morning and a grass min of -12! Not sure whether to believe that one or not (sensor buried in the snowpack most likely).

[Manukau heads obs]

I see there is now a Blog on the M/S site about the cold snap

[Nev]

Nice aerial pic of Chch yesterday in The Press this morn, plus a few of yesterday's stats for the record...

'Black ice poses 'huge risk'' - Stuff
Last updated 05:00 27/07/2011

...Minimum temperatures around Canterbury yesterday dropped to -5 degrees Celsius in many places and came close to -10C in a few locations. Christchurch Airport had a low of -6.3C, with -7.4C at Culverden, -7.8C at Timaru Airport and -9.3C at Pukaki aerodrome.

Elsewhere in the South Island, Manapouri set a new record with -10C. Queenstown had its third-lowest temperature since 1871, -8.9C, and Greymouth also had its third-coldest morning since 1947, with -2.3C.

Can't really verify the Manapouri record, as I can only find data going back 20 years. However, I think that should be 'Greymouth Aero's fifth-coldest morning since records began in 1947'.

The Queenstown claims are also very misleading, as the readings at the Airport in Frackton began late in 1968 and seem to read around 4-5C lower than the older Queenstown Gardens site. It also appears that readings from 1871 were only for the 10 years to 1881 (?) and the records at Queenstown Gardens actually began in 1930. So if Ranfurly recorded NZ's lowest ever temp in 1903, I wonder what Queenstown would've recorded?

Below is a list of dates which have exceeded those on Tuesday. It's possible however that some of these T-mins may have occurred around the 9am cut-off time, i.e. occurred on the same calendar-day...

Code: Select all

Queenstown - T-mins (°C)
Date            Aero   Gardens

2011 Jul 26     -8.9      ?

1995 Jul  2    -11.0    -6.8 
1995 Jul  3    -12.2    -8.4 
1995 Jul  4    -12.0    -8.0 

1992 Jun 19    -10.3    -4.8 
1992 Jun 20    -10.0    -6.3 
1992 Jun 21    -10.2    -6.0 
1992 Jun 25     -9.2    -5.4 
1992 Jun 26     -9.7    -5.1 

1972 Jun 20     -9.0    -4.5 
1972 Jun 21     -9.0    -4.0 


Greymouth Aero - T-mins (°C)
Date            Aero   

2011 Jul 26     -2.3 

2005 Sep 20     -2.4 
2001 Jul 04     -2.4 
1998 Aug 24     -2.4 
1978 Jun 22     -2.5 

[Storm Struck]

Sometimes you can get an isothermal layer close to 0C between the freezing level and the ground. That can give surprise snowfall when the freezing level height or thickness do not suggest low level snow. That kind of effect is not unusual in inland Otago/Canterbury.

Yes, I have observed ice pellets in the UK.

On Monday something i had never observed before more around midday there was breif spells of a mix of these rain pellets different to grapel a much softer form coming down horizontal with the wind but bouncing off the jacket.
I suspect the deeper areas of the CB shower clouds where precipitation on the radars was heavier would be when the heavy snow fell with large flakes and the lighter areas would perhaps produce these more pellet/light snow flake form.
It was quite good in some ways it created a light base for when the snow flakes fell again making for a faster settling to continue.

[Razor]

Well we certainly had CB's given the lightning and thunder that ripped over my head!

[NZstorm]

Well we certainly had CB's given the lightning and thunder that ripped over my head!

yes, I would say all the precip was from cb.

The ice pellets is something that is more likely to occur with nimbostratus along an occluded front where you have warm and cold air mixing. You really need an intrusion of relatively warm air to get the ice pellets.

[Simon Culling]

Richard, thanks for the SST information. The chart in the link below gives a good idea of the SST around the whole of NZ. I was surprised at how cold the water is even though it is winter, but I presume you have nothing like the Gulf Stream shovelling warm water towards you.

http://magicseaweed.com/msw-surf-charts ... starttime=

On another tack, when NIWA publishes its climate report for July, it will be interesting to see how many climate sites fell below -10*C on the morning of the 26th.

[Manukau heads obs]

cold sea temperatures?


its 14 to 15C water temperature around the top half of the NI during winter

thats only 2C colder than summer time temperatures for the UK or water temperatures for Los Angeles (northern california has only 10C water temperatures)

summer time sees 22C around the NI

the SE coast of the SI has cold water year around, coming up from the southern ocean

[spwill]

Swimming at St Clair Beach, Dunedin in summer is similar to a polar blast but the waters around Northern NZ in summer are warm enough to attract tropical fish like Blue Marlin .

[Richard]

The past snowfall last weekend has reinforced my belief that snowfall to sea level within the Pegasus Bay area has become more common over the last 19 years.I grew up in Rangiora and as a young fella i remember that snow laying in the town was a rear event and most winters we would ask the oldman to drive us up to Loburn so we could play in the stuff,but since 1992 it now seems like every other year it snows and right down to the beach too,where in those earlier years most of the big snowfall events stayed above 100+m.

[Manukau heads obs]

a warmer earth (warmer sea and temperature) will mean more moisture availble to create more intense snow events

the satellite pictures of the canty snow shows that it was too sea level where it first came ashore south of Lincoln
instead, it actualy intensified, the event, as it moved north
most likely due to the influence the topography (banks peninular), and the river of wind against the further inland air mass (convergence) (this is a documented effect)
and other factors like the more moisture, the more latent heat released and then so more cooling, i.e feedback
and so the snow ends up being too sea level at the other end of the whole event more than when it started

and the difference is due to more energy in the system (more overall warmth) in the first place

thats just my thinking

[Richard]

the satellite pictures of the canty snow shows that it was too sea level where it first came ashore south of Lincoln

No it wasn't,i spoke to a bloke i know of lives at Birdings Flat and he had no snow there,only rain,in fact he said in the 50+ years living there he's only known snow to fall & settle only once.Like ive said in this thread before is where snow falls to sea level is all to do with offshore winds


There is clearly an area of snow free land along the south coast then its 10-15km inland before the snow starts,an onshore wind
http://rapidfire.sci.gsfc.nasa.gov/imag ... .terra.1km

[Manukau heads obs]

I meant was not too sea level, typo there

[spwill]

i spoke to a bloke i know of lives at Birdings Flat and he had no snow there,only rain,in fact he said in the 50+ years living there he's only known snow to fall & settle only once

Interesting Richard. The odd year Otago gets snow to beach level in an onshore flow. The sea is colder along the Otago Coast with more exposure to cold air however Chch seems to have had more snow events than Dunedin in recent years.

Looked to me like a convergence line running onto Chch on Monday, SW v onshore flow.

[Manukau heads obs]

yes, convergence line was what I was alluding to
there was a thread about the dymamics of that here ....or was it a M/S blog?

[Richard]

Interesting Richard. The odd year Otago gets snow to beach level in an onshore flow. The sea is colder along the Otago Coast with more exposure to cold air however Chch seems to have had more snow events than Dunedin in recent years.

Looked to me like a convergence line running onto Chch on Monday, SW v onshore flow.

Yes i would think that far south would,but this far north there's only two areas that get snow to settle at sea level, Pegasus bay and around Timaru and southwards,again because of offshore SW or SSW winds.

I personally think that no there was no convergence line running onto Chch causing the snow there,it was just that from CHCH northwards the wind direction was SW,even where i am on the day the wind was more WSW if anything.

Hey i must say we're are getting our money's worth outa this thread

[Manukau heads obs]

the convergence was there alright I would say
its a convergence between the land sheltered flow inland and the river of wind runnning up past otago peninsular

[Richard]

I think more of a deviation than a convergence in the wind flow which at lower levels is the norm for the Pegasus bay area in a southerly air flow,any convergence between these two winds would take place further out to sea

[Storm Struck]

There was obviously some form of land based development when the CB's went onto land and over Christchurch they intensified becoming much more deeper shower clouds as was seen on the radar.
Quite often this happens in a S-SW flow ive noticed, the showers come through that channel between Ellsemere and either carry on up the coast towards Amberley or cut off the coast just north of Woodend.
Also known as the chain shower form, there something that gets them going over land more than the sea is that the convergence?

[Manukau heads obs]

found that blog
http://blog.metservice.com/2010/06/rain-or-showers/

i.e

Topographic effects can lead to preferred places for showers in southerly flows. For example, have another look at the radar animation of showers above. Note the band of showers over the Canterbury Plains at the western edge of the radar echoes. This is caused by a low-altitude convergence effect that favours upward motion there. There are no showers farther to the west due to sheltering from the eastern Otago ranges. If you are beneath that band of showers, then it will seem more like continuous rain than passing showers.

[spwill]

I personally think that no there was no convergence line running onto Chch causing the snow there,it was just that from CHCH northwards the wind direction was SW,even where i am on the day the wind was more WSW if anything.

:

You will have noticed the West Coast had clear skies Monday, the WSW at your place was due to local affects. Air flow running off land ( SW) meets the general Southely flow creating a line of more inhanced shower activity, Christchurch was under this line of convergence.

[Richard]

You will have noticed the West Coast had clear skies Monday, the WSW at your place was due to local affects. Air flow running off land ( SW) meets the general Southely flow creating a line of more inhanced shower activity, Christchurch was under this line of convergence.

Yes,point taken.So the air flow running off land ( SW) meeting the general southerly flow creating the line of more enhanced shower activity still carried on under this line of convergence which happened though out North Canterbury as well as the Pegasus bay area.