Sunshine Coast Supercell Hailstorm – November 17, 2019

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A strong upper trough tracked through NE NSW and SE QLD on Sunday the 17th of November, 2019 which generated numerous severe hailstorms over coastal areas of these regions.  The largest was a destructive high-precipitation supercell that tracked over the Sunshine Coast region with widespread cricketball hail occurring.

Thunderstorms developed early over the northern Brisbane Valley by midday thanks to the forcing of an approaching surface trough and a weak cap.  Initially storms were disorganised over the northern Brisbane Valley due to the weaker instability in the region.  These began to intensify as they moved eastwards due to higher levels of moisture and instability present close to the coastline (drier air moved in from the west during the middle of the day which contracted instability closer to the coastline).

Around 1pm, the thunderstorm began to interact with the seabreeze airmass closer to the coastline.  In some cases, this seabreeze airmass can be more unstable but storms struggle to survive in this airmass due to a stronger cap (stable layer above the surface) due to the cooler air.  However as the storm had already become quite intense prior to interacting with the seabreeze, the strong outflow with the storm helped support and develop new updrafts (while the strong seabreeze kept the outflow from advancing too far ahead of the storm).

Eventually the storm developed a Bounded Weak Echo Region (BWER) which is normally only found in the most intense thunderstorms and is where the updraft is so strong that no precipitation reaches the ground at all, and grows to such an extent that it begins to “fall over the top” of the updraft and down the other side!

After 1pm, the storm entered an even more unstable environment with CAPE values reaching about 2.5 of its origin.  More favourable wind shear was also encountered with strong NE winds at the surface and very strong westerly winds several kilometres up in the atmosphere.  This type of turning helps the thunderstorm rotate and become a supercell.  The development of rotation signifies the supercell (the most intense and destructive type of thunderstorm).  This high level of organisation in thunderstorms help cause them to become much more intense than non-supercell thunderstorms.

It’s unfortunate that this is the same time the thunderstorm crossed the Bruce Hwy and entered the very population Sunshine Coast region.

The thunderstorm carried large amounts of hail high in the atmosphere for an extended period of time.  This meant that the hail had a long time to sit in the thunderstorm and grow (thanks to the strong instability preventing any smaller stones from reaching the ground).  The result was that when the hail finally fell through the updraft it was incredibly massive!

The supercell crossed the coast just before 2pm with what is known as a “Rear Flank Downdraft” helping to create a hook echo.  A hook echo often occurs just before the development of a tornado (which would be the most destructive type of feature a thunderstorm could produce).  Fortunately this didn’t eventuate, though the Rear Flank Downdraft still generated damaging winds that brought down trees and powerlines.