Tasman Low To Bring Significant Rain Accumulation

A low pressure system is expected to develop significantly in the Tasman Sea over the next two days. As this low pressure system gradually moves eastwards towards the high pressure that is currently situated over the country, moisture from the tropics begins to be drawn into the low. This phenomenon occurs largely due to the opposing wind directions generated by these two differing airmasses. Anti-cyclones, which we commonly refer to as high pressure systems, produce winds that flow in an anti-clockwise direction. Conversely, low pressure systems generate cyclonic winds that rotate in a clockwise manner. The image below illustrates the high pressure system positioned out east of New Zealand, driving winds back towards Australia, while concurrently, the low pressure system in the Tasman Sea is sending air back in the opposite direction. This movement is also assisted by the presence of a high pressure area sitting over South Australia, effectively creating a vacuum or an atmospheric river that looks to suck tropical moisture down towards New Zealand.

In our detailed Outlook, we have meticulously produced forecasts along with corresponding risk levels for the upcoming significant rain event anticipated in the coming days. Unfortunately, there still remains a considerable amount of uncertainty regarding how much rain is expected in total and which specific locations will be affected the hardest. At this early stage in our assessment, we have identified the western and northern areas of the South Island as the regions likely to bear the brunt of the rain, while the central and upper North Island are also being closely monitored for any developments. As the low pressure system advances closer to the coastal regions of the country, there is a possibility that northeasterly winds could usher in heavy rain to the East Coast of the South Island on Friday. Below this section, we have included detailed rain maps that provide projected daily rainfall totals spanning from Tuesday to Friday. These images serve to illustrate the potential severity of this weather system, utilizing one of the advanced weather models we have at our disposal, and they SHOULD NOT be interpreted as an exact forecast. We strongly advise everyone to keep checking our Significant Weather Outlooks and In-Depth Analysis pages for accurate, timely, and up-to-date forecasts as the situation evolves.

Tuesday 

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Wednesday

Friday