New South Wales has just had its coolest summer in 11 years, but now March is on track to be the hottest on record.

Confused? You’re probably not alone.

Daily maximums this March have climbed as much as 15 degrees Celsius above the long-term average.

In some cases, they have been the highest in decades, but it is not the extreme days that have been unprecedented, it has been the consistency of the heat.

Sydney’s daily highs are running at a very warm 29.1C, well above the long-term March average of 24.8C.

In fact, the March highs people are experiencing now are 3C above the long-term summer average.

Central Sydney has already recorded five days over 30C in March, while Penrith is up to 13 days.

The Observatory Hill station near the southern end of the Sydney Harbour Bridge has 165 years of data, the longest continuous site in NSW, and this March’s average is 2C above the previous record of 27.1C from 2006.

At this rate, it would take a prolonged cold snap during the last week of March to prevent the record.

Western Sydney suburbs are also on track to break a record, along with much of the Hunter and Illawarra.

Penrith’s March highs are averaging 31.8C, Newcastle 28.1C and Wollongong 27.6C, all 4C above the long-term average.

A handful of towns in western NSW are also running at 4C above the long-term average, including Cobar, Walgett and Bourke. While in the state’s north-east, Scone, Merriwa and Mangrove Mountains have been 5C higher than usual.

What’s behind the March heatwaves?

The reason autumn is doing its best interpretation of summer is complex.

It is essentially a combination of factors playing out simultaneously, including:

• La Niña’s demise combined with a break in the monsoon has allowed a lengthy stretch of sunny skies to generate a broad mass of hot air over northern and central Australia
• The westerly winds which circumnavigate Antarctica have expanded during the past few weeks, extending to our southern latitudes and dragging the hot interior air across NSW.
• Water temperatures in the Tasman Sea are at nearly 27C, theoretically warm enough to support a tropical cyclone. This means that, even when winds are onshore, it is still warmer than normal

The most surprising aspect is the westerlies, which have spent the past few warm seasons banished to the Southern Ocean.

The recent absence of westerly winds was a critical factor in Sydney’s record wet 2022, a year dominated by moist easterly winds and rain.

When will autumn arrive?

The heatwave rolls on this weekend, and parts of western NSW will even climb to about 42C, challenging all-time March records.

Cooler southerly winds will arrive on Monday, but modelling is showing the potential for more bursts of warm weather towards the end of next week, meaning we may have to wait until April to see a more permanent change of seasons.