November 17, 2024
There's good news in store for storm-weary residents of Brisbane, Southeast Queensland, and Northeast NSW. After weeks of relentless hailstorms and supercells, a pattern shift is occurring that will briefly reduce the number of storms in the region. Thanks to a more persistent surface ridge along the east coast, slightly more stable southeast winds are expected to make storms less frequent—at least for now.
Storms have provided a visual feast to residents across Southeast Queensland with their dramatic scenes across the suburbs and adjoining regions.
It’s a much-needed break for not only residents but also emergency services, who’ve been kept busy responding to the storms. And for good reason. Using Weatherwatch’s HailTracker, we’ve been able to track the number of large hail events around Brisbane each spring dating back to 2010. Not surprisingly, the numbers support what everyone’s been saying—this spring has been significantly more active than usual for hail and severe storms. In fact, Brisbane has just experienced the second most hail-active spring for hailstorms in 15 years!
Above Average Hail Days for Spring
So far in 2024, a total of 9 days have recorded large hail (>2 cm) within 50 km of Brisbane. That’s nearly double the annual average (2010-24) of 5 days for spring, and there are still two weeks left in the season! The most active year over the last 15 years was 2021, with 12 large hail events for spring. But this year isn’t far behind. Areas around the nearby mountain ranges tend to be even more hail-prone, but this is outside the 50km radius.
The number of large hail days within 50km of Brisbane has been higher than usual so far for spring. Source: Weatherwatch HailTracker
The reasons for this uptick in activity come down to a combination of factors, but the primary driver has been the high number of supercells in recent weeks. This brings about a multitude of questions which we've answered below - including what's a supercell, what's caused them and why this season has seen an uptick in supercell numbers:
What’s a Supercell?
The hallmark of a supercell is a persistent, rotating updraft known as a mesocyclone. This rotation organises the storm, allowing it to persist longer and produce more severe weather. Supercells have several characteristics that make them particularly intense:
Stronger Updrafts: Rotating updrafts can enhance lift on one side of the storm which creates stronger updrafts enabling the storm to lift and sustained larger hailstones and sometimes create intense downdrafts.
Wind Shear: Supercells often form in environments with strong wind shear, which keeps the warm inflow and cool outflow of the storm separate. When these mix, storms typically weaken, but supercells avoid this, helping them stay stronger for longer. As supercells last longer, they cover more ground so have the potential to impact larger areas.
A supercell has a rotating updraft. It can sometimes appear rounded or circular due to the rotation within the cloud.
What Causes Supercells?
Supercells require instability (a common ingredient for all thunderstorms) and sufficient wind shear to develop. Wind shear refers to changes in wind speed and direction with height, and it’s often the determining factor between supercell and non-supercell thunderstorms. Supercells require opposing winds, that is strong winds in the lower atmosphere from one direction (ie northeast) and strong winds in the upper atmosphere from the opposite direction (ie southwest), but can still occur in linear wind shear when the speed is very fast.
In recent weeks, several key factors have contributed to supercell development in the region:
Strong Upper-Level Winds: A broad region of cold air in the upper atmosphere across southern Australia has clashed with warm air over the north, creating strong pressure gradients and strengthening winds at high altitudes.
Southeast Wind Changes: Frequent but brief southeast changes along the coastline have injected deep low-level moisture into the region, a crucial ingredient for instability.
Opposing Low Level Winds: A combination of southeast changes and gusty seabreezes has helped bring strong easterly winds at the surface that assist in providing rotation to updrafts.
Perfect Instability Mix: Warm air drifting down from northern Australia and humid air from the ocean have combined to create strong instability—essential for producing large hail.
An atmospheric sounding for the Brisbane November 1, 2024 supercell that produced golfball hail over the suburbs. Note the opposing wind direction in the bottom half of the atmosphere which assists in rotation and the supportive CAPE environment (~2000j/kg).
Fast-rising updrafts are critical, too. A cricket-ball-sized hailstone requires updraft speeds of over 150 km/h to stay suspended in the storm! For this region, higher levels of instability is also favourable for large hail (CAPE typically >1000-1500j/kg)
The Role of Seabreezes
November is also prime time for gusty seabreezes along the coastline. These breezes enhance low-level inflow into thunderstorms. During this month, the ocean is still relatively cool, while the land heats up quickly, creating a strong temperature difference that strengthens the seabreeze. This additional inflow can help fuel supercells.
Are Hailstorms Becoming More Frequent?
Over the past 15 years, there’s been a slight upward trend in the number of hailstorms recorded. However, 15 years is a relatively short time in climatological terms. Brisbane and its surrounds have always been prone to hailstorms due to their location, which is ideally situated to experience:
Warm, humid conditions at the surface that fuel instability and thunderstorm development.
Cold fronts and upper troughs from the south, which enhance the atmosphere and make it conducive to severe thunderstorms and supercells by bringing increased wind shear.
Upper troughs are regions of cold air in the upper atmosphere. These nudge into the region to assist in creating instability and providing cold air in the upper atmosphere for the formation of hail.
How is Hail Tracked in Thunderstorms?
Tracking hail within thunderstorms involves a combination of cutting-edge technology and atmospheric analysis. By ingesting 3D radar data, detailed weather models, and complex radar analytics, we can can estimate and map hail size with remarkable precision.
This information is invaluable for industries like insurance and hail repair, as it allows them to pinpoint exactly where hail has fallen, reducing the guesswork and improving response times. Tools like Weatherwatch's HailTracker also enable proactive hail alerts, giving people and businesses advance notice of large hail. This means valuable assets—like vehicles, solar panels, and outdoor equipment—can be protected before the hail arrives.
An assortment of hail maps from recent large hail days from Spring 2024 across Brisbane and Southeast Queensland. Source: Weatherwatch HailTracker.
Will the Trend of Active Hail Continue?
Long-range forecasts are inherently variable, but the immediate outlook suggests a quieter period for at least the next week and possibly even for the remainder of spring (at least for Brisbane and the surrounds). Thunderstorms often occur in clusters, and after an active few weeks, we’re likely to see a temporary lull as storm activity shifts to other parts of the country. This is thanks to a blocking high in the Tasman helping to establish a prolonged ridge along the east coast which will contract any instability well inland, possibly even favouring western to central Australia. While the intense storms may become less frequent, the same pattern can enhance rainfall potential.
Forecast models suggest a slow-moving high will take up residence in the Tasman which tends to result in storm activity favouring the western, central and northern areas of Australia. While this may mean decreased supercell activity, it's a pattern that can bring increased rainfall. Source: MetCentre.
As we move further into the warm season, severe thunderstorms typically become less frequent in Southeast Queensland. This is due to the reduction in wind shear during summer compared to spring. However, December often remains an active month for hailstorms, with the number of hail days usually tapering off in January and February.
For now, residents can enjoy a much-needed breather—but remember, the storm season isn’t over yet.
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