Prior to the electric telegraph, meteorologists had few tools to forecast the coming weather.

Sometimes signs presented themselves: pre-frontal clouds, red skies in the morning, falling barometric pressure, but without them forecasting wasn't far off guesswork.

This gave rise to an approach to forecasting called the Persistence Method, which could be summarised as 'tomorrow equals today'. Asked what the weather tomorrow will be like, weathermen of yore would describe what they saw out the window today.

When the weather is in a steady state - such as what the Eastern Seaboard has been like the past month or so - it's easy to believe that the weather tomorrow, next week, or even next season will simply follow on with more of the same.

Hence, many people are expecting the Persistence Method to apply to the coming spring and summer - meaning warm and dry.

Unfortunately it won't be the case.

As the Eastern Seaboard continues to dry out under crisp westerly winds and clear sunny skies, it’s hard to imagine that we’re staring down a wetter than normal end to spring and summer. 

Blue skies, clear weather: Enjoy it while it lasts

This year’s La Niña was forecast to arrive much earlier, but the dynamics at play between the ocean and atmosphere took time to develop, though they are finally starting to bear fruit.

Stronger than normal trade winds over the past month have been slowly upwelling cold water across the eastern Pacific Ocean which is now making its way further west. In the image below, the blue 'tongue' of cold water - which is a characteristic of La Niña - can clearly be seen spanning the central and eastern Pacific.

SST anomaly as of 16th September (NOAA)

This development has finally dropped the critical Niño 3.4 monitoring region below the 0.8°C threshold required by the BOM.* If it stays there for three months, La Niña is declared.

The writing, however, is on the wall.

With continued trade-wind activity and a plentiful source of cold water sitting beneath the surface, we can expect this region to stay below thresholds as we round out spring and head into summer.

On paper, it only looks to be a weak La Niña, but as local meteorologists are learning, the temperature of the surrounding ocean - which drives local climate - will also play its part and lift the relative intensity, especially across the coastal ribbon.

What does this mean for Australia?

As cool water surfaces along the equator, warm water is currently piling up across the Western Pacific and Coral Sea and now starting to extend south in the East Australian Current.

All the clear blue water Queensland and New South Wales surfers have seen this last week..? That's the beginning of it.

This will lead to increased convection and rainfall across the north-east of the country, extending down the Eastern Seaboard, with it really kicking in from November.

The below chart shows the precipitation forecast for December to February, with the dark green indicating wetter than normal conditions for much of eastern Australia.

Rainfall anomaly forecast by ECMWF and UKMET for the period December 2024 to February 2025 (Ben Noll Weather)

As for the surf, I think we all know what to expect. If not, cast your mind back to the summers before last.

On the East Coast there was an increase in easterly trade energy as the subtropical ridge shifted south, also subduing the typical north to north-east wind patterns. Local troughiness will likely bring lighter local winds to southern NSW with a slight southerly bias.

In the southern states, swells from a more southerly angle are expected, albeit with less size plus winds from the eastern quadrant - which is good for the exposed beaches.

In the west, it'll make for a summer of clean, hot, but small surf.

Things still have while to play out but we’ll continue to monitor and provide updates on these developments.

// CRAIG BROKENSHA

*NOAA uses a different threshold whilch was reached at the start of the month. Hence US meteorologists calling El Nino/La Nina are slightly out of synch with Australian colleagues.