Atmospheric river to bring a wet week to New Zealand
The north and west of New Zealand will cop several days of heavy rainfall and strong winds during the first week of May due to an atmospheric river of tropical moisture and a barrage of low-pressure systems.
Image: 7-day accumulated rainfall to the night of 6th May according to the ECMWF model.
The above chart shows the forecast rainfall totals during the next seven days. The areas expecting the heaviest falls are those with the blue shading, indicating falls of 200-300mm. Some areas could receive even more than 300mm.
The first system to impact the country is a subtropical low-pressure system to the north that is tracking southward and spreading a thick band of cloud over the North Island.
Image: Himawari-9 visible satellite imagery at 3:50pm (NZ time) Saturday 29th.
This subtropical low is drawing down a stream of tropical moisture called an atmospheric river. The amount of water vapour flowing in these ‘rivers in the sky’ can exceed the amount of liquid water flowing in the world’s largest rivers.
The following chart shows the forecast moisture (with tropical moisture in purple and blue shading) for tomorrow evening. The moisture is measured by the millimetres of rainfall that would fall if all the water vapour were converted to liquid water (think of squeezing a wet sponge until it’s empty). This quantity is called precipitable water.
Image: Atmospheric moisture (precipitable water) on Sunday night according to the ECMWF model.
As you can see, there is also tropical moisture flowing just to the east of NSW which will be contributing to heavy rainfall along the south coast this weekend. However, that river of tropical moisture will quickly move away from Australia while the one over New Zealand will remain until at least the next weekend. A series of low-pressure systems and fronts and persistent northeasterly winds will convert this abundant amount of water vapour into hundreds of millimetres of rainfall.
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