An unprecedented rainfall event from the 21st to 24th took place across the region. Starting around 12.30 am on Saturday 21st, rain began to fall in Geelong. By 9 am, 8 mm had been registered at Grovedale. Continuing light showers fell for the rest of the day, with some respite during Saturday afternoon.
But in the early hours of Sunday 22nd it began to get heavier and more constant. Around 3.30 am heavy rain commenced, so that by 9 am the 24-hour total had reached 52.8 mm, a record for an April day in the district. Not letting up, the downpour continued right through until 6 pm when there was a lull for two hours in which only light drizzle persisted.
By 6 pm on Sunday 22nd the progressive 9-hour total had reached 52 mm and more was approaching.
Meanwhile news arrived that the Otway and Moorabool water catchments were also benefiting from the deluge. Mt Sabine in the West Barwon catchment reported an incredible total of 478 mm from 9 am Saturday to 9 am Tuesday, triggering flood warnings for Geelong in the following days.
Geelong’s 72-hour total to midnight on Monday 23rd reached 165 mm and even more rain was forecast, albeit lighter, for the next day. When the rain stopped around 9.30 am on the 24th, Geelong had received more rain than four times the April average and almost one third of our yearly normal.
What was the reason for this freakish circumstance? Geelong Weather Services believes that, while the rainfall totals were freakish, the causes were not. Basically the reason for the high totals in the region was the growth of a low pressure trough over eastern Victoria which developed a slow-moving wrap-around cloud band moving from the east and southeast over the Bellarine Peninsula, Geelong and the eastern Otways. The system had been made more than normally unstable with the effects of an upper level cold pool which had been fed in from the southwest in the previous few days.
The westward-moving winds brought so much rain to the Otways-Surf Coast region because the lie of the coast is directly perpendicular to the wind flow, causing rapid lifting of the air mass and the resultant precipitation.
Normally we could expect such an event to happen once or twice in a year. But we have had three such events with stalled low pressure systems in the last seven months.
Some rainfall totals for the event (a period of 91 hours) up to 9 am Tuesday 24th were: Mt Sabine 478 mm, Mt Cowley 414 mm, Benwerrin 310 mm, Boonah 274 mm, Weeaproinah 227 mm, Aireys Inlet 209 mm, Highton 180 mm, Forrest 177 mm, Leopold 172 mm, Grovedale 170 mm, Anglesea 164 mm, Manifold Heights 161 mm, Hamlyn Heights 158 mm, St Albans Park 153 mm, Lara 150 mm, Meredith 146 mm, Corio 144 mm, Avalon 134 mm, Bannockburn 133 mm, Cressy 123 mm, Birregurra 102 mm, Colac 101 mm, Scotsburn 94 mm, Laverton 92 mm, Lal Lal 88 mm.
Average daily maximum and minimum were close to normal (the month was approximately 0.5 degree cooler than normal) but there were some interesting variations. From a high point of 28 degrees on the 7th, maximum daily temperatures fell to around 15 on the period 20th – 23rd when overcast skies and rain dominated the conditions. The 19th’s maximum of 22.6 degrees was recorded at 12.30 am after a high of 26.7 on the previous day, while the minimum overnight temperature dropped to a low of 3.1 degrees on the 13th.
Severe wind storm damage occurred in several suburbs on the 12th, with an early afternoon maximum gust of 105 kph at Point Lonsdale. Lightning lit up skies to the south on the evening of the 18th after minor rains and on the 21st several monstrous thunderclaps occurred over Geelong between 3.30 pm and 4 pm during the early stages of the record-breaking rain event.