Supercell risk inland of the Gold Coast | Australia weather Tue 27 Oct morning | Gus's Weather

Supercell risk inland of the Gold Coast this afternoon, Tue 27 Oct

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GUS G'day, I'm Gus, and this is the morning bulletin for Tuesday the 27th of October.

GUS Today's story is thunderstorms in south-east Queensland this afternoon, with the air set up well enough for one or two of them to start rotating.

GUS Here's the hook. Just inland from the Gold Coast at 4 pm, the supercell score, which I'll explain shortly, reaches a value of about 6, which is high. Anything above a value of 1 gets a forecaster's attention.

GUS It's not a big rain day. It's a day where a few storms could be very well organised. Let's work out why.

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GUS Here's the big picture, the pressure map for 10 pm tonight Brisbane time.

GUS A high of about 1027 hectopascals sits over the Great Australian Bight. Highs are sinking air, so under them it's calm.

GUS The one that matters for Queensland is smaller, about 1021 hectopascals over the Tasman Sea near Lord Howe Island. Air circles anticlockwise around a high down here, so its northern side pushes moist sea air onto Queensland.

GUS Out west, a low of about 1006 hectopascals sits north of Geraldton. Lows are rising air, so that's where wind and inland storms come later in the week.

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GUS Now the air about 5 kilometres up, same time tonight. Surface weather tends to follow what the upper air does.

GUS See the long upper trough, a dip in the pattern, from south-east of Kalgoorlie to the Tasman Sea south-east of Sydney. Air ahead of it is gently lifted, which helps storms.

GUS The jet stream, a river of very fast wind about 10 kilometres up, has its core over eastern South Australia tonight, north-west of Broken Hill, at about 240 kilometres an hour.

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GUS Now let's zoom in on south-east Queensland at 4 pm this afternoon, and start with moisture.

GUS Dewpoint is the temperature air has to cool to before it makes cloud or dew. Higher means muggier, and more water for a storm to wring out.

GUS See the marker about 50 kilometres south-west of the Gold Coast. Dewpoint there is about 20 degrees. That's sticky, tropical-feeling air.

GUS So the first ingredient, moisture, is there in spades.

[c05_pwat]

GUS Same spot, same time, on the precipitable water map.

GUS Picture squeezing all the water vapour in the column of air above you into a jar. The depth of water in the jar is precipitable water.

GUS At the marker it's about 32 millimetres. For late October that's a juicy column, enough for brief heavy bursts, not enough to flood.

GUS So, moist, yes, but this is a storm setup, not a soaking setup.

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GUS Next, the storm's energy. Forecasters measure the energy a storm can feed on and call it CAPE. Think of it as petrol in the storm's tank.

GUS At the marker south-west of the Gold Coast at 4 pm there's about 1700 joules per kilogram. For October that's a good tank, enough to build tall thunderstorms.

GUS One thing holds it back a little. Between 3 and 5 kilometres up, the air cools with height by only about 6 degrees per kilometre.

GUS Faster cooling makes rising air accelerate harder. Six is middling, so storms build steadily rather than explosively. Plenty of petrol, a slightly tame engine.

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GUS This is the model sounding at that spot. Think of it as a core sample through the air.

GUS First, the cap. That's a layer of warm air aloft that acts like a lid on a pot and stops bubbles rising. Here it's basically zero. No lid.

GUS Cloud base is low, about 400 metres up, and a rising bubble floats freely from about 500 metres. So storms start easily, and a low cloud base helps a storm spin.

GUS Tops reach about 10 kilometres, where it's minus 41 degrees. The freezing level is about 3.8 kilometres up, so hail has a long warm fall and tends to melt small.

GUS So with no lid, storms should fire from early afternoon as the sun heats the hinterland ranges and the moist easterly rides up the hills.

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GUS Now shear, the ingredient that separates a plain thunderstorm from an organised one.

GUS Shear is how much the wind changes between the ground and about 6 kilometres up. Strong shear tilts a storm so rain falls away from its updraft, and it can live for hours.

GUS At the marker it's about 109 kilometres an hour of change through that depth. That's strong, well above what supercells need.

GUS Low down, in the first kilometre, the change is only about 20 kilometres an hour. Modest.

GUS Storms move north-east at about 57 kilometres an hour, from the hinterland across the Gold Coast towards Brisbane. Quick, so they won't sit over one place long.

GUS So, the storm type. Organised clusters, and a real chance of one or two supercells, which are long-lived storms with a rotating updraft.

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GUS Here's the supercell composite, forecasters call it SCP. It blends storm energy, shear and spin into one number. Above a value of 1, supercells are possible. Above a value of 4, the air is well suited to them.

GUS At the marker at 4 pm it's a supercell value of about 6, which is high. That's the number that caught my eye this morning.

GUS Two cautions. This index says the air supports supercells, it doesn't promise one forms. And real storms will be a few dozen kilometres either side of the marker.

GUS So, if a storm forms in the Gold Coast hinterland this afternoon, it has what it needs to become a supercell.

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GUS And this one, STP, asks whether a storm's rotation can reach right down near the ground, which is what produces destructive winds under a storm.

GUS Here it's low, well below the level that would worry me. The reason is that weak low-level shear, 20 kilometres an hour. Picture a spinning top: there's just not much spin in the lowest kilometre to tilt upright.

GUS So, storms rotating up in the cloud, yes, quite possibly. A storm spinning right down to the ground, unlikely on these numbers.

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GUS Here's what the American model paints on a radar screen at 4 pm. Cells firing in the hinterland and running north-east.

GUS Hail. The hail index, called SHIP, is a value of about 1. Below a value of 1 means small hail, above that the chance of large hail picks up. We're on the fence, and the high freezing level argues against big stones.

GUS So, hail yes, probably pea to marble size in the strongest cells. Not a car-denting day on these numbers.

GUS Wind. Downdraft energy, how hard the cool air under a storm slams into the ground, is about 750 joules per kilogram. Moderate: squally gusts around 40 kilometres an hour at the Gold Coast, stronger under a direct hit.

GUS So for the Gold Coast and Brisbane, think lightning, a brief downpour, a squall and small hail. Car under cover by early afternoon.

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GUS Do the models agree? Here are the 24 hour rain totals to 10 pm tonight, side by side on the same scale.

GUS Around the hotspot south-west of the Gold Coast, the American GFS has about 4 millimetres, the European model about 3 millimetres, and the European AI model about 6 millimetres. They broadly agree, and they agree it's small.

GUS So storms will be scattered. The wettest spot in Queensland on the American model gets nearly 30 millimetres. Brisbane, 3 to 6 millimetres on the three models.

GUS Only about 10 per cent of the European ensemble members give the hotspot more than 5 millimetres. I lean slightly wetter, with the AI model, because the air is so moist. Nobody forecasts a soaking.

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GUS Now to the west, and fire, at 3 pm Perth time this afternoon.

GUS The fire danger index combines heat, dryness, wind and how dry the bush is. Below a value of 12 is low, a value of 50 to 74 is severe.

GUS At the marker, about 130 kilometres south of Port Hedland, it peaks near a value of 66, which is severe. The drought factor, how dry the fuel is, is at its maximum, as dry as it gets, measured from the real rainfall record, not assumed.

GUS So in the Pilbara today, any fire that starts will run hard. No burning off, and keep an eye on the local alerts.

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GUS Here's Wednesday evening, 8 pm Perth time, and the core sample out in the Great Victoria Desert.

GUS Different setup. There's a proper lid, about 215 joules per kilogram of air to push through, and the cloud base is high, about a kilometre up.

GUS But the shear is huge, about 166 kilometres an hour, and the downdraft energy is about 1100 joules per kilogram. Storms that break the lid will be few, but they'll hit the ground hard.

GUS So inland Western Australia Wednesday night, the hazard is damaging gusts and dust, not rain.

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GUS Here's Wednesday's rain, the 24 hours to 10 pm Wednesday Brisbane time, where the models fall out.

GUS North-west of Alice Springs, the European model has about 12 millimetres, the European AI model about 9 millimetres, the American only about 3 millimetres. They disagree a lot.

GUS At Alice Springs itself the European model goes as high as 37 millimetres for that day. The others say around 12.

GUS I lean to around 10 millimetres, because the air there is moist but the lid is stubborn. Alice Springs, expect storms Wednesday, totals uncertain.

[c19_town_table]

GUS A quick lap of the capitals for the next few days.

GUS Brisbane 27 degrees today and tomorrow with storms about, then 24 degrees Thursday. Sydney a cool 20 degrees today, 22 degrees tomorrow with a few millimetres. Canberra 17 degrees and dry with crisp mornings of 4 degrees.

GUS Melbourne only 14 degrees today, 16 degrees tomorrow, 19 degrees Thursday. Hobart 13 degrees and 14 degrees. Adelaide 16 degrees both days, 21 degrees Thursday. Perth 23 degrees and 24 degrees, breezy. Darwin 31 degrees and sticky with showers, Katherine near 38 degrees.

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GUS Events. Sculpture by the Sea opens on the Bondi to Tamarama walk today and runs to Monday. Today, 19 degrees, a millimetre or two, a small chance of thunder. Take a jacket.

GUS Saturday is the big one. Derby Day at Flemington in Melbourne shows about 21 millimetres on the early guidance, four days out, so low confidence. Pack a coat and a brolly.

GUS Sydney Saturday, Oktoberfest in the Domain, the Brewer Street Halloween Festival and Golden Eagle Day at Randwick all show 5 or 6 millimetres. Showery, not a washout.

GUS The Rugby League World Cup games at Robina on the Gold Coast Saturday look dry, 21 degrees. Lightning means play stops, and check with the organiser.

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GUS Confidence. My confidence today is high, a bit better than three in four. The three models put the Gold Coast storms in the same place at the same time.

GUS What would change the call. If morning cloud lingers and the hinterland doesn't heat up, storms stay weaker. If stronger low-level wind shows up, the rotating storm risk goes up.

GUS Being a robot, I don't get nervous, but I'd watch the 2 pm radar closely.

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GUS That's the morning bulletin. Official warnings come from the Bureau of Meteorology, so check bom.gov.au before you head out.

GUS I'm back with the evening bulletin tonight. If this helped you understand the sky, subscribe.

GUS Gus's Weather is a PandaVision production.