Average trends but more extremes

No ongoing trend towards less rainfall yet, although the 10-year rolling average may be impacted by recent shortfalls.
No ongoing trend towards less rainfall yet, although the 10-year rolling average may be impacted by recent shortfalls.

Tenterfield has seen it all in the past few years, weather-wise. Floods, drought, heatwaves, snow, but it all falls within natural variability, according to the Bureau of Meteorology's Luke Shelley.

Speaking to a Tenterfield audience last Wednesday in a weather workshop hosted by the Northern Tablelands Local Land Services, Mr Shelley said to identify weather trends you need a minimum of 30 years of data.

While there has been a warming trend for most of Australia of 1-1.5 per cent since 1970, Mr Shelley said that trend does not play out at every location and Tenterfield is one of them.

Over the same period average temperatures have remained fairly steady for Tenterfield and most surrounding weather stations.

Mingoola is the exception, showing a marked increase in average temperatures post-1990 and an increase in minimum temperatures. Mr Shelley said this could have implications for farming businesses, and that the BOM is keen to know what more it can do to help farmers process this information.

While the thermometer rarely sits on the average mark on any given day but tends to hover somewhere around it, the district has experienced more extremes in hot and cold days.

While a weather station can't detect frost as its sensor sits a metre above ground, it can predict the potential for frost when the air temperature measures two degrees at that height, following warmer, drier days (not the cold, wet days as one might expect).

In the last 30-year period compared to the previous one, Tenterfield is experiencing more earlier and fewer later frosts.

"This is very generalising, a minor shift earlier but nothing to write home about," Mr Shelley said.

Drake, on the other hand, has on average been seeing far fewer frosts through to August with a similar number thereafter, potentially impacting planting times.

The severity of frosts was of interest to farmers at the workshop, and Mr Shelley agreed this was information the BOM could make available.

On the rain front, most of Australia including Tenterfield is getting average or more summer rain over the past 20 years than before. The eastern edge of Western Australia received the highest on record but as Mr Shelley points out, there's not much rain to begin there so the bar is set quite low.

Winter rainfall for Tenterfield used to be more reliable than it has been in the past 30 years, but the district receives little precipitation during winter anyway.

One exception was a mid-winter cyclone that hit the district in the mid-1960's dumping inches of rain, demonstrating how single weather events can skew data if a broad view isn't taken.

The records show that summer rainfall in this district is more reliable than it used to be (gasp), but there's great variability from year to year.

"Talking averages is fruitless," Mr Shelley said.

He said the BOM needs to work on rainfall days data, but that he doesn't see any rainfall trend and no indication of it declining in Tenterfield.

"And I think that's good news."

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