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Upper air, why do we care?

Jess Miskelly, Saturday June 10, 2017 - 12:08 EST

There's been over 130mm of rain over the Northern Rivers district since 9am Friday and there'll probably be at least that much more before the long weekend is out. What's causing it? The commonly viewed Mean Sea Level Pressure (MSLP) chart shows a surface low pressure system off the coast of NSW, but it doesn't look much different to the chart on a day when there'd just be a few isolated showers. Why?

The answer lies, in part, in the upper air. Currently, in addition to the surface trough, there is also a low pressure trough or pool of cold air in the upper atmosphere over NSW. This makes the whole system deeper, hence stronger and more long lasting.

In fact, the persistence of this upper trough will lead to rain and/or showers along the northern NSW coast for most of the next week, showers extending well inland and the chance of thunderstorms over the northeast. Another surface low should develop during the week and focus heavy falls over some part of the north coast, though it's difficult to know exactly where at this stage.

That upper trough is also helping to drive cool southeasterly winds well north over the tropics, which lead to the coldest night in nearly two years for parts of the West Kimberley. Just 8.7 degrees for the normally very muggy Broome, 5.3 for West Roebuck and 7.1 at Fitzroy Crossing.

Sometimes it's helpful to look beyond the surface.


- Weatherzone

© Weatherzone 2017

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