Weather News

Remarkably dry La Niña November in NSW

Ben Domensino, Wednesday December 2, 2020 - 12:53 EDT

NSW just had its driest November in 18 years and its warmest in six years, which was blatantly at odds with what was expected to happen. So, what happened?

Despite La Niña being active in the Pacific Ocean in recent months, a lack of rain-bearing cold fronts and low pressure troughs saw NSW remain unusually warm and dry in November.

The state as a whole registered its driest November in 18 years. That's right, last month was even drier than November 2019, which came near the end of the state's driest year on record.

Image: Rain deciles in NSW during November 2020. Source: Bureau of Meteorology.

NSW also just had its warmest November in six years based on the state's monthly mean temperature, which is calculated by combining overnight minimums and daytime maximums across the state throughout the month.

Prior to November, all signs were pointing towards a wetter-than-average month for NSW and near-to-below average daytime temperatures for much of the state.

So, what happened?

Last month was a perfect example of how abnormally warm and dry months can still happen in eastern Australia during La Niña events.

While La Niña significantly increases the likelihood of wetter and cooler weather in eastern Australia, it doesn't guarantee that it will occur in any given month.

The images below show that in November, abnormally high pressure over northern and eastern Australia limited the amount of moisture-laden air flowing over NSW from the east. This anomalous pressure pattern also enhanced the strength of northwesterly winds over central Australia, which helped transport hot and dry air into NSW.

Image: Australia's mean sea level pressure (top image) and 850 hPa wind (bottom image) anomalies during November 2020.

Towards the end of November, NSW felt the full brunt of this exceptional late-spring heat, setting new day and night time state heat records for spring.

Smithville's 46.9 degrees last Saturday was the highest temperature ever reliably recorded in NSW during spring. The next morning, a weather station to the east of Wanaaring (Delta AWS) registered a minimum temperature of 33.8 degrees, which was the highest minimum on record in NSW during spring.

This exceptional heat has persisted into the opening days of summer for northern NSW, with Brewon (near Walgett) reaching 48.0 degrees on Monday.

The past week's heat has also caused elevated fire danger ratings over large swathes of NSW, particularly west of the ranges where fields of long, dry grass are now facing searing summer heat.

With each hot day, the land and vegetation are becoming even drier and further heightening the risk of fires.

Fortunately, La Nina is still here.

Despite the unexpectedly dry and hot November, there is still a good chance that NSW will see above-average rain and below-average daytime heat this summer. The impacts of La Niña are likely to be most significant when other rain-inducing climate drivers influence Australia's weather in the coming months, such as the Madden Julian Oscillation (MJO) and the Southern Annular Mode (SAM).

It's also worth keeping in mind that while the outlook may still favour wet and relatively mild weather, it is still summer. Like every year, there's still going to be hot days, warm nights and fires. Be sure to stay up to date with the latest warnings and forecasts throughout the season to be prepared for dangerous weather events.

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