The answer is simple: when it's over land.
This morning the ABC's Steve Austin invited Senior BOM* Meteorologist Rick Threlfall – who's from my neck of the woods in the North of England, except he supports the wrong football team – to answer an important question. Where did Oswald get its energy from if it wasn't absorbing moisture from a warm ocean?
In Far North Queensland, Rick explained, Oswald the low-pressure system was driven by the northern monsoon trough (which arrived late this year, I know for a fact, and obviously wished to make its presence felt now it was here). 'It developed a fair bit of spin,' Rick embellished, and high humidity meant lots of moist air rose rapidly and then condensed. Cairns copped it.
As Oswald moved south there was an extratropical transition. An upper trough (different from the northern monsoon) interacted with Oswald the low-pressure system, feeding off the high degree of moisture in the atmosphere once again (90-100 per cent humidity). The system then sat over Queensland's eastern highlands for about 24 hours, during which moisture was sucked up and condensed, then dumped over North Burnett and Bundaberg, and as it moved on south.
The huge amounts of rainfall and extremely strong winds over South East Queensland and northern New South Wales were the 'very same ingredients' as you would get in a category 1 tropical cyclone, Rick conceded. Possibly even a category 2, by definition. Technically, that's what we experienced: we just can't call it that because it was over land. That's the rule.
Thank you, Rick Threlfall, for setting the record straight.
I think I'm obsessing about Oswald.
* Bureau of Meteorology
This post was last edited on 2 February 2013