THE WRONG SYSTEMS COMING TOGETHER AT THE "RIGHT TIME"
Since October 11, 2021 there's been a marked increase in severe thunderstorm activity across NSW and Queensland including multiple tornadoes and record breaking hail. While this coincides with the historical start of the severe thunderstorm season, there's no denying that this period has been very active - particularly for hail but there's been an above average number of tornadoes reported.
Hail requires several factors to form, but a critical factor is the healthy dose of cold, dry air in the upper atmosphere. This has been in abundance recently - if we look at the gargantuan (15cm>) hailstone event north of Mackay on October 19, 2021, we can see that the upper atmosphere is unusually cold for this time of year:
The 300mb temperature anomalies (this is the temperature approximately 10km above the surface) are well below average for mid October, with temperatures 7-8C below average - largely associated with a broad upper trough across eastern Australia.
While the upper atmosphere temperature anomalies are well below average, the lower atmosphere temperature anomalies have been above average. Given the fundamental cause of thunderstorms is hot air rising, the presence of these two factors gives rise to the strong instability. Particularly when combined with the high levels of tropical moisture.
The sounding north of Mackay yesterday yielded very strong SBCAPE (Surface Based Convective Available Potential Energy) with values exceeding 3000j/kg. In addition, wind shear was supportive for the development of supercells with 15kt NE winds in the lower levels, swinging to the WSW at 40kt at 500mb. This provides bulk shear values exceeding 50kt (the supercell threshold is 40kt)
Perhaps what is most unusual about this setup is the moisture. Normally a strong upper trough in October would result in drier, westerly winds pushing over the coastline, causing conditions to be too dry. However at the same time of this upper trough we saw two climatic influences play their role in our October Hail Outbreak. The first is the developing La Nina in the Pacific Ocean. Weatherwatch subscribers would know that this has been building since June earlier this year, but since early September has began to intensify more rapidly.
La Nina events often cause an increase in the easterly flow across eastern Australia which can inject more moisture in the region. As such, La Nina's are more prone to bringing earlier severe storms to the region (though this can tail off towards the summer period and contract inland as the easterly influence becomes too strong).
The final "nail in the coffin" was the collapse of the weakly negative SAM (Southern Annular Mode) event. Negative SAM events increase the westerly flow across southeastern Australia which can push well into Queensland at times. As the SAM weakened, it allowed the easterly influence of the Pacific Ocean (assisted by the La Nina) to push through quicker than normal.
The result was higher levels of moisture combining with an already warm lower atmosphere and a very cold upper atmosphere - the "perfect" recipe for hail. More storms are likely this week with further hail likely in some areas.
Map of hail sizes from Sydney October 11, 2021 HailStorm (Weatherwatch HailTracker)