Thread by @althecat

The severe south-westerly gales Wellington experienced yesterday were just a foretaste of a series of, tropical origin, extreme weather events forecast for NZ over the coming 16 days.

#ClimateChangeNow #AdaptationMitigation #COP26 @COP26

Here is a wider angle view showing the atmospheric river set up of NZ's climate as the ICTZ (monsoon belt) is moving south for summer.

1. Warm wet air comes in from the north heading south east.
2. Cold air comes in from the south heading north west.

The storm currently affecting NZ contains a combination of warm and cold streams merging south east of NZ and curling back around to target NZ from the South West. This set up is similar to the severe easterly Canterbury Gales back in May.

The next two animations are precipitable water simulations. Atmospheric water, because it has mass, is at the core of the global physics simulations that power modern weather forecasting.

Here we have the Australian ACG simulation for the coming 10 days.

In the first few frames we see what is shown in the satellite imagery earlier in the thread. With a combined stream of wet and cold air hitting NZ from the south west.

Meanwhile over Indonesia/Oz we see the source of the next tropical atmospheric rivers which are forecast.

In the ACG (Access G) simulation the next atmospheric river is due to arrive on Saturday around midnight and sweep up the west coast through Monday evening. This rain event is similar to the West Coast flooding event which caused massive damage to Westport in August.

The third atmospheric river is the largest and hasn't quite arrived at the time this 10 day forecast ends on midday October 23.

From what we saw during 2021 we know large atmospheric rivers are damaging. Fortunately for NZ there is usually plenty of warning in the forecasts.

This second long range forecast is from the global U.S @NOAA GFS model. It's different in details, but similar in impact & timing.

[Correction: Here's the missing picture from the ACG model of the the third anticipated tropical atmospheric river - which is forecast to arrive shortly after October 23rd.]

@NOAA Here's the next storm about to arrive on Saturday afternoon, October 16th, and then leaving late on Monday October 18th after sweeping up the entire NZ West Coast.

@NOAA And as the GFS model is longer range [16 days] we can also see a forecast prediction for the third and probably largest atmospheric river, and consequently extreme rainfall event. which is forecast to begin around Oct. 23 - and persists in this forecast till at least Oct. 29

@NOAA And as you can see there it is possible that there is a fourth extreme rainfall event following fairly swiftly behind that one.

Regarding the reliability of these forecasts:

While forecasts of local weather events over four or five days out (especially rain) are not reliable. Large scale events like these [provided they are not affected by cyclones] are now remarkably accurate.

We have two additional global simulation models we can reference for these long range predictions. Canada's CMC and the EU's ECMWF models.

Forecasts for the beginning and end of the Oct 16th event:
1. ECMWF (start) 2. (finish)
3. CMC (start) 4. (finish)

Forecasts for the beginning and end of the Oct 23rd event:
1. CMC. October 22nd (setup)
3.ECMWF. Ocober 23rd (setup)

While the models differ on the precise nature of this event, all of them look fairly dangerous from a flooding potential.

The MetService's Severe Weather outlook (metservice.com/warnings/sever…) is warning about this coming weekend's severe weather. But 23rd-29th Oct. event is not yet on the radar, and possibly should be. Then people can prepare & for example not plan holidays.

But the bigger question posed by this, in the run up to COP26, is the extent to which NZ is prepared for and budgeting for Climate Change adaptation measures.

While 2021 has certainly been special in some respects, particularly wrt the scale and consistency of extreme/unusual weather events across the northern hemisphere, that consistency points to climate change rather than random chance, & the trajectory for warming remains upwards.

There are signs that ENSO is moving into a drier La Nina pattern, and it is possible that these forecast storms are a reminder and not indicative of what we should expect this NZ summer, but that is also no reason for complacency.

1000s of people have been killed this year from extreme rain events all over the globe. And 100s of billions of damage has been done to housing/infrastructure - especially in East Asia.

NZ needs to start taking the threat posed by a wetter southern hemisphere seriously.

It would be nice if climate change for NZ was just a matter of accounting for emissions and negotiating with stakeholders about the transition, but it clearly is not that simple. Climate Change is here now and as @antonioguterres says we all need to wake up.