March Monsoon Season West Coast 25-6 March

[Razor]

Metservice going biblical with 400-600mm predicted for much of the West Coast. Some severe gales over east too. A vigorous autumnal equinox starter this year.

[tornado]

metservice says a risk of thunderstorms too

[NZ Thunderstorm Soc]

Interesting to see how this pans out, with road closures, slips, whatever

[Chris W]

The MetService weather warning refers to 500-600mm over the ranges and 250-300mm nearer to the coast, just for Monday and Tuesday. There is more rain due on Wednesday. It's quite incredible to view the 3-day rainfall chart on their site.

In addition they have high or moderate confidence of severe northwest gales across everywhere east of the ranges. There is a watch out for Chch city and the plains, so we'll see if we are spared relatively as we often are, or if the wind retains its strength through to the coast.

[spwill]

Dew point temperatures over the NZ region, an unusual amount of moist air. On the humid side in Dunedin this morning relative to their normal weather, currently 17.5C there with a dp of 16C.

[NZstorm]

Another useful way of looking at the moisture is the precipitable water. PW values upto 50mm in the mid Tasman which is unusualy high atmospheric moisture. Something to do with the tropical system over Queensland.

[Razor]

Remarkable weather warnings issued today, its actually ramped up further. Civil Defence are going to get busy I fear

Heavy Rain Warning
Heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous.

Area: Westland south of Otira
Valid: 38 hours from 9:00am Monday to 11:00pm Tuesday
Forecast: Heavy rain, with thunderstorms possible. Expect 500 to 700mm to accumulate about the ranges, and 200 to 300mm near the coast. Peak intensities of 20 to 30mm per hour about the ranges today (Monday) and 30 to 40mm per hour on Tuesday.
Changes: Increased range of rainfall totals about the ranges.

Area: The headwaters of the Canterbury Lakes and Rivers south of Arthurs Pass
Valid: 38 hours from 9:00am Monday to 11:00pm Tuesday
Forecast: Heavy rain, with thunderstorms possible. Expect 400 to 600mm to accumulate near the main divide, and 200 to 300mm within 20km further east. Peak intensities of 15 to 25mm per hour today (Monday), and 25 to 35mm per hour on Tuesday.
Changes: Decreased warning area to south of Arthurs Pass only. The headwaters further north are expected to see much less rainfall and in a shorter time period, and the area is now under a heavy rain Watch.

Area: The headwaters of the Otago Lakes and Rivers
Valid: 32 hours from 9:00am Monday to 5:00pm Tuesday
Forecast: Heavy rain, with thunderstorms possible. Expect 300 to 500mm to accumulate near the main divide, and 150 to 250mm within 20km further east. Peak intensities of 15 to 25mm per hour today (Monday), and 25 to 35mm per hour on Tuesday, especially near the divide.

Area: Fiordland
Valid: 28 hours from 9:00am Monday to 1:00pm Tuesday
Forecast: Heavy rain, with thunderstorms possible. Expect 300 to 400mm of rain to accumulate north of George Sound, and 150 to 250mm further south, on top of what has already fallen. Peak intensities of 15 to 25mm per hour today (Monday), but 30 to 40mm per hour on Tuesday.

Strong Wind Warning
Strong wind gusts could damage trees, powerlines and unsecured structures. Driving may be hazardous, especially for high-sided vehicles and motorcycles.
Area: Canterbury High Country
Valid: 20 hours from 3:00am to 11:00pm Tuesday
Forecast: Severe gale northwesterlies gusting 130 km/h in exposed places.

[Razor]

Milford Sound 113mm already so far today, projecting to be 450-500 next couple of days

Webcam on Metservice site looks amazing

[Andy]

https://fenz.harvest.com/?cmd=home&sid=7644&group=FENZ

[TonyT]

A lot of rain in a relatively short amount of time. Some of the highest intensities I have ever seen from model guidance.

[melja]

I hope they remove all the associated stuff with the waimak bridge widening or it will be washed away.

Sent from my Redmi Note 4 using Tapatalk

[Bradley]

The MetService weather warning refers to 500-600mm over the ranges and 250-300mm nearer to the coast, just for Monday and Tuesday. There is more rain due on Wednesday. It's quite incredible to view the 3-day rainfall chart on their site.

In addition they have high or moderate confidence of severe northwest gales across everywhere east of the ranges. There is a watch out for Chch city and the plains, so we'll see if we are spared relatively as we often are, or if the wind retains its strength through to the coast.

Interesting that EC has 15-25mm for much of the plains and Christchurch overnight Tuesday and Wednesday whereas GFS only going for 2-5mm - EC over exaggerating the moisture from the N-NW flow perhaps?

[TonyT]

Interesting that EC has 15-25mm for much of the plains and Christchurch overnight Tuesday and Wednesday whereas GFS only going for 2-5mm - EC over exaggerating the moisture from the N-NW flow perhaps?

More likely to be due to resolution differences and different implementation of the rain physics between the two models, than significant differences in moisture input. This is an interesting test case, because this is an area where the models often dont do a particularly good job of getting the rain pattern east of the ranges correct.

[Bradley]

Interesting that EC has 15-25mm for much of the plains and Christchurch overnight Tuesday and Wednesday whereas GFS only going for 2-5mm - EC over exaggerating the moisture from the N-NW flow perhaps?

More likely to be due to resolution differences and different implementation of the rain physics between the two models, than significant differences in moisture input. This is an interesting test case, because this is an area where the models often dont do a particularly good job of getting the rain pattern east of the ranges correct.

Thanks for the explanation Tony, do you find in the past 6-12 months it has swung around a little between GFS and EC accuracy and GFS is now the better performing model, particulary for rainfall?

[TonyT]

More likely to be due to resolution differences and different implementation of the rain physics between the two models, than significant differences in moisture input. This is an interesting test case, because this is an area where the models often dont do a particularly good job of getting the rain pattern east of the ranges correct.

Thanks for the explanation Tony, do you find in the past 6-12 months it has swung around a little between GFS and EC accuracy and GFS is now the better performing model, particulary for rainfall?

No, definitely not.

[Bradley]

Thanks for that, I'll be keeping a close eye on which model performs the best, I've always found that EC is too bullish in any spillover rain scenarios but we all know the limitations that GFS has as well so it will be fascinating to see which one comes out on top

[Razor]

Milford Sound now around 200mm, with another 250+ now modelled by Metservice

[NZ Thunderstorm Soc]

A "Monster Storm" now as advertised by the media, Newshub

[NZstorm]

Useful ways to assess spillover rainfall potential is to look at the instability as well upper level divergence.
The best measure for instability is the L57 which is the 700mb to 500mb lapse rate. The strength of the winds is another parameter that contributes to it.

[Simon Culling]

Looking at the MetService page for Milford Sound Airport and adding up the two hourly totals gives the following rainfall figures for this site:

24th March, 1200-1800 3.4mm
24th March, 1800 to 2400 7.0mm
25th March, 0000 to 1200 131.0mm
25th March 1200 to 2400 146.8mm

This gives an event total so far (to midnight on Monday 25th) of 277.8mm. With another 215.5mm predicted to fall between midnight and 1400 today (26th March), that would give an event total (over 2 days) of approx. 490mm. An impressive total and comparable to falls recorded in the Lake District, Snowdonia and the NW Highlands of Scotland in the past.

Is there the same expectation that these so called 'atmospheric river events' are likely to increase in New Zealand in a warmer world?

EDIT: I note that the airport is at an altitude of just 3m which makes the rainfall figures all the more impressive. There must be double this amount falling on the higher slopes.

[Chris W]

Milford has had 99mm before dawn today, over 60mm of that between 2am and 4am. They are now forecast 238mm for today taking them over 500mm for the two days, and the rain does not stop tomorrow although it lessens in intensity.

[Razor]

Milford has had 99mm before dawn today, over 60mm of that between 2am and 4am. They are now forecast 238mm for today taking them over 500mm for the two days, and the rain does not stop tomorrow although it lessens in intensity.

Milford is actually 141mm since midnight as at 6am going off Metservice. Nuts.

[snowstormwatcher]

Most of the heavy rain seems to be south of Mt cook so far looking at the ecan website. Mild night here with a low of 17c and a light NW wind.

[Chris W]

Milford has had 99mm before dawn today, over 60mm of that between 2am and 4am. They are now forecast 238mm for today taking them over 500mm for the two days, and the rain does not stop tomorrow although it lessens in intensity.

Milford is actually 141mm since midnight as at 6am going off Metservice. Nuts.

Yes, I was looking up to 4am but you're right. Phenomenal intensity.

The Waiho river was up to 8.2m according to someone on RNZ this morning, the 2016 flood got up to 8.3m.

[NZstorm]

Is there the same expectation that these so called 'atmospheric river events' are likely to increase in New Zealand in a warmer world?

EDIT: I note that the airport is at an altitude of just 3m which makes the rainfall figures all the more impressive. There must be double this amount falling on the higher slopes.

I believe its hard to say whether NZ will be wetter or drier in the future. Firodland rainfall is reliant on the polar jet stream and if that is forced south by a warming planet then less rain.

Rainfall is always a lot higher on the upper slopes/valleys.