NZ Weather Forum

Significant disturbance now showing up on EC in the 2nd week of July around the lower and mid stratosphere that projects coupling into the troposphere. This looks like a stronger disturbance than the twin sister set up in mid June, with the lower part of the stratospheric polar vortex being displaced off the Antarctic continent. A polar streamer parade will cut loose into the mid latitudes around the 3rd week of July if this verifies, finer details to come.

A top shelf fully coupled twin sister is now appearing on EC tonight around 14 July over the Antarctic continent. The recipe for a significant long wave trough in the 3rd week of July launching towards the mid latitudes continues to be the signal.

GFS now picking up the signal. Beastly cold core cyclone. Too early to take serious but its becoming interesting.

A nice cold long wave would be good for us! Don't think we have had a significant long wave in winter since the 2011 Canterbury snow?

Early days but this July's long wave trough is wiggling around, timing is projecting to balloon equatorward around Tasmania on most of the main models with a cut off low scenario in the Tasman, the July 2011 trough launched a direct hit on NZ. Lots of time for change of course

Today midday, EC has fully coupled stratospheric / tropospheric twin sisters over the Antarctic continent.

Cyclone Tracy wrote: ↑Fri 09/07/2021 11:42 Today midday, EC has fully coupled stratospheric / tropospheric twin sisters over the Antarctic continent.

So a bigger chance of some of that super cold air escaping the polar vortex then and heading north?

Bradley wrote: ↑Fri 09/07/2021 12:06

Cyclone Tracy wrote: ↑Fri 09/07/2021 11:42 Today midday, EC has fully coupled stratospheric / tropospheric twin sisters over the Antarctic continent.

So a bigger chance of some of that super cold air escaping the polar vortex then and heading north?

Powerful long wave troughs are now propagating northward in the Southern hemisphere. Most ensembles suggest the timing on one of these Rossby waves into the Tasman / NZ region is from 18th July. If a ridge blocking develops in the Australian bight during this time, there could be a prolonged Southern ocean barrage towards NZ.

GFS now getting on this train as well from mid-month (after the current initial nudge of the vortex towards NZ). Looking possible that it might favour cut-off lows and a sort of double belt of westerlies - one north and one south of NZ - rather than ice shelf southerlies. Looking more likely as we go on that we'll wind up with a fairly negative SAM after it was positive for most of June.

GFS and EC ensembles dialling things back a little today, there's still a bulge of the SPV towards/over NZ around the 18th-19th but it models a weaker tropospheric trough that is semi cut-off and messy. Plenty of modelling road to travel before next weekend however.

https://www.weatheronline.co.nz/cgi-bin ... MO=&ZOOM=0

Possibly looking more significant from the 21st onwards, GFS op and ensembles are modelling a significant 500mb/surface longwave trough which seems to relate to bulge in the SPV. Could be severely cold if that verifies. EC ensembles don't seem as keen looking at surface pressure but the GFS scatter isn't that huge before the 20th.

The above didn't really happen in the end, and there are signs from the GFS that the vortex is contracting away from NZ with wave activity more towards Australia, South Africa and South America. So potentially more benign conditions over the next couple or few weeks than the wild ones we've had through June and into July so far. More likely westerly/northerly than southerly/southwesterly.

EC now showing the affects of equatorial heat flux propagating from the La Nina pattern in the Pacific 10 days from now on the stratospheric polar vortex. viewtopic.php?p=201654#p201654 . The planet's short term weather pattern is changing.

Astonishing how far north the vortex is reaching in the Pacific!
https://earth.nullschool.net/#current/w ... 65,-28.793

Down below, the polar and subtropical jets have also merged north of us, likely contributing to Auckland's wild ride this morning:
https://earth.nullschool.net/#current/w ... 02,-33.120

Looks to be a vortex breakout over NZ at the weekend, should drive something cold around then. After that as CT noted it looks like a warming occurs over the Southern/Indian Ocean and that then tightens the vortex back up.

Vortex being affected by the warming area over the Indian/Southern Oceans that then gets distributed clockwise around the vortex (eastwards). This pushes the vortex away and destabilises it, so then it looks currently like there's a cold outbreak possible for NZ around the 22nd-24th of August after some westerlies/anticyclones. GFS modelling via Metcheck below:

https://www.metcheck.com/SEASONAL/strat ... ecasts.asp#

Another potential warming from the Andes at the end of the current run (27th August).

I dont see any cold outbreak in the models for the 22nd, opposite, warmer NE flow instead

As I said, 'possible' and also this is the stratospheric modelling which the tropospheric modelling doesn't always immediately follow. Not quite seeing the same stratospheric profile over NZ for the 22nd-24th so perhaps surface cold is less likely then, but just after that the Andes-induced warming seems to still be on the cards going by GFS. CT, your thoughts on that?

GFS still wants this Andes-induced warming next week which could be interesting:
GEFS not on board, not sure about EC as I don't pay for their charts. GFS may be the outlier:
https://stratobserve.com/ens_ts_diags

Yes, EC is aligned to that theory with a equatorial heat flux attack on the SPV at the end of August. The polar vortex normally successfully defends such an attack at the end of August but its definitely worth sitting down with a bowl of popcorn over the next 2 weeks.

GFS carrying on with the theme today and it puts at least the 10hPa vortex into difficulty, just 5-6 days away now. End of August could be quite cold here in NZ.

GEFS still not interested though.

GFS showing up a heat flux attack on the Stratospheric Polar Vortex around 10 days from now.

Starts in just a few days but impacts really start to be noticed next weekend. Cold end to winter/start of spring if it holds.

GFS showing this is brief as the warming navigates around to us then peters out as the PV flexes back early Sept, but potential for a chilly blast/active weather late month.

Lower stratospheric twin sisters appearing in 5 days on EC. September polar streamers are entering the game soon.

Warming currently underway at 10hPa, just southeast of South Africa/Madagascar.

Another strong warming being modelled for 10 days' time by GFS.