Topic: Australian tornado climatology and information / Bulahdelah Tornado Report 1/1/1970

[Jimmy Deguara]

On slightly another tangent, the difference between Australia (BOM) and the United States (NWS) is that all tornadoes are investigated and rated in the US.

The description from eyewitness of the Bulahdelah tornado refer to rapidly turbulent scud action and low bases. Giant hail was reported that would indicate supercells on this day with smashed windows around Newcastle if I recall and the Mid North Coast.

Regards,

Jimmy Deguara

[Michael Bath]

The Bulahdelah tornado report written by B.W. Shanahan at the Bureau of Meteorology is now online.

---> PDF link


An interesting read.  55 knots from the NNE at 850 from the Coffs Harbour sounding - not bad lol

Plug a bit more boundary layer moisture into that 9am plot and you'd end up with a rather high CAPE figure.


David Croan replied in an email:

Definitely Michael, I'd say richer moisture would, given those 22 degree dewpoints at noon near Taree, would have advected down so the 16 dewpoint at noon is off.  From the model there is not a great deal of 500mb cooling between 0 and 6z, not sure how accurate that is,  but the very strong CAP was breached so something changed through the day in addition to just heating.  Very dynamic system, those winds 3pm at Coffs ...wow  --0-3 SREH would have been high end, and presume 0-1 km too if we had that information.

It seems Australia is a tipping point with these sorts of systems. We have had nothing really like this in the last 13 years, in fact it is getting horribly boring here as far as chasing. Was the fact that this Brisbane and then Sandon tornadoes occurred soon after just a coincidence, or does it reflect subtle shift in longer term patterns. Always hopeful it will tip back in favour of more intense upper systems with this sort of surface cyclogenesis, in the warm season of course.  We must be overdue surely!

then Jimmy Deguara responded:

Yes to me the 850 winds and moisture are the key ingredients to low bases and inflow dominant structure. The upper level cooling would have been in place further west at the least. Nevertheless, so long as you have uplift and some lifting mechanism and great shear we are business.

and Michael Thomas

From what I see, there appears to be some pretty big differences between the wind profiles given in the report versus the reanalysis. In the reanalysis, the 850 mbar winds are in the 20 to 30 knot range over the Northern rivers/mid-North Coast whereas in the report the 850 mbar winds much stronger at Coffs Harbour (55 or 60 knots at 3 pm?). I also suspect that the LI's were considerably lower than shown in the reanalysis given that dew points were in 20 to 22 C range with 500 mbar temps around -12 to -14C. How would you rate this outbreak to a classic US setup (with the limited information available)?

[Michael Thomas]

I will add a few things here. From what I understand, the temperature/dew point at Taree was 26/22 at 1200 EST. Taking a surface-based parcel of 26/22 from the surface and lifting to 500 mbar gives a temperature of -4/-5.  The 500 mbar temperature on the Williamtown sounding was -13. Therefore, a rough estimate of the surface-based lifted index (LI) would be around -9/-8. Using a surface-based parcel instead of a mixed-layer parcel usually gives an overestimation of the instability. However, given the direction of the lower-level winds in the afternoon and the observation of scud cloud around the time of the Bulahdelah tornado I would think that moisture would have been quite “deep” in the afternoon. Also, if the observation of 26/22 was taken at midday then there may have been some further heating during the afternoon.

Taking a look at the upper wind profiles from Williamtown and Coffs Harbour in the afternoon, a couple of things are quite interesting. At Coffs Harbour, the lower-level winds are extremely strong and turn from NNE to NW.  This would give an enlarged hodograph with strong anticlockwise turning favouring left-moving supercells. The only flaw with the Coffs Harbour upper-level wind profile is the relative weakness in the 700 to 500 mbar winds (40 to 45 knots, not much of a flaw really). The wind speed at 500 mbar at Williamtown is very strong at approximately 65 knots. However, the lower-level winds are not as strong as at Coffs Harbour. For example, the 850 mbar wind speed is only 20 knots from the NE (still pretty good in Australian standards). I wonder whether between Coffs Harbour and Williamtown there was a blend between these two upper-level wind profiles giving both strong lower-level and mid-level winds. If so, that would be a near-perfect profile for strong to violent tornadoes given the strong instability and low LCL’s present that day.

[enak_12]

Ah very fascinating reading thank you for putting this online. This was an incredible event, they even make observations to the setup being exceptional even if this was in the USA. If only we could get one setup a year like this or similar, at least for the purpose of storm chasing. It wouldn't be fun to see a tornado like that go through a town though!

You can actually make out deforestation in the background of the first photo even in that photo copy.

I wonder if the cricket ball hail at Urunga was from the same storm as that which destoyed the house at Frederickton, which is just east of Kempsey, would have loved to chase that storm also.

Now when are we going to get a setup like this again? haha

[Jimmy Deguara]

Hi Enak,

This event may have been briefly comparable to a US style tornado in upper scale but probably not a May 3 1999 Oklahoma City or Joplin style event. We are still talking paramaeters being sketchy for the day and I agree with Michael Thomas and David Croan in terms of downplaying the event a slight touch perhaps F3 to F5 range.

I wonder if the cricket ball hail at Urunga was from the same storm as that which destoyed the house at Frederickton, which is just east of Kempsey, would have loved to chase that storm also.

If trajectories on this day were from the NW, then definitely not. I would perhaps suggest there were quite a few supercells on this day.

And yes, such events are quite rare in this country in my opinion. Tornadic events of varying intensities and significance are simply part of the Great Plains climate!

Regards,

Jimmy Deguara

[Michael Thomas]

Hi Jimmy,

You mention of the parameters being sketchy for the day. What do you feel was lacking in this storm outbreak?

For most of the big storm days in Australia, there always seems to be something missing. Probably fair to say that moderate to high CAPE with strong deep-layer shear is not exactly rare. These conditions are favourable for supercells with very large hail. The low-level shear is however, usually quite weak and LCL's too high which probably explains the lack of tornadic storms.

I thought I would bring up the September 2009 duststorm too since that was quite an interesting event. Here is the Woomera sounding-

http://soundings.bsch.au.com/skew-t.html?source=wyoming&lat=-31.1558&lon=136.8054&gribdate=&month=09&day=21&year=2009&hour=00&window=on

The wind profile on this sounding is quite amazing with an 80 knot NW'y at 500 mbar and a 50 knot NNW'ly at 850 mbar. Mid-level lapse rates are also very steep (though the 300 mbar temp is a bit warm in this sounding). The biggest problem though is the low-level moisture. The surface dew points were only in the low teens that day in the warm sector so instead of a tornado outbreak we had a dust storm.

[Michael Bath]

I think that point has been raised in the past a few times. We do see the strong shear from time to time - with decent lapse rates, but those setups are typically always bushfire risk days rather than thunderstorms.

[Jimmy Deguara]

Michael Thomas,

You mention of the parameters being sketchy for the day. What do you feel was lacking in this storm outbreak?

I think your previous post above cover things well. I meant to suggest that I was confirming the difficulty to gather sufficient information to achieve a sufficient profile for the time the storm occurred. Was there an increase in the windshear profile. Given the dynamics, were the old profiles and 're-analysis accurate' for so long ago given the lack of data.

I guess what I look for are:

- instability and steep lapse rates. The delta H7-H5 temperature difference > 19C is a rule of thumb for violent tornadoes. Was this the case on this day?

- inflow - one would require to know the pre-storm environment but at least the low level profile was favourably directional

- mid-level moisture profile being dry to enhance clean structure and less upper level loading in terms of precipitation. This should overly deep moisture to 700hPa. Together, with upper level windshear can lead to classic supercell profiles.

- existance of boundaries can assist and this region particularly further south extending to the region west of Bulahdelah have natural boundaries. Were there other outflow boundaries on the day

- upper trough short wave passing through to break a lower level cap

Seriosuly though, the lower level wind shear is the most common missing ingredient that differs between the US and Australia regularly. I feel that deep moisture can also be an issue - the lack of dryline dynamics also tends to be present here.

Whenever we have had ingredients in place, tornadoes occur. Population density or not, someone sees a tornado under the intense storms on the day.

Regards,

Jimmy Deguara