Darwin recorded its first double-digit daily rainfall total since April in the 24 hours to 9 am Wednesday.

While the 16.8 mm that fell in half an hour around 5 pm on Tuesday was far from a major downpour by local standards, it was a clear sign that the wet season is imminent, especially with possible showers on the forecast almost every day in the week ahead.

To understand the transition from the dry season to the wet season in Darwin and the Northern Territory's Top End, it's important to distinguish between two key terms: the build-up and the onset.

What is the wet season build-up?

The wet season build-up is the term used to describe the return of humid conditions to the Top End ahead of the first significant tropical downpours, after months of predominantly dry weather.

The build-up typically begins around mid-to-late September and is caused by the shift from winter synoptic patterns to summer patterns.

As the Southern Hemisphere transitions from winter to spring, the high pressure systems which bring clear, stable weather to much of interior and northern Australia tend to retreat towards the Southern Ocean. This causes the dry southeasterly trade winds over the Top End to weaken.

Meanwhile humidity increases as sea surface temperatures start to rise in northern Australian waters.

But the build-up doesn't immediately mean that regular rainfall kicks in. Indeed, it can be quite a frustrating period for locals, as September, October and November are typically Darwin's three hottest months and the humidity can be stifling without the relief of afternoon showers and storms.

What is the northern rainfall onset?

When the northern rainfall onset arrives, it means the wet season is officially underway.

The onset is reached when 50 mm of accumulated rainfall has been recorded since September 1. The figure of 50 millimetres is considered a key threshold because it's roughly the amount which stimulates plant growth.

So when you've got 50 millimetres in the gauge (whether from a single storm or from several days of rain), the onset has officially arrived.

As the chart at the top of this section shows, the onset tends to arrive later the further south you travel in northern Australia. It also tends to arrive a little later during El Niño years like this one, as the chart below indicates.

Image: Median arrival of the Top End rainfall onset in El Niño years. Source: BoM.

Interestingly, as of 9 am this Tuesday, October 28, Darwin has accumulated 37 mm of rainfall since September 1.

So it will be a close call to see whether this year's Darwin onset officially occurs during the typical period for all years (the last week of October) or during the typical period in El Niño years (the first week of November).

It's also worth noting that there is virtually no correlation between an early or late northern rainfall onset and a wet season which is relatively wet or dry by historical standards.

Rain also likely in Central Australia this week

Image: Predicted rainfall accumulation across the NT from Wednesday, October 28 until the morning of Saturday, October 31, 2026, according to the ECMWF model. Source: Weatherzone.

The Top End is not the only part of the NT expecting wet weather this week.

As the chart above shows, significant falls are possible over large parts of the territory over the next two days, including in Central Australia.

Alice Springs is just one town expecting heavy showers and storms. We'll keep you posted on the weather in the southern NT as this event unfolds.