Southeast Queensland 2023 Christmas Day "Derecho"
Extreme Wind Event Analysis
Destructive wind events in thunderstorms are often associated with the passage of tornadoes due to the extreme wind speeds that are experienced in these phenomena. However there is one type of thunderstorm wind event that’s even more destructive – a derecho. True derechos are incredibly rare outside of the USA – with the most notable recent derecho event occurring in August of 2020 causing over $US11 billion in damage. This was the most expensive thunderstorm event in modern US history – dwarfing the damage bill of many significant tornadoes.
Concrete power pole snapped due to winds on December 25, 2023 SE QLD windstorm. Source: Energex Facebook page.
Derechos are long lived severe wind events in thunderstorms that occur over a minimum distance of 240 miles (385km). As they are often many (and sometimes tens) of kilometres wide, they can carve long-lived swathes of damage. They can be identified on radar from a series of bow echoes in a progressive line of thunderstorms. As to whether the extreme wind event that swept through Southeast Queensland on Christmas night of 2023 can officially be classified as a derecho is one of semantic definition. However this event is most similar to a derecho and can be closely compared to one – noting that derechos can also contain smaller, and weaker embedded tornadoes and vortexes within the system.
A quasi-stationary upper low pushed into southern NSW which stalled a trough system through eastern Victoria, NSW and Queensland. High levels of moisture were present to the east of this, with some remnant moisture flowing down from ex-TC Jasper. This provided a mix of high temperatures, high humidity and cold air aloft – a perfect combination for high levels of instability. In many ways, the surface was a typical late December “hot and humid” setup, while the upper atmosphere was more akin to conditions we expect to see in October and November in terms of wind shear and cool air aloft. The result was a period of unusually high levels of instability and supportive wind shear that occurred over a 3 day period (excluding the December 23 supercell hailstorm in Gatton, and then subsequent storms that still occurred in Sydney and parts of the Queensland coastline on December 27, all a part of the same trough system).
Upper cold pool and dryline from Christmas storm system. Source: metcentre.weatherwatch.net.au
The morning of December 25, 2023 was hot and muggy across Southeast Queensland with Amberley recording 29C with a DP of 21C by 9am. By 3pm this has increased further with temperatures of 34C and a DP of 24C. This yielded very high levels of surface-based CAPE (SBCAPE) of 5600j/kg with lifted index values falling to below -12C in what was a very strongly capped atmosphere. Very little storm activity occurred in the afternoon, though some isolated stronger storms developed about the Border Ranges late in the afternoon and evening where the elevation of the ranges likely assisted in the development of thunderstorm activity.
Analysed live CAPE from December 25, 2023. Source: metcentre.weatherwatch.net.au
Further inland the cap was weaker – likely assisted by the presence of a surface trough and higher surface temperatures. By 1pm, some isolated showers and storms began to develop south of Roma where temperatures were reaching 35C with dew points in the high teens yielding CAPE of 2500-3000j/kg. Wind shear here was stronger than in the Southeast Coast region (courtesy of the closer proximity to the upper trough) and this allowed for storm activity to move more quickly towards the E/SE over the coming hours.
Modified Roma sounding for December 25, 2023. Source: metcentre.weatherwatch.net.au
Between 2:30 and 3pm, the storms gathered further intensity and became more intense based on radar signatures. By 4pm, the system had transitioned into a QLCS (Quasi-Linear Convective System) while continuing to track eastwards and pick up speed. This is likely attributed to the increase in outflow winds and the development of a stronger forward cold pool that accelerated the storm activity. Around 5pm, a bowing of the activity can be seen to the south of Miles and while the system likely experienced damaging winds prior to 5pm, is a good indication of a broader, stronger wind field developing.
Between 5pm and 6:30pm, the storm system rapidly moved from Miles to approach Dalby where wind gusts of 93km/h were observed. On the radar, the strongest bow seems to have occurred south of Dalby which suggests winds were likely stronger than this south of the town. Over the coming hours, the QLCS advanced rapidly eastwards – moving over Oakey, Wellcamp and Toowoomba with all three stations exceeding severe wind thresholds (96km/h, 102km/h and 91km/h respectively). During this time, the Southeast Coast region continued to experience very little development resulting in an atmosphere full of convective energy prior to the arrival of the Darling Downs QLCS.
Just after 7pm, the line began to move into the Southeast Coast region. Despite the extreme levels of instability, storm activity had been almost exclusively confined to the areas close to the southern ranges at this time. This resulted in much of the energy from the day still being present – using the Amberley observations of 27/24 during the evening yielded SBCAPE still exceeding 4000j/kg.
Modified Ipswich sounding for December 25, 2023. Source: metcentre.weatherwatch.net.au
Strong outflow from the storm (90-100km/h>) approaching a capped area with SBCAPE of 4000j/kg> is akin to a strong trough and cold front moving through the area. The result was significant updraft development along the leading edge of the QLCS. Further bow echoes occurred across the Southeast Coast region, the most significant bow echo begun around Amberley (82km/h gust) and tracked southeast through the Ripley – Jimboomba – Tamborine – northern Gold Coast region gaining rapid forward momentum.
3D Radar 8:20pm, December 2023. Source: metcentre.weatherwatch.net.au
The movement was so sudden and significant, it created a possible book-end vortex on the southern side of the bow (which provides the impression of rotation in the thunderstorm). However upon examining the southern side of the bow with 3D radar, it does not appear to yield any updraft (let alone a mesocyclone) over the area meaning that it’s unlikely to be associated with deep convective rotation that would typically be expected during a tornado event.
3D Radar 8:35pm, December 2023. Source: metcentre.weatherwatch.net.au
3D Radar 8:35pm, December 2023. Source: metcentre.weatherwatch.net.au
3D Radar 8:45pm, December 2023. Source: metcentre.weatherwatch.net.au
3D Radar 8:45pm, December 2023. Source: metcentre.weatherwatch.net.au
Further 3D radar analysis over Coomera / Oxenford shows heavy surface precipitation across the region with no updraft or precipitation occurring aloft. This is a signal of a very strong rain foot – that is extreme winds in the storm pushed the rain well ahead of the main storm itself. This does not support the occurrence of a tornado – rather the presence of straight line winds. However, it does not exclude the possibility of a tornado-like event (gustnado or embedded vortex(es)) may have occurred within the wind event in itself.
3D Radar 8:50pm, December 2023. Source: metcentre.weatherwatch.net.au
3D Radar 8:55pm, December 2023. Source: metcentre.weatherwatch.net.au
3D Radar 9:00pm, December 2023. Source: metcentre.weatherwatch.net.au
3D Radar 9:05pm, December 2023. Source: metcentre.weatherwatch.net.au
Straight line winds in thunderstorms are traditionally localized and at the lower end of severe thresholds (90-125km/h). The fact the event was so widespread and extreme was in itself unusual. A private weather station at Mt Tamborine recorded a 160km/h wind gust – which is actually consistent with the damage observed which suggests the presence of very destructive (165km/h>) wind gusts in the system.
An analysis of satellite imagery was undertaken to examine for defoliation in vegetation over the region. Extensive defoliation was present across the Mt Tamborine region – mostly favouring the western slopes with the main damage path approximately 3-4km wide. The physics and meteorology required for a long lived (30-50km+) and 3-4km wide tornado is beyond that of what we could reasonably expect from the conditions on the day. The official widest measured tornado was 4.2km in the USA in 2013. Put simply, the fact that the path is so wide in itself almost rules out a tornado for the cause of the damage through the region. However as mentioned, this does not exclude the potential of embedded smaller tornadoes or vortexes that may have occurred within the QLCS as it moved through. However it’s likely the main cause of the wind damage was primarily straight line winds.
Satellite imagery courtesy of the EO Sentinel Hub Browser: https://apps.sentinel-hub.com/
This event can be considered most akin to a “Derecho” – a rare but violent wind event that mostly is experienced in the USA. Derechos are long lived, destructive wind events that typically have a continuous path of 240 miles (385km) or more where wind speeds exceed 90km/h. They are characterized by multiple bow echoes on the radar that continually regenerate. While tornadoes are often considered the most powerful winds in thunderstorms, derechos are typically the most destructive wind events that thunderstorms can produce. This is due to the wide damage path that is often associated with these events.
The system become a QLCS prior to crossing the A5 highway south of Miles. Assuming that wind gusts exceeded 90km/h in the first bow out stage, this provides a path of approximately 350km between south of Miles and the Gold Coast. While this is technically just shy of the 385km required, based on radar it’s very likely that damaging wind gusts continued well out to sea. Conceptually, this wind event is likely to have been much closer to a derecho rather than a tornado (though again, derechos are well documented to contain smaller and weaker tornadoes embedded in them due to the extreme vorticity that can occur around these events).
Revisiting the sounding, the Brisbane airport 2pm (04Z) sounding shows some interesting clues as to how such an extreme wind event occurred. A deep layer of moisture is present in the bottom 1300m of the atmosphere, with a strong cap present around the 800-850mb level. A small inversion was present here and likely restricted the depth of the boundary layer on this day ensuring deep, well mixed moisture in the lower atmosphere. The result is very little CAPE dilution, meaning that while SBCAPE can often overestimate mixed-layer CAPE (MLCAPE), in this case the 4000j/kg SBCAPE was likely representative of the actual instability present. This is significant as many Australian setups suffer from high SBCAPE values but much lower MLCAPE values due to the lack of a cap around 800-850mb causing the low level moisture to mix with the drier air aloft and diluting the instability.
Further to this, there is a very dry pocket of air between 700mb and 800mb. The presence of dry air here allows for precipitation to evaporate as it falls to the ground. This evaporative cooling can generate significant negative buoyancy, causing it to fall rapidly to the ground. This works in a similar way to a microburst, but in this case this is a downburst (which occurs at a larger scale than a microburst).
Analysis Brisbane AP sounding from 2pm, December 25, 2023. Courtesy www.bom.gov.au
There have been several well documented straight line wind events across Australia over the years. Most notable was the recent event on December 15, 2023 when Archerfield recorded a 169km/h wind gust from straight line winds. However other events include The Gap during November 16, 2008 (likely winds 160-170km/h, a similar damage to the December 25, 2023 event), and Double Island Point during December 16, 2006 with a wind gust of 196km/h. This demonstrates that while rare, straight line wind events can produce wind gusts that are capable of producing damage similar to that of a tornado in Australia. The difference between this event and the aforementioned events is that these events contained extreme winds for a much smaller area and over a shorter duration. The Christmas Day 2023 event (or derecho) was far more extensive and covered significantly more ground than any of these events. It therefore makes sense that it was far more destructive due to the large area covered.