This was some of the best supercell structure that I've ever seen!  A chase that will surely be remembered for many years to come...

September loves to tease - it's rare to get great setups during September as normally there's just not enough moisture.  However a slow moving high in the Tasman (and a previously fairly moist SE change several days prior) helped load up the atmosphere with unusually high amounts of moisture for September.  Dew points weren't overly high - still in the mid to high teens.  But as an upper trough approached (-15/-16C @ 500mb), and temperatures rose towards 30 degrees - the unusually stable September dynamics suddenly changed.

Shear was a little different on this day, no level was overly strong - but it was great directional shear!  There were two main areas to chase - the first was the Western Downs (along the main troughline), here LIs were forecast around -8 to -9, and shear was very linear (Surface to upper levels was NW to W, increasing to 40-50 knots by 500mb).  In the east, a weaker trough was situated along the ranges.  Shear was forecast to be slightly weaker in the upper levels, but this time from the SW (30 knots SW @ 500mb).  GFS suggested NW winds for the surface too - but September still has the ocean fairly cool, so any seabreeze is normally likely to build fairly quickly and rush inland.  With the SW shear, storms were already going to be favoured to move in a northerly direction, hence by the time the seabreeze reached inland it should be fairly warm, and add an extra 90 degrees of shear as the winds swung from the NW to NE!

After much umming and ahhing, I decided to try and go for the coastal trough, better directional shear and the threat of the low levels in the western downs mixing with drier air aloft helped with that.  LIs were still nice, around -7 - so there still wasn't a huge difference in instability!

A friend of mine Julian came over around lunch time to chase.  We headed off and went to Rosewood for lunch and then sat in a field to watch.  There were two main areas of convection, one to the NW (building near the usual Crows Nest to Esk corridor), and the other near to our south (around the Cunningham ranges).  Initially the northern convection showed the best promise.  It was nice, it had plenty of punch and there were a few nice overshoots.  However you can only chase another 30-40km or so north of Esk before either needing to go W or E for about 40km before getting any options.  The developing cells to the south, although not as high, always had a fairly large area of updrafts.  In the end we decided to head south and watch the convection there.  We had chaser convergence with Chrissy and David Smith.  It was good fun and we had a good laugh while we waited for the storm to do something!

At one stage the cell nearly gave up the ghost - but we were all encouraged by the northeasterly which had picked up to around 15 knots (presumably the seabreeze).  Once this got going, the storm began to finally take shape with a nice little base and defined boundary between the rain shaft and the updraft.  The RFB grew - but soon a much larger RFB grew to its west - in fact it was massive!  Winds began picking up too, reaching 20-25 knots from the NE and things started to get interesting very quickly.  Large CGs were dropping out of the new RFB base, and Julian and I quickly scaddeled out of there and shot northwards.  We were initially on the NE side which was fairly outflow dominant, but we could see interesting features on the NW side and on a hunch the storm would move more N or NNW, we repositioned to another side of the storm - and that's when things really got cranking!!!

When we first arrived about 15km south of Amberley, it was already a nice mesocyclone...however the meso kept "winding up" and getting larger, nicer and more separated from the main storm. 

At one stage it looked almost to become an outflow feature from our angle (when the hailshaft nearly came over us), but that wasn't the case!

Narrowly avoiding the main hail shaft (when we turned back onto the highway, the direction of the storm was a complete whiteout 400-500m down the road)...a few redlined gear changes and we were shooting northwards.  It was very soon evident that there was nothing outflow about the meso at all!  Quite the opposite...about 7km south of Amberley we stopped again - and what a site!!!!!!!!!

To date, the best mesocyclonic structure that I've ever seen in Australia!  It was incredible.  We were getting frequent close CGs shooting just ahead of the meso, along with a fair amount of 2cm hail that was falling from the overhang of the updraft.  At one stage the wallcloud underneath the meso seemed to tighten up and spin incredibly quickly!  However it wasn't to be - the Doppler wind velocities seemed to support it with a small region of 70km in/outbound side by side in the storm.  Unfortunately we couldn't stay long and had to head towards Amberley, and eventually towards Walloon.

Before taking the Walloon turnoff though we had to stop for a few more photos, this time the meso was tending a little more outflowing, but still incredible! 

From Walloon we got a few more shots before trying to shoot to Marburg and stay north of the storm.  The problem with September chasing is that the sun goes down so early and it was already nearly dark!  That also means it gets cool quickly, and as expected the storm began to weaken quite quickly as it went over Marburg (thankfully for the residents!)  We tried for some lightning photos, but no go (however we got some nice lightning footage on video driving back).  Not to worry - it was a fantastic day nonetheless and one that will surely be remembered!!!