Storm Australian Severe Weather Forum
Severe Weather Discussion => General Weather - all topics not current severe weather. => Topic started by: Carlos E on 01 November 2007, 04:04:17 AM
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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.
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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
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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.
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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!
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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. >_<
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http://www.bom.gov.au/weather/nsw/sevwx/windfact.shtml take a look for some BoM info with a NSW emphasis
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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!!
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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
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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."
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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...
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Interactive radar loops of the event:
Gympie 128km (http://australiasevereweather.com/storm_news/2007/radar/20071029/gympie128.htm) / Gympie 256km (http://australiasevereweather.com/storm_news/2007/radar/20071029/gympie256.htm)
Pity there are some missing images in the archives.
MB
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Intersting the change of direction as it got towards Gayndah.
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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.
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May I ask, are you from up that way Carlos?
Regards,
Dave
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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
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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.
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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
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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
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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.
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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
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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.