SE QLD severe storm analysis for Sunday 17th November, 2019

So, let's look back at this day and how it all unfolded.

Firstly, this day should have come as no shock to people, as numerous agencies forecast thunderstorms, including the potential for severe thunderstorms, days in advance.

A surface trough was draped from the Gulf down into northern NSW, taking on a more north-to-south orientation across SE QLD and NE NSW (this orientation will be important later on). One significant uncertainty on this day would be whether drier west-to-south-westerly winds may push too close to the coast. Regardless of this, it was evident that thunderstorms were more than likely at least across the Sunshine Coast region.

The morning started sunny, with temperatures quickly warming up. The 18z analysis showed a nice tongue of 700-800mb moisture from the southern Wide Bay north of Brisbane. This was likely to be the trigger for more elevated/high-based convection during the early afternoon hours across parts of the Brisbane Valley. 18z ACCESS C 1hr precipitation was on the money for this event across this region, blowing up activity at around lunchtime.

Image 1: ACCESS C 1hr precipitation field 12pm. WeatherWatch

Image 2: ACCESS C Esk Sounding 1pm EST WeatherWatch

On Sunday, 17th November 2019, a significant thunderstorm outbreak occurred across South East QLD. Unofficial hail to the size of softballs (10cm diameter) fell across parts of the Sunshine Coast, with hail larger than golf balls reported across parts of Logan and the northern Gold Coast.

It was evident during the morning that an impressive kinematic and thermodynamic profile existed across SE QLD with bulk shear profiles in the 40-45kts range & ML CAPE (Convective Available Potential Energy) across coastal regions was in the order of 2000-3000 joules, with a strong north-easterly feed developing during the afternoon hours. A strong short-wave trough would enter SE QLD with a relative vorticity max across the region. This vorticity max would produce an area of PVA (Positive Vorticity Advection), which would contribute to upper-level divergence in the PVA, thus rising air. Of note is that the axis of this short wave trough was also perpendicular to the surface trough across SE QLD. 18Z ACCESS C soundings showed surface to 2km lapse rates at impressive values, sitting around 9.5C/km (dry adiabatic is 9.8C/km), and the 0-6km lapse rate around 7.0C/km across SE QLD during the afternoon hours (see images below).

Image 3: Images via Alicia M Bentley

Image 5: GFS Surface streamlines and CAPE

Image 6: 500MB, WeatherWatch MetCentre

Image 7: Bulk Shear, Pivotal Weather

18z GFS analysis also showed that drier westerly winds would push very close to the SE Coast from Brisbane to the NSW border, inhibiting the amount of convection that might develop across this region. However, forecast soundings via ACCESS C were still very impressive and showcased the potential for supercells should activity fire in this corridor. One potential issue was CIN (Convective Inhibition or a cap) across coastal regions. However, the lift from the short wave aided in breaching this cap, enabling storm activity to tap into the quality moisture and instability across coastal locations and enabling thunderstorms to intensify and become supercellular rapidly.

Image 8: Brisbane sounding, Sunday morning BoM

Image 9: ACCESS C sounding, Sunday afternoon (Brisbane to Sunshine Coast). WeatherWatch

Image 10: Sunshine Coast sounding. WeatherWatch

Image 11: Sunshine Coast sounding. WeatherWatch WeatherWatch

Image 12: Sunshine Coast AP observation, Sunday 17/11/2019

Image 13: Tewantin Observation, Sunday 17/11/2019

As the storm across the Sunshine Coast intensified, it took on classic supercell characteristics. A large hook echo began to form along with a well-defined V notch on the radar, the sign of a powerful updraft. It was evident on 3d radar that this storm was impressive, showing a clear and well-defined BWER (Bounded Weak Echo Region). As seen in the observation images above, strong inflow was now feeding this supercell, with north-to-north-easterly winds gusting to 70km/h. A clear and well-defined couplet was now evident on the radar. Evidently, this storm could have destructive winds and giant hail. With such a powerful storm exhibiting these characteristics, tornaodgenesis couldn't be ruled out. However, it would only be a slim possibility considering a marginal LLJ (850mb winds) was present. No considerable damage consistent with tornadic activity was reported/occurred.

Image 14: Images via Radar Scope showing inbound and outbound winds, along with a well-defined hook echo

Image 15: Images via Radar Scope showing inbound and outbound winds, along with a well-defined hook echo

Image 16: Hail to cricket ball size over the Sunshine Coast region

Image 17: A strong couplet begins to develop NW of Caboolture

Image 18: Strong inbound and outbound winds near Woodford

Image 19: A classic V notch appearance takes shape on radar

Image 20: Classic BWER signature on 3D radar

Splitting supercells developed across the Logan Village-Tamborine-Ormeau-northern Gold Coast regions, with a particularly impressive right-moving supercell producing hail between a golf ball and cricket ball size. The sounding below was taken from the Tamborine area via ACCESS C. WE can see that this profile exhibits large cape, good shear, and moisture depth to nearly 800mb along with steep lapse rates from the surface to 900mb, 800-600mb, along with a pronounced dry slot in the mid-levels. Right-moving supercells in SE QLD tend to be in the LP (Low precipitation) mode and usually produce large hail. Given the impressive set-up, it was easy to see why a thunderstorm in this environment would rapidly intensify and produce large hail.

Image 21: Sounding from the Tamborine region, Sunday afternoon 17/11/2019

Image 22: Golf ball sized hail from Tamborine