Topic: Questions about the Gayndah Thunderstorm 29/10/2007

[David C]

Have to say I don't see anything resembling what I would consider a hook echo in there. In fact, other than the period between frames 4 and 5 (one scan was not taken in between) it looks pretty ugly and looks to be forming a bow echo as it hits town. I don't find a spotter report of a 'two funnels' all that compelling either.

[Michael Thomas]

Interesting storm I believe. Just a quick point, I thought that a tornadic supercell was a supercell that produced a confirmed tornado. How can you tell if a supercell is capable of producing a tornado if it doesn't produce one (or a least a confirmed tornado)? I think that we must be careful in analysing radar images, even the presence of a hook does not imply that a storm is capable of producing a tornado. Anyway, I'd be interested in knowing more about bow echos eg. what causes a storm to transition into a bow echo?

Michael

[Jimmy Deguara]

Hi,

Has a damage survey been conducted - I mean the area in question is not that far from Brisbane. If this is such a significant event and that the radar had significant features as described in Jonty's statements - I would have thought that a damage survey is in order even to verify one way or another.

We must be careful not move off track here in regards to hook echos and the definition of tornado warned storms. My impression from US warnings is that Doppler radar picks up "tornadoes" but not always is not known if the tornado is on the ground. Now the definition of tornado is a funnel connecting a cloud base with the ground. So this often confuses me in warnings but I am now used to it.

Regards,

Jimmy Deguara

[Jeff Brislane]

For winds of 180kmh the report from Jonty Hall of a bow echo with rear inflow jet makes the most sense imo. For those who don't understand what a rear inflow jet is, to put it simply, when you get a bow echo from a High Precipitation storm you can sometimes get a situation where the jet stream at high altitude is forced down to ground level and that is where you get the 180kmh winds from, literally the jet stream comes into contact with the surface at the middle of the bow on the radar. You then get vortices on both sides of the both which can produce tornadoes.

As for the discussion about the type of microburst which it probably isn't, it would have been a wet microburst not a dry one seeing as it was a high precipitation storm.

Jeff.

[Macca]

Hi all,

Thought I'd keep this one going for a little while longer.  When I first saw the AWS with the 180km/h gusts, I was automatically skeptical about the possibility of a tornado.  I analysed the wind speeds and directions leading up to the maximum gust and it just didn't make sense that a "normal" tornado had passed over/close to the AWS.  This was using the assumption (without looking at radar at all) that it was a supercell and that the "normal" tornadogenesis mechanisms were in place.

Following Jonty's comments regarding the bow echo and the reports of funnel clouds, the wind directions and speeds recorded by the AWS may actually support an anti-cyclonic bow echo tornado.  (Anti-cyclonic as Gayndah was on the northern end of the bow which favours anti-cyclonic rotation).  I eagerly await further confirmation from the BoM.

Something else mentioned by Jonty was that the rainfall as indicated by the publically available data from the AWS was not (as) accurate and that the more detailed data received by the BoM showed that 37mm fell in just 5 minutes and 74mm in just 10 minutes!  If this is the case, I think this may break some Australian records for rainfall rates.

Macca

[Carlos E]

Hi again.

I found some footage of a storm from the same day that went through Emerald (kill me if someone has already posted this).

http://www.youtube.com/watch?v=2LdexvwySBA&feature=related

That storm had gusts to 140km/h, and was on the same day the Gayndah one occurred, so I would assume the storm formation was similar.