This week’s weather map has a low at the tip of the Cape York Peninsula. It looks fairly innocent (just 1006hPa which isn’t very deep), and is right at the tip of the Cape — but this low is actually the remains of Tropical Cyclone Maila, which less than a week ago was a category 5 cyclone, the strongest on the scale.
Since then it has lost the typical cyclone characteristics of terrifying winds and storm surge — but it hasn’t lost its moisture.
It may also be up at the very tip of the Peninsula, but the rain from the remains of this cyclone should stretch far and wide over the northern half of Queensland and the Northern Territory.
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In the next few days falls more than 50mm is likely across a huge area, with some parts seeing well over 100mm.
We’re nearing the end of the wet season, so rain like this is starting to become less common in the tropics, but this may be one last hurrah for the season.
The weather pattern on Wednesday morning. Credit: Bureau of MeteorologyPotential rain over the next few days. Credit: 7NEWS
Farmers in the south follow these systems with interest, particularly at this time of year.
April and May mark the traditional time of “the break” which is when the high-pressure systems that dominate the weather pattern over summer start to break up, allowing stronger low pressure to move through.
This is the change from generally dry conditions to a return to regular rainfall.
If low pressure can break through (seen on the map as a front or trough, and sometimes as a proper low) then that is a handy ingredient to produce rainfall — but it is missing the key ingredient: moisture.
If a front or trough moves from the Southern Ocean over southern Australia it brings moisture from that ocean with it. That ocean has temperatures about 10C to 15C which doesn’t deliver much moisture to the air above it. That’s why the rain decreases as you head inland with a standard trough or cold front.
But if that trough or front can link up with moisture from a tropical ocean, then it is a very different story.
Tropical oceans are up about 20C to 30C which push a lot more moisture into the air. Think about how different the air feels as you walk onto a plane in cold and dry Melbourne, and walk off the plane in humid Cairns.
If tropical moisture can feed a trough or front then it can produce widespread, soaking rain, not just at the coast but anywhere the weather system travels inland.
Over the next five to eight days, this current surge of tropical moisture isn’t likely to link up with any systems that cross the south — but it is worth keeping an eye on the remains of this moisture to see if any connection can be made further down the track.
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