Twenty-six years ago, on the evening of April 14, 1999, Sydneysiders watched in awe as a seemingly ordinary Illawarra thunderstorm exploded into a record-breaking supercell. It hurled 9–10 cm hail across the eastern suburbs, causing devastation and going down as one of Australia’s most destructive hailstorms.
Tonight, April 21, 2025, a line of coastal storms is once again illuminating the skies over eastern NSW. While these storms are far less severe—hail has mostly been in the 1–2 cm range and the most intense activity has remained offshore—the atmospheric setup bears an uncanny resemblance to that infamous 1999 event.
Both outbreaks occurred in mid-autumn, during Australia’s so-called “second severe storm season.” This is when lingering summer warmth clashes with the first intrusions of cool Southern Ocean air, creating a volatile mix. In fact, tonight’s storm comes almost exactly 26 years to the day after the 1999 event. This time of year often produces sharper temperature contrasts than summer, enhancing the potential for explosive convection.
Image: Lightning activity over coastal NSW (via EWN GIS Platform)
In both cases, storms fired up just after sunset. On April 14, 1999, the parent storm developed near Nowra around 4 pm and rapidly intensified after 6 pm. It peaked between 7 and 8 pm as it neared Sydney. Similarly, radar tonight is capturing blossoming updrafts in the same post-sunset window—driven by residual surface warmth and rapidly cooling upper levels.
Sea surface temperatures off NSW are currently running 2–3 °C above average, much like the warm eddy of the East Australian Current that energized the 1999 system. This marine heatwave is saturating the boundary layer with moisture, enhancing storm longevity—especially over water.
Today, Sydney reached the mid-20s under northwesterly winds—4–5 °C above average for April—before a southerly change arrived this evening, associated with a developing offshore low. The 1999 supercell also formed ahead of a strong southerly. These setups steepen low-level lapse rates and sharpen frontal boundaries, providing ideal conditions for storm development.
Both events feature a cold air pool in the mid-levels, sliding northeast along the coast. This added instability supports hail formation. In 1999, it was a key ingredient behind the supercell’s grapefruit-sized hail. Tonight’s system is more modest, but the presence of a cold pool remains a crucial factor.
By Michael Thompson Uploaded by Daniel at en.wikipedia
In both episodes, storms have moved northeast along the coast, steered by mid-level southwesterlies aligned with the frontal boundary. The 1999 supercell famously veered inland near Wollongong, cutting across Bondi and the CBD at peak intensity. Tonight’s cells have mostly hugged the coastline, staying offshore—but a shift in steering flow could bring them onshore later this evening.
The 1999 storm’s leftward pivot into Sydney’s eastern suburbs was the turning point in its destructiveness. Tonight’s system hasn’t made a similar move—yet.
Autumn remains dangerous: Even after Easter, late-day instability can turn a warm, humid afternoon into a hail-risk evening.
Tasman warmth is a silent contributor: Elevated sea temperatures fuel stronger and more sustained convection.
Upper-level cold pools are stealthy players: These instability drivers often emerge in forecasts only 12 hours out.
Stay alert: With the cold pool still moving northeast, conditions could evolve rapidly overnight.
While tonight’s storms aren’t rewriting history, they’re certainly rhyming with it. That alone is reason to keep an eye on real-time radar and the latest EWN warnings. Nature has a habit of reminding us that the past is never too far away.