A series of very long period swells will bring deceptively powerful waves to NSW's coastline.

A very active period with powerful low pressure systems crossing well to the south of Australia over the past weekend and early week have generated very large waves thousands of kilometres south of the continent.

Image: Satellite loop of powerful lows well to the south of Australia on Monday, March 17.

As seen in the figures below, significant wave heights exceeding 10 metres were generated on Monday, March 17, over 3000 kilometres away from NSW. The US’s National Oceanic and Atmospheric Administration (NOAA) satellite altimeters verified this data with wave heights of 11 metres recorded in the region.

Image: DTN's OneFX Significant Wave Height forecast on Monday, March 17, afternoon.

Most of this energy will be sent due east, towards New Zealand's South Island and into the South Pacific. However, some of the energy will radiate north, lapping into parts of the Australian coastline. In the 3000 kilometres journey to NSW, the swell height will dramatically flatten, but the swell period will lengthen significantly.

The first of these long period swells filtered into NSW and the Sydney region this morning, on Wednesday, March 19. The plot below shows a number of different swells in the water, with a peak period of 14-15 seconds.

Image: Directional Spectrum swell analysis for Sydney on the morning of Wednesday, March 19, with a dominant swell from the south with a peak period of 14 seconds. Source: Manly Hydraulics Laboratory

This initial groundswell will ease in period to about 12-13 seconds on Thursday. A reinforcing groundswell should filter into the region on Thursday morning with a period of 17-19 seconds.

Why period matters?

While each swell train should only have average heights of about 0.5 metres, the wave power of an 18 second period wave is double that of a wave with a period of 9 seconds. Along with that, the longer period swell ‘feels’ the ocean bottom four times deeper than the 9 second swell. This allows the waves to turn into the coast at a much deeper depth through the process of refraction, impacting the coast more head on, as seen in the figure below.

Image: graphic showing how various swell periods interact with the ocean floor at different depths.

On the beaches, the distant nature of these swells will lead to big lulls with generally calm ocean conditions. Every 10-15 minutes, abnormally large waves will break across the coast. The size of these waves will not be huge, but surf riders may find inconsistent head high waves at surf spots exposed to the south. Stronger rips may also suddenly form as swimmers enjoy the warm and mostly sunny conditions.