Severe thunderstorms, large hail, flash floods may impact Hawkesbury next several hours – BOM

Severe thunderstorms are likely to produce heavy rainfall that may lead to flash flooding in the warning area over the next several hours, says BOM in an Amber Alert they put out at 4.45pm Wednesday.

At 5.12pm BOM updated their warning to say large hail and heavy rain likely to hit Blaxland, Londonderry areas.

You will all have noticed there is a fair amount of thunder and some showers around but it could get worse according to BOM, with the severe weather set to impact Penrith, Bowral, Campbelltown, Katoomba, Goulburn and Taralga.

According to our friends at Penrith Storm Chasing who watch storms closely, it is about to (at 5.10pm) impact Penrith.

Here’s the warning map from BOM.

And just an update on Warragamba Dam levels because we haven’t done that for a while – it’s now sitting at 98.2% full, that’s 0.4% up on the past week, and rising.

It is not likely the dam will spill anytime soon – certainly not today or tomorrow – but it is steadily creeping up.

WaterNSW, which runs the dams system, recently explained how Warragamba is managed and why water is not let out early.

Here’s WaterNSW’s explainer, which includes the fact their operating protocols allow for storage levels to be reduced by one metre after reaching 100% full. Of course, by then the dam is beginning to automatically spill.

“Once the storage approaches 100% WaterNSW specialist teams at Warragamba move to 24-hour rosters and prepare critical equipment such as dam gates in case the storage exceeds capacity and spills,” says WaterNSW.

“Exceptional La Nina weather conditions over the past 12-18 months have resulted in huge volumes of water flowing into Warragamba Dam, from across a catchment that covers over 9,000 square kilometres, keeping the storage at or near capacity since August last year.

• Since March 2021, Warragamba Dam – which holds 2,000 gigalitres (GL) – has received approximately 4,200 GL, more than twice its storage capacity (and more than eight times the volume of Sydney harbour).

• Since November 2021 WaterNSW has pro-actively made controlled releases of 1,000 GL – the equivalent of half the dam’s storage – to reduce the storage level as much as is permitted under its operating rules.

“As the primary water supply for Greater Sydney, Warragamba Dam does not have a dedicated flood mitigation function, but its operating protocols allow for the storage to be reduced by up to one metre after reaching 100%.”

“Once the dam reaches 100%, WaterNSW continues releasing water through the gates to reduce the storage by one metre, to 96.5%, and create capacity to capture 72 GL of inflow.

During this La Nina period recurring rainfall has resulted in available storage capacity often being rapidly re-filled with inflows from rivers in the dam’s saturated catchment, an area which stretches from north of Lithgow to south of Goulburn.

“Warragamba is not the sole contributor to water in the Hawkesbury-Nepean Valley. During significant rain, large tributaries entering the river system downstream of the dam are a major contributor to the height of the rivers.”

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