January 17, 2001 was an unusual thunderstorm event for Northeast New South Wales and Southeast Queensland due to the different timing and structure of the general system.  Traditionally in typical severe thunderstorm setups, thunderstorms often develop on the ranges before finally moving off the ranges and producing and enhancing other thunderstorms on the “plains” with their outflow as they move across Southeast Queensland.  However this was unusual in that thunderstorms did not develop until very late in the afternoon, and when they did develop they kept going through the morning!  In one particular instance, two lines of thunderstorms went through Brisbane overnight (one at approximately 11pm, another line from 3-4am), and then at 5:30am more severe thunderstorms formed and also went through Brisbane!  This is unusual in that after two lines of thunderstorms going through overnight, the amount of heat and moisture available to produce thunderstorms let alone severe thunderstorms would have been greatly diminished.  This is an informal report that aims to look at some of the meteorological factors that played a key role in such a significant event.  It will also look at how the same mid-latitude system moved up into the tropical Queensland region there, and also produced severe thunderstorms resulting in large damaging hail and tornadoes in the tropics!

A surface trough was situated from Southern Queensland to the Mid North Coast region of New South Wales.  This was moving fairly rapidly northwards, assisted by a strong S’ly jet behind it.  An upper level low was moving into the Tasman Sea, and a strong upper level ridge was pushing in behind this, producing a very strong jetstream due to a strong pressure gradient as a result from the tight isothermal gradient.  This assisted in moving the system further northwards across the Northeast New South Wales and Southeast Queensland regions.

A weak high was situated to the NNW of Southeast Queensland, this resulted in W to NW winds inland, drying out some of the inland regions.  It also assisted in maintaining an 850mb ridge over the general area, bringing a strong inversion at this level that would prevent any convection from breaking through in normal conditions.  Streamlines indicate an 850mb high over the surface high enforcing this.  Streamlines also indicate that a 850mb low also moved through the Northeast New South Wales region between 06z and 12z, and moved through the Southeast Queensland region between 12z and 18z.  Within twelve hours 850mb temperatures dropped from 21C over Brisbane to approximately 16C.  This is a significant drop, considering during these twelve hours the SE’ly change had not moved through yet!  (Although was close behind).  Also during the day and overnight, a surface low formed over the Northwestern Slopes & Plains and moved to the WNW of Brisbane.  This brought in an E’ly flow (Pseudo-NE), onto the Darling Downs region which was another significant factor as the region had been starved for moisture during the day, but was able to receive this moisture overnight.

The 850mb low resulted in moderate SW to WSW winds (15-20kn) at that level.  This played a part in assisting the wind to back with height.  Within the lower 2km the wind had backed nearly 180 degrees simply by the 850mb low moving through!

Interestingly, while there was an 850mb low over the region, the 850mb temperatures remained relatively constant, hence the simple thought that the 850mb low moved over the region and removed the inversion may not be entirely the case.  It did play a part, but more so in the severe thunderstorms that developed in the following six to twelve hours.

The 500mb charts shed some light onto how the thunderstorms broke such a strong cap, and how most thunderstorms that formed actually went supercellular and so severe.  An upper level trough was in the Tasman Sea associated with a cold pool.  This was moving northwards and intensifying into a defined upper level low on streamlines.  The upper level ridge was pushing in from a large upper level anticyclone over South Australia.  The 500mb jet was unusually strong, 40-50kn for most of the region from the SW.  The positioning of the main upper level low to the east out in the ocean was unique, as was the positioning of the upper level anticyclone.  It resulted in a moderate diffluence region over the area at 500mb, as well as a defined left exit region.  The tilt of the upper level trough was also changing quickly, from originally SW to NE, to S-N to eventually become SE-NW over the next day!  Vertical velocities of –20 to –25mb/hr (i.e. ascending air movement) at 06z suggests that this played a significant role aiding in further lift.  These actually increased to –25 to –30mb/hr at 12z, and then an area of –20 to –25mb/hr vertical velocities was also positioned out towards the Darling Downs region between 12z and 18z.

The upper levels were also getting significantly colder during the day, from –8C at 500mb during 06z dropping down to –12C over the next six to twelve hours.  The atmosphere was already conditionally unstable at –8C, but the further drop intensified the CAPE (Convective Available Potential Energy) in the region.  The 300mb levels also cooled significantly during this same time period from –33C at 300mb, to –37 and –38C over the region.  This suggests that the entire atmosphere cooled, not just these levels, further enhancing CAPE.

However it is the 300mb levels that appear to play some extreme significance in the situation.  The upper cold pool extended well up into the atmosphere, with a strong 300mb trough sitting over the Tasman Sea and Northeastern NSW and Southeastern Queensland.  However the 500mb upper level anticyclone over South Australia also was present at the 300mb level.  Between this a very strong jet was formed due to the pressure gradient resulting from the very strong temperature gradient.  The maximum speed analysed was 115kn, however this was not situated over the main regions.  At the time of most of the Northeastern New South Wales thunderstorms, the 300mb jet was from the SW at 70kn, but very defined diffluence was present, and the right exit region of the jet was sitting just behind this and moving over the region between 06z and 12z.  Again, due to the unique nature of this system, the 300mb trough was sending a W’ly jet just offshore from the region, and to the west there was an E’ly jet due to the curvature of the 300mb anticyclone!  This resulted in extreme diffluence and a very defined right exit region of the jet moving over Southeastern Queensland.  Of importance is also the way the jet does not become very diffluent until later in the afternoon.  This is noticed by the 300mb vertical velocities going from –5mb/hr under the diffluent area at 00z, to –20mb/hr at 06z, increasing further to –30mb/hr at 12z!  The effects of the jetstream played a significant role in the severity of the thunderstorms, aiding in further lifting by a “pulling” mechanism through diffluence of the jet exit region, and further divergence from the right exit region which existed over the area.

If the timing of all these synoptic and mesoscale events is looked at closely, they tie in with several significant meteorological events.  It is difficult to gain an exact sequence of events as I personally was not able to see all of the radar.  However some events can be tied in.  A very large cap was present due to the 850mb ridge being present over the region, this prevented all convection until very late in the afternoon.  At around 05z, the cap was breaking in Northeastern New South Wales, at the same time a 500mb and 300mb jet with moderate to strong diffluence was moving over the area, as was the right exit region and colder air aloft.  A trough, and SE-E’ly surge and a low pressure system to the west combining with the extensive surface heating (low-mid 30’s and DP’s around 20) finally allowed thunderstorms to develop.  Slightly cooler air was drifting in at 850mb, the surface triggers were applying a lot of forcing to the cap, and the upper levels were providing an area of low pressure over the region in all levels due to shear and cold air mechanisms.  When thunderstorms did develop, they had 180 degrees backing in the lower 2km of the atmosphere, from NE’ly at the surface to approximately SW’ly at 850mb.  The strengthening jet ensured that there would be plenty of outflow for the thunderstorms, and the lifting and “pulling” mechanisms ensured that the CAPE present that day would be enhanced.  By studying a variety of different Skew-Ts, CAPE can be calculated for this day between 2500 and 3000, although some areas may be locally lower or higher.

One of the first supercells of the day moved north in Queensland through Rathdowney, but quickly dissipated.  Other cells (most were supercellular) also moved in Queensland later that evening however many went out to sea.  The largest cell of the day (Casino Tornadic Supercell) also moved northwards into Queensland and although took longer to dissipate, eventually did as it moved over the Gold Coast hinterland and out to sea.  At this time signs of destabilisation were occurring in Southeast Queensland  with Congestus and Towering Cumulus forming on the Great Divide, these formed into weak thunderstorms during the course of the evening.  However the main upper level mechanisms that had enhanced thunderstorms significantly in Northeast New South Wales still had not moved into this area at this stage.

Another line of thunderstorms had formed towards Moree late in the afternoon, this moved northeast towards Northeast New South Wales and Southeast Queensland.  This resulted in a squall-type line that moved through with widespread strong to severe wind squalls and rainfall.  At this stage the main jet and right exit region was moving into Southeast Queensland.  A probable supercell also occurred on the WNW edge of this squall line with a prominent hook echo on radar.  This line passed through Southeast Queensland giving widespread minor damage.  Large portions of Brisbane were blacked out.  Further thunderstorms formed in the Darling Downs region soon after this line had moved through and they began moving eastwards.  It was unusual that these thunderstorms moved eastwards, however they occurred on the right exit region of the jet, and the jet was curving so significantly that the jet over the Southeast Queensland region was westerly, and several hundred kilometres north was actually southerly!  These thunderstorms were able to utilise the heat from limited convection during the day over the Downs, as well as increased moisture that east to northeast winds brought in as the surface low moved north-northeast.  The thunderstorms moved eastwards towards the Southeast Coast region and large to very large hail was reported across the northern suburbs of Brisbane from these thunderstorms!  Larger than golf ball size hail was reported near Samford (NW suburbs of Brisbane), and cricket ball size hail was reported at Redcliffe at 4am in the morning!  This was a phenomenal result given that temperatures around were largely around 21-23C with DP’s around 20-21C.  Resulting in CAPEs between 300 and 800 – not quite indicative or suggestive of cricket ball hail supporting updrafts.  However with other factors such as a deep upper level trough, diffluent jetstream and extra divergence resulting from the right exit of the jetstream, it assisted greatly in updraft strength.

These factors remained over Southeast Queensland for a while, and after 5am another line of thunderstorms developed on the border ranges.  These thunderstorms moved towards the northeast, and under the influence of the strong upper level trough and jet, quickly intensified into severe criteria with damaging wind squalls and large hail.  These thunderstorms went through Brisbane, causing creeks to flood over roads, more trees down and further disruption to power services.

After this, a weak SE’ly had moved through and the most favourable upper level atmospheric conditions continued to move northwards.  The main thunderstorm threat for Southeast Queensland had passed – but next in line were the northern Wide Bay & Burnett and the Capricornia regions.

Normally most upper level troughs approach from the west or southwest and are weakening by the time they reach Southeast Queensland, and tend to glide southeastwards.  This was not the case however for this system, as a high pushed a ridge up the coast, subsequently pushing the surface low with it up the coast.  The upper level trough also extended into these regions and produced an impressive MCS (Mesoscale Convective System) over the Capricornia district.  It is not in itself unusual to see extensions of upper level troughs into the Capricornia region however perhaps the main difference is that it continued northwestwards, well far North Queensland and into the tropics during Summer.

The upper levels were becoming highly meridional, and the upper level trough over the Coral Sea produced a low pressure system to east of Brisbane.  Since this low was not of tropical original, rather mid-latitudinal origin and it acted as a type of blocking mechanism. 

The upper level ridge was trying to dominate over Australia, and since the cold air was actually cutoff somewhat from the upper level low to the south, the upper level ridge was able to encroach into the Tasman Sea, leaving an upper level low by itself in the Coral Sea.  This resulted in strange tilting of the upper level trough (NW to SE) so while the low pushed the ridge up the coast with the high in the Tasman, it also pushed the surface disturbance.  This is not unusual and does happen, and results in increased tropical thunderstorm activity.  However the tilting of the upper level trough aligned itself with the Queensland coast perfectly, thanks to the upper level ridge encroaching in towards the south and the upper level low that had been cut off. 

A 50kn to 80kn jetstream was present just behind the upper level trough from the southeast.  This resulted in lower shear and would explain why an MCS formed over the Capricornia district on January 18.  Shear was moderate, and isolated thunderstorms originally did develop, some may have been supercellular however there was not enough shear to prevent an MCS.  The next day an even larger complex formed over the Townsville to Cairns region, this could be classified as a MCC (Mesoscale Convective Complex).  The system eventually was actually split into two separate complexes by a highly divergent jet.  This can be seen clearly on a WV animation of the MCC.  Further convection results in the middle and goes well into the night.

The result was an anvil shield larger than the size of New South Wales!  With an injection of colder air, and very hot and moist tropical air CAPE values calculated were extremely high, from sounding analysises of the area and past observations, CAPE can be calculated to between 5000 and 6000.  Hail up to golf ball size was reported to the west of Cairns!  A tornado was also videoed at Port Douglas the January 20 too.

Further to this system, another MCS formed over the southern Gulf of Carpenteria and northeast Northern Territory as a result of the remnants of the cold air being pushed into the Northern Territory by the southeasterly jet!  This took place on the 20th of January.

The main factor that rendered this system unusual was the fact that the cold air drifted so far north, especially during January.  It resulted in widespread coastal severe thunderstorm activity from the Mid North Coast of New South Wales to the Far North Queensland coast!  Quite often it is rare to have so many favourable variables over a particular region.  This was a highly unusual, yet extremely interesting and fascinating system to watch develop, not to mention gave us amongst the best thunderstorms of the 2000-2001 thunderstorm season!