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Tuesday 23 May 2017

 Unusual May rain event covers eastern Australia from Gulf to Tasmania

A sweeping band of rain that began on Wednesday 18 May in tropical north QLD and the Gulf Country and travelled south to cross four states and the ACT finally exited the East Coast of TAS on Sunday morning after taking over four days to make the journey. What makes it notable is the expanse of country it traversed, almost the whole nation from north to south, and this in a month not normally associated with large-scale rain bands or events.

Many people have commented that the warnings and forecasts ahead of the event were overstated, and it is true that in some areas the expected rainfall amounts didn't happen. Yet widespread May one-day rainfall records were broken in the 24 hours to 09.00 on each of 18 May, 19 May, 20 May and 21 May, and many locations had more than their monthly average rain in 24 hours.

BoM Senior Forecaster Scott Williams commented, as the event was beginning, that "it's unusual to get a band that extends right across the country this late, outside the traditional wet season. The average rainfalls in western QLD, western NSW and northwest VIC for May are only about 25 to 40mm, so this event in many places should deliver more than the average monthly rainfall in May." He added that further east, as well as in VIC and TAS which are moving into winter rain in May, amounts are normally heavier anyway. "So the real unseasonal rain with this is the northern parts over inland NSW and QLD in particular."

The BoM rainfall map for the week to 09.00 on 21 May shows the distribution of the rainfall well. Very little rainfall, other than from this system, fell in the week. The map clearly shows the patchy and irregular nature of the rain, which is why some people were commenting that the forecasts had overstated the expected rain. Many of the areas that received heavy falls were under thunderstorms or one of the long, rather thin but long-lasting rainbands that were the main mechanisms for delivering the rain. As a result, while some areas received no real rain to talk about, many from the north to south of the country had their heaviest May rain in decades.

The heaviest rain was on the QLD tropical coast and inland to the Gulf. On and near the coast between Townsville and Mackay, many locations received between 150 and 250mm in the 48 hours to 09.00, Friday 19 May, with the heaviest of the official or hydro recordings 244.6mm at Alva Beach. Bowen Pump Station gauge was not far behind on 220mm and minor flooding was reported in the Don River inland form the city. Over the two days, Townsville recorded 154.8mm (May average 39mm).

Some notable long-term records were broken. Mingela Post Office, in the hills 70km SSW of Townsville, which has continuous rainfall records going back 1899, set a new record on 18 May with 108.0mm, then surpassed that on 19 May with 110.6mm. Other records on 19 May at stations with long histories were Ravenswood, near Mingela, 127.4mm with records back to 1887, Townsville Aero 115.2 (1941), Collinsville 120.4 (1939) and Croydon 118.0 (1889). Croydon's previous record was only 63.5, set in 1956, and its average May rainfall is 7.8mm. For the statisticians, Croydon's median rainfall is a mere 0.4mm.

While the wet weather was winding down in northern QLD on Friday 19 May, rain and thunderstorms were pushing into western NSW and VIC and across to the SE QLD coast. During the day, these spread across much of NSW and VIC with the heaviest rain in a band that spread from SW NSW and NW VIC to the NSW southeast. Even far southeastern SA saw some action with falls over 25mm E and SE of Murray Bridge; Parrakie's 38.0mm was the highest.

While VIC had no rain in record territory, NSW broke many one-day rainfall records over Friday and Saturday with the most significant 56.0mm to 09.00 on Friday 19 May at Gunbar in the far western Riverina (records back to 1949) and 51.2mm (1971) at Berriedale west of Cooma to 9am Saturday 20 May. Canberra had a drenching with 33mm in the city centre to 09.00 Saturday, which triggered emergency crews to prepare sandbags [The Age]. Tuggeranong recorded 52.2mm, its highest May total in a 22-year history, and Mt Ginini, on the Divide west of the Capital, recorded 88mm in the same period. The heaviest falls in SE NSW were on the South Coast where Central Tilba recorded 151.0mm, the highest since it began recording in 2003.

Rain began falling in NW TAS overnight into Saturday 20 May, and by 09.00 had deposited a record 50.2mm in the gauge at Marrawah (records began 1971) and 61.6mm at Smithton, the highest May fall since records began in the town in 1911. During Saturday and into Sunday, the rain spread across NE TAS and Flinders Island and down the East Coast with most locations picking up 50 to over 120mm.

Flinders Island Airport recorded 73.8mm to 09.00 Saturday, its heaviest May fall since 1969, and a further 36.6mm during the day. Notable new records were set in the NE to 09.00 on Sunday 21 May where Fingal recorded 102.0mm (records began in 1888) and Gladstone 83.0mm (1914). The heavy rain in the northeast produced minor flooding in the South Esk River. Only ten emergency callouts were required in the north, but blustery winds helped produce widespread though small power outages [The Advocate].