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

[Carlos E]

Hi all, this is my first (well actually my second post) here.

My question is in regards to storm which brought Gayndah some powerful 180km/h winds, as well as 70mm of rain in a vert short period.

I'm going to C&P the email.

The other day, on the 29th, I noticed that a SuperCell developed, and went near the town of Gayndah, I checked the weather observations on the BOM site after this, and it said the town had just recorded wind gusts of 180km/h (which is scary, Category 3 Cyclone matching winds). My question is how on earth did the storm become that powerful, because from what I saw on the radar images, it wasn't actually at it's peak (far from it) when it struck that area, it was a massive blackish type storm before that, and while it was still intense, it was way more intense beforehand. There was also another storm that had tremendous winds, at Emerald I believe. I also notice that the BOM put up the observation on an updated Severe Thunderstorm Warning.

My question is this: Is it possible that this storm, was associated with a strong Tornado? I'm aware that Tornadoes have funnels, and the winds inside of them do not normally extend that far (certainly not as far as Cyclones), but I find it ridiculous that it was able to summon that powerful of winds, as well as dump 70mm on the area within about 40 minutes, and it did this AFTER it had weakened.

Sorry, just a little bit fascinated by this. Any response and opinion with regard to this storm will be greatly appreciated (the Emerald one is also pretty fascinating).

Many thanks.

[David Brodrick]

Hi Carlos,
In Jan 2005 we had a storm here in Narrabri with 150km/h winds, some hail, almost no rain however.

There's some info about it here if you are interested:
http://narrabriweather.net/events/20Jan2005.html

The winds here were caused by a microburst which is a localised downdraft of cold air descending from a storm. When the downdraft hits the ground the wind spreads out and can reach speeds >200km/h.

I've read that these kind of events generally happen on days when there is very dry air at higher altitudes.

Maybe the Gayndah winds were also caused by some kind of downburst? Just a guess..
Regards,
  Dave

[Carlos E]

The winds here were caused by a microburst which is a localised downdraft of cold air descending from a storm. When the downdraft hits the ground the wind spreads out and can reach speeds >200km/h.

I've read that these kind of events generally happen on days when there is very dry air at higher altitudes.

Maybe the Gayndah winds were also caused by some kind of downburst? Just a guess..
Regards,
  Dave

Yeah, I have done a little reading on that as well. I'm still pretty new with some weather types (although the majors like Tornadoes are of course the first thing that come into my mind), I looked around, and you're probably correct, that it was a downburst. I looked at the video in the link that you provided as well, that's scary, it looks like the town is having a Cyclone come off the sea, but the actual town is no where near the tropics.

Very impressive.

[David C]

180km/h straight line winds (resides in the upper F1 on the Fujita scale) can cause substantial damage, but such winds are certainly not unheard of with severe thunderstorm straight-line winds, as per example David noted at Narrabri. We had 170km/h + gusts with elevated severe storms some years back at Richmond, and there have been many other such events over the years. Most notably, the 1991 northern Sydney hailstorm produced high-end F2 damage (winds were estimated at 230km/h) and this almost certainly due to a microburst.

Rain rates of 70mm / 40 minutes etc are also impressive enough but certainly not extraordinary. The Jan 91 storm which caused massive damage across northern Sydney recorded 35mm of rain in only 6 minutes!

Was this event a strong tornado? Maybe, but most probably not? Just had a look at Gympie radar and it looks like a messy HP storm. At the end of the day, there will probably be a damage assessment that will determine the type and extent of damage. This would also take into consideration other evidence, such as public reports of a tornado from the area at the time. We'll wait and see!

[Carlos E]

I'm now obsessed with Microbursts as well (dunno if that's good or bad). I'm trying to find the strongest Australian microburst; I found some for the US that were as strong, and the strongest was at Andrews Air Force base (240km/h!). Some of the US ones that were listed as notable were also considerably weaker, so if something like this ever happened over Brisbane or Sydney (which I believe could both have the conditions favourable for it to occur), it would be messy.

I'm guessing this one measures up with the strongest of Australian ones. And David C, 230km/h in northen sydney? Sheesh.... and 35mm in 6 minutes? That's like a bloody waterfall. >_<

[David C]

http://www.bom.gov.au/weather/nsw/sevwx/windfact.shtml  take a look for some BoM info with a NSW emphasis

[David Brodrick]

Im not entirely sure dave where you heard about the dry air at high altitudes. I was pretty sure it was more related to the dryness of the air between the base of the storm and the ground together with fairly cold air aloft.

I had to resort to textbooks for this.. I have a copy of "Microbursts, A Handbook for Visual Identification" by Caracena, Holle and Doswell on loan from Rob McNaught.

After some grokking, I think the answer is that the situation you describe with dry air below the storm is the environment for 'dry microbursts', while the 'wet microburst' environment has the dry layer of air at high altitudes.

Given the 80mm in an hour at Gayndah I think it is in the wet microburst category :-)

To quoue from the book:

The wet microburst environment is marked by a deep, nearly saturated layer with a nearly moist pseudoadiabatic lapse rate that is topped by an elevated dry layer. The equivalent potential temperature of the dry layer is cold enough and the layer is sufficiently high above the surface that, when it is reduced to its wet bulb temperature by saturation and mixed (in equal parts) with the warm updraft, there is still enough negative bouyant potential energy to create a severe downdraft through the sinking of such a mixed parcel to the surface.

I've attached the GFS sounding for Gayndah near the time of the microburst. It is characterised by almost saturated air to about 500mb and then a dry layer as the text suggests.

Very interesting!!

[Jimmy Deguara]

The tendency to a tornado report most likely stems from the media publicity from the Dunoon tornado in the previous days. Microbursts wet or dry are far more prevalent in this country than tornadoes.

Regards,

Jimmy Deguara

[Michael Bath]

Two quotes from Jonty Hall, severe weather meteorologist at the Queensland Bureau of Meteorology :

"We have visual reports from a storm spotter in the Gayndah area of two funnels on the ground - not sure of details, but seems to be pretty solid evidence."

"The radar data shows pretty clearly that the Gayndah storm had evolved into a bow echo as it approached the town, the weak echo channel into the rear of the storm is quite prominent at low to mid levels, clear evidence of the development of the rear inflow jet, with tornadogenesis likely occurring as this descended to the ground."

[David Brodrick]

Wow that's amazing. I didn't notice it before but the ABC News article quotes residents describing it as a "mini-tornado", he he.

http://www.abc.com.au/news/stories/2007/10/30/2074296.htm

The skew-t I posted shows great low-level shear.. but WOW! Surely someone must have snapped a few photos...

[Michael Bath]

Interactive radar loops of the event:

Gympie 128km / Gympie 256km

Pity there are some missing images in the archives.
MB

[Richary]

Intersting the change of direction as it got towards Gayndah.

[Carlos E]

Pity there are some missing images in the archives.


I don't think anything annoys me more than a storm that is very good to watch on radar, then the radar just "does nothing". It did it with that storm and I felt like crying.

Although, I shouldn't complain, it did look very impressive regardless, and I believe the fact it weakened a little bit before Gayndah does suggest a Microburst, but still curious about the Tornado.

Thanks for the images, I lost the ones I saved.

[David Brodrick]

May I ask, are you from up that way Carlos?

Regards,
  Dave

[Jimmy Deguara]

John,

In previous posts:

Strictly I don't believe that this storm had weakened, rather the radar signal was providing a false image. Black precipitation rates can indicate far higher precipitation than 70mm in 40mins(though this is rare), so im guessing this was the case.

and

Very interesting dave, thanks for pulling that one up. I see now exactly how it works, identical theory to that I aforementioned: Throw hailstones and ice into the dry layer within the cloud, generates cooling by absorbtion of heat to melt the crystals, results in a moist body of cold air rapidly descending, so I guess the case i described is that for the classic dry microburst.

I assume you were leaning towards microburst here, yes?

Then recently?

Thanks for posting those loops Michael, great as always. Amazing storm scale rotation there and definitely some evidence to suggest that a tornado may have occured. While the archive is not complete the remnants of a hook echo look to be present in at least one frame. Just goes to show you that tornadoes arent as rare as they seem then i suppose.

Are you leaning towards tornado now? How did you detect storm scale rotation from this conventional radar imagery?

These two funnels all the way to the ground certainly does not conclusively - were they scud formations. I mean a rear inflow cutting into a bow echo certainly does not provide easy visual opportunities within an HP supercell?

Thoughts John or anyone?

Regards,

Jimmy Deguara