Desert deluge? Storms and flash flood risk | Australia weather Tue 27 Oct afternoon | Gus's Weather

Desert deluge? Storms and flash flood risk for the NT on Wednesday

[c02_mslp_fronts]

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

GUS Today's story is water in the desert: thunderstorms build over the Northern Territory tomorrow, and the European model drops a flood-sized dump on the Tanami Desert.

GUS Here's the hook. Alice Springs, a town that lives on dry air. For Wednesday the European model has about 28 millimetres falling there. The American model has about 4 millimetres.

GUS That gap is the whole bulletin. Before I work through it, a quick look at how yesterday's call went.

[c01_verification]

GUS Yesterday the American GFS had the best rain record of the three models. It picked rain correctly at 25 towns out of our 50 towns, and it only cried wolf at 7 towns.

GUS The two European models each cried wolf 9 times. The big miss was the Gold Coast: the models had 7 to 11 millimetres and 29 millimetres actually fell.

GUS Overnight minimums ran a little under a degree too warm on average. Ceduna was the worst, the American model 6.6 degrees too warm.

GUS So the lesson: in this pattern the models have under-done the coastal showers and over-done the warmth at night.

[c02_mslp_fronts]

GUS Here's the big picture as of this morning. The lines are isobars, lines of equal air pressure. Highs are calm, sinking air. Lows are rising air, and rising air makes cloud and rain.

GUS See the marker over the Indian Ocean south-west of Perth: a high of about 1028 hectopascals. Another, about 1026 hectopascals, sits over the Great Australian Bight. Those keep the south settled.

GUS Now the lows. About 1010 hectopascals inland in Western Australia, west of Newman. About 1014 hectopascals over the Gulf Country. And about 1016 hectopascals over the Coral Sea east of Brisbane.

GUS The Western Australian low is the one to watch. It's the engine for the next two days.

[c03_upper_500]

GUS Now let's go up to about five and a half kilometres, the steering level. Think of it as the river the weather floats on.

GUS See the marker over the Indian Ocean about 180 kilometres north-west of Geraldton. That's a cut-off low, a pool of cold air aloft that has broken away from the westerlies and is wandering on its own.

GUS Cold air aloft over warm ground is unstable, like a pot that's hotter at the bottom than the top. It bubbles.

GUS The marker over eastern South Australia is the jet stream, a ribbon of wind near 10 kilometres up doing about 252 kilometres an hour. That ribbon drags the whole system east over the next two days.

[c07_dewpoint]

GUS Now let's zoom in on the Gulf of Carpentaria for this evening, about 7 pm Queensland time. This map shows the dewpoint, the temperature air has to cool to before it fogs up. It measures how much water is in the air.

GUS See the marker over the Gulf: a dewpoint of about 25 degrees. That's sticky, tropical air, the kind that makes your shirt cling in Darwin.

GUS Off Bundaberg the marker shows about 22 degrees. Still juicy.

GUS So the water is there. The question is whether storms can use it.

[c09_cape]

GUS Same place, same time, now the storm energy. Meteorologists call it CAPE. Think of it as the petrol in a storm's tank.

GUS See the marker over the Gulf: about 2100 joules per kilogram. For October that's a full tank, enough for very tall storms.

GUS But a full tank doesn't mean the engine starts. For that I need to look inside the air column.

[c11_skewt]

GUS This is a model sounding over the Gulf for 7 pm tonight. Think of it as a core sample through the air, from the sea surface up to the stratosphere.

GUS First, the lid. That's warm air aloft holding the bubbles down, like a lid on a pot. Here it's about 140 joules per kilogram, a reasonably firm lid. It takes a push to break it.

GUS Cloud base is low, about 500 metres, because the air is so wet.

GUS The downdraft energy, the shove a storm gives the ground when its rain-cooled air falls out, is about 1240 joules per kilogram. That's enough for damaging gusts.

GUS So over the Gulf tonight: mostly over water, lid on, but any storm that breaks through could throw down a nasty gust. The coast near Normanton gets only a few millimetres.

[c02_mslp_fronts]

GUS Here's Wednesday morning. The Western Australian low has deepened to about 1006 hectopascals, now about 250 kilometres north of Geraldton. See the marker.

GUS A second weak low, about 1012 hectopascals, has formed over the Tanami Desert. That's the one feeding the storms over the Northern Territory.

GUS The high over the Bight is still there at about 1026 hectopascals, so Adelaide, Melbourne and Hobart stay quiet.

GUS Lines close together between that low and the Bight high mean wind, and that wind drags warm, moist air into the centre of the country.

[c09_cape]

GUS Now Wednesday lunchtime, about half past twelve in the afternoon, Northern Territory time, and the storm energy map again.

GUS See the marker about 300 kilometres south-west of Alice Springs, near Yulara: about 1100 joules per kilogram. Half the Gulf's tank, but for the desert that's plenty.

GUS And the marker in central Queensland, east of Longreach: also about 1100 joules per kilogram.

GUS So two storm zones on Wednesday: the centre of the Territory and inland Queensland.

[c10_skewt]

GUS Here's the core sample south-west of Alice Springs for Wednesday lunchtime.

GUS The lid is about 5 joules per kilogram. Basically no lid. The afternoon heat alone, about 32 degrees, will pop the storms.

GUS Cloud base is high, about 2 kilometres up, because the surface air is drier than the Gulf, a dewpoint of about 15 degrees. Total water in the column is about 35 millimetres, a lot for the desert.

GUS High cloud base plus dry air underneath means rain falling through dry air, cooling it and crashing down. The downdraft energy is about 1125 joules per kilogram. That's the damaging wind risk: dust and gusts out of these storms.

GUS Between 3 and 5 kilometres the air cools about 7 degrees for every kilometre you go up. That's steep, and steep means the bubbles rise fast.

[c13_shear]

GUS Now wind shear, how much the wind changes between the ground and 6 kilometres up. Shear tilts a storm so the rain doesn't fall into its own updraft and choke it.

GUS See the marker south-west of Alice Springs: about 106 kilometres an hour of change. That's strong, supercell territory if the energy is there.

GUS East of Longreach, about 76 kilometres an hour. Also enough for organised, long-lived storms rather than quick pop-up showers.

GUS So Wednesday's storms will be organised and will last hours. The Territory cells move north-east at about 37 kilometres an hour.

[c14_index_map]

GUS This map puts energy and shear together into one number, the supercell composite. A low value, around one, means supercells are possible. A value around four or higher means the environment is very favourable.

GUS See the marker in the Great Victoria Desert, in Western Australia's far east, for late Wednesday night, about 11 pm Perth time: a supercell value of about five and a half, which is high.

GUS That's driven by strong low-level turning. Picture a spinning top: a storm can tilt that spin upright and start rotating. The models do flag rotating storms across the inland this week.

GUS But the energy there is only about 470 joules per kilogram and the air is capped. So it's a small, late-night chance over empty country. An index describes the environment, it doesn't promise a storm.

[c16_rain_wind]

GUS Here's the rain on Wednesday evening, about half past seven in the evening, Alice Springs time. See the marker about 120 kilometres north-east of Alice Springs.

GUS The American model has rain falling at about 47 millimetres an hour there. That's a real downpour, a bucket on your head, the kind that fills creeks in minutes.

GUS Desert ground is hard and crusty, so water runs rather than soaks. The soil there is rated moderate after recent rain, not bone dry.

GUS So if you're on the Stuart Highway or the outback tracks Wednesday night, flash flooding across the road is the risk.

[c20_model_compare]

GUS Now the key question: how much rain, and do the models agree? These are 24 hour totals to Thursday morning, same scale.

GUS Over the Tanami Desert, well west of Tennant Creek, the European model pours about 105 millimetres. The American model's biggest Territory total is about 47 millimetres. The European AI model tops out near 19 millimetres.

GUS South-west of Alice Springs, same pattern: European 16 millimetres, AI model 10 millimetres, American 4 millimetres. The models disagree a lot.

GUS Here's my call. The European model has been best at Territory temperatures lately, but on rain it's one voice in three, and only it reaches flood totals.

GUS I lean to 20 to 50 millimetres in the heaviest storms, with 100 millimetres an outside chance, not the base case.

[c19_ens_prob]

GUS Here's the European ensemble, about 50 runs of the same model with small tweaks, each asked: does 25 millimetres fall in the 24 hours to Thursday morning?

GUS See the marker east of Longreach: about 15 per cent of runs say yes. South-west of Alice Springs, about 5 per cent.

GUS For 50 millimetres, the best any spot on the map gets is about 57 per cent.

GUS So the ensemble sees the rain, but not many runs put the heavy stuff in one place. That backs my lean: real storms, patchy totals, flooding where a storm parks, not widespread.

[c17_index_map]

GUS Now the fire side. This is the fire danger index, a mix of heat, dryness, wind and how dry the fuel is.

GUS See the marker about 160 kilometres south-east of Karratha in Western Australia, for this afternoon about 3 pm Perth time: a fire danger index of about 67 points, which is severe.

GUS The dry air is remarkable, a dewpoint of about minus 3 degrees, so humidity near 6 per cent, with winds about 36 kilometres an hour.

GUS The drought factor is at the very top of its scale, as dry as the scale goes, and that was measured from recent rainfall records, not assumed. There's nothing wet in that country to slow a fire.

GUS So if you're in the Pilbara this afternoon, no burning off, and check the official fire warnings.

[c22_town_table]

GUS Here's the rest of the week for the capitals. For most of us it's mild and quiet.

GUS Brisbane about 28 degrees Wednesday with a few millimetres, 27 degrees Thursday and drying out. Sydney warms to about 25 degrees Wednesday, back to 22 degrees Thursday.

GUS Canberra 19 degrees Wednesday after a cold start near 2 degrees. Melbourne 17 degrees Wednesday, 19 degrees Thursday. Hobart 16 degrees. Adelaide 18 degrees then 21 degrees.

GUS Perth about 26 degrees Wednesday, breezy but dry. Darwin 33 degrees and sticky, with a storm or two worth about 7 millimetres Wednesday.

GUS The European AI model has beaten the American on Perth maxima on 12 days out of the last 12 days, so I'm leaning on it there. A robot admires a perfect score.

[c90_events]

GUS Now the events board. The one to watch is Victoria Derby Day at Flemington on Saturday, a crowd of about 86 thousand people, about 13 millimetres of rain in the models and a top of 20 degrees. Low confidence this far out, so pack a coat and check again Friday.

GUS Sydney on Saturday has Oktoberfest in the Garden at the Domain and the Halloween Festival on Brewer Street, both with 5 or 6 millimetres and about 24 degrees. A shower, not a washout.

GUS Sculpture by the Sea runs all week along the Bondi walk, a crowd of about 450 thousand people, and only about 2 millimetres in sight. Looks kind.

GUS Melbourne Cup next Tuesday: about 4 millimetres and 20 degrees, nothing to worry about yet.

[c02_mslp_fronts]

GUS How confident am I? My confidence is high, about 89 per cent. The three models put the lows and highs in nearly the same places, so the pattern is solid.

GUS What would change the call is the rain amount. If tonight's European run drops the Tanami total, the flood risk fades. If the American model comes up to meet it, I'll say so tomorrow.

GUS Official warnings come from the Bureau of Meteorology, so check bom.gov.au before you travel or light anything.

GUS I'm back tomorrow morning. If this was useful, subscribe, and if you've got a weather question, put it in the comments and I'll get to as many as I can during the day.

GUS Gus's Weather is a PandaVision production.