Author Topic: Mythbusters: Northwest Slopes and Plains - a tornado alley?  (Read 7354 times)

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Offline David C

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Mythbusters: Northwest Slopes and Plains - a tornado alley?
« on: 08 November 2007, 10:43:49 AM »
I got thinking about this while reading the thread on Doppler at Tamworth vs. Grafton.

In 1997, I laid my hands on a book "Willies Willies and cockeyed Bobs - tornadoes in Australia" ( a book which, ironically, I lost on my way back to Australia after seeing my first tornado in 2001!) the information in it suggested to me the presence of a great tornado hinterland west of the northern and central tablelands of NSW.

If any of you have looked through the NSW severe storms database, there does appear be a disproportionately high number of tornadoes reported from the NW Slopes and Plains region, some of which were evidently quite destructive. Even in recent times (~2005), a substantial tornado was reported to cross the Oxley HWY near Coonabarabran. In 1996 there were 3 tornadoes reported from the general area. In 1999, Jimmy and Michael saw what looked like a funnel cloud to the ground through rain. Jimmy chased a storm with a rotating wall cloud and low base near Coonabarabran in Dec 2005. There are localised, narrow swathes of tree damage through the Pilliga scrub (so I am told). The region sits in a good location to benefit from the typical upper level system. Big storms are part and parcel of life out there. But tornadic supercells?

I must say though that over the last 8 or so years that we have had good access to computer models, I have not seen many situations that would support the presence of any 'tornado alley'. Most notably, there does appear to be issues with T/Td spreads such that on many days tornadoes are effectively ruled out. Good 0-3 km shear seems lacking and deep N-NE flow is a rare event on storm days.

So, are these 'tornado' tracks through the scrub the result of dry microbursts (we know there are plenty of those out there!) and/or'
Are the eyewitnesses reporting 'scudnadoes'? or;
Is the accumulation, in the database, of genuine tornado reports over time the result of 'freak' events that occur only rarely
Or is our meteorological vision of this part of Australia, and indeed much of eastern Australia, tainted by what has been a dry climate cycle over the last decade?


Personally, I suspect the latter is at least partly responsible. I always say that Sydney's climate, well over the last decade or so is drier and less stormy  then a 10-year period that you might pluck out of the 70s and 80s. Whether such wetter phases translates to more tornadoes or simply more rain events / thundery weather, I dont know. It is also worth noting that the traditional chase areas of the US southern Great Plains appear to be going through a quiet phase (many veteran chasers will tell you this with the halcyon days being in the 70s and 80s). I doubt these observations are simply the case of nostalgia. 
Let's hope whatever factors drive such climate cycles swing back to our preferred direction and we enter a wetter, longer-term cycle in our climate (and for more important reasons than seeing tornadoes  of course). It would be an interesting period to chase the NSW slopes.
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Offline Rhett Blanch

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Re: Mythbusters: Northwest Slopes and Plains - a tornado alley?
« Reply #1 on: 08 November 2007, 02:48:49 PM »
Hi David,
Interesting question you pose - I've read the book your referring to. I think the dry climate cycle has a bit to answer for in not producing the right conditions in the "model period".  I think the area can produce the right conditions, but even so I wouldn't say that it would do so frequently, even outside a dry climate cycle.  I've been through many of the Tamworth papers and there seems to be about a 10 year return time events that could probably be called tornadic in the areas immediately surrounding Tamworth up until the 70's, some reports below (I must get around to putting the others up on day):

http://www.wilgatree.com/reporting/19321008_klori.htm
http://www.wilgatree.com/reporting/19140109_tamworth.htm

Reports get a bit sparse after that which is odd as I would expect that reporting rate would have actually improved. Certainly there have been major storm events, predominately large hail and significant straight line winds - the area is VERY VERY good for that.  But not that many appear to be tornadic, so i dare say calling the North West Slopes and Planes a tornado alley is a bit much.  Spectacular anvil crawling severe storm paradise on occasion, but not a tornado alley.

Offline Jimmy Deguara

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Re: Mythbusters: Northwest Slopes and Plains - a tornado alley?
« Reply #2 on: 08 November 2007, 02:58:36 PM »
David,

You hit the nail on the head with your reference to lack of moisture. When moisture does eject inland, the northwest slopes and plains with access to intense heat can become unstable over a vast area. Given the climatological positioning of cut off lows fitting favourably and perfectly in New South Wales at times, this region often is in the firing line for progressing negative tilt short wave troughs. The tendency therefore is to allow for rapid destabilisation releasing such energy explosively into the atmosphere.

One key ingredient in this region is the low level trajectories: a low level jet often develops during the early evening with a nice deep and moist northeasterly airflow. Also this region does not succumb as regularly to the dreaded dry hot northwesterly airflow. This was certainly the case on the 20th January 2005 tornadic supercell event near Coonabarabran David refers to in his post above.

I feel this region deserves further investigation as given its distance from the main population areas, many of the storm events are often just a little out of reach for the typical one day drive wonders.

Regards,

Jimmy Deguara
« Last Edit: 09 November 2007, 02:46:33 AM by Michael Bath »
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Offline Jimmy Deguara

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Re: Mythbusters: Northwest Slopes and Plains - a tornado alley?
« Reply #3 on: 08 November 2007, 03:14:42 PM »
Rhett,

In relation to the "Tornado Alley" comment - I guess it is all relative to what is experienced here and the United States. Obviously much less. What one must consider is the frequency of tornadoes anticipated / reported in other parts of the country as compared to this region. Well the sparseness and the visual opportunities can create issues in the region amongst the encrouching Pilliga scrub or should I say forest! Remember this region did host a nice little outbreak not so long ago -29th September 1996 Bearbung and -29th September 1996 Bundella when at least 3 tornadoes were reported.

As David suggests, the lengthy drought surely must have had a significant role in the characteristics of storms observed over the past one or two decades as compared to what may have occurred in the past. Remember, there really is a fine line between the tendency to outflow dominant supercells and tornadic supercells. A simple well oriented boundary can take care of that (as we saw in the Dunoon case where the supercell was not that impressive by any means). And this region being relatively flat particularly westward can certainly produce favourable and uninterrupted boundaries seemlessly. Satellite imagery in the past certainly show evidence of this.

I guess time will tell - but this region may be over due for a series of significant events.

Regards,

Jimmy Deguara
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Offline David C

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Re: Mythbusters: Northwest Slopes and Plains - a tornado alley?
« Reply #4 on: 09 November 2007, 03:57:53 AM »
Hi Rhett, Jimmy.

Thanks for posting those links Rhett - good to get a "expat's" perspective :)

Yes, just to clarify, there is only one Tornado Alley - not in any way suggesting we have a Great Plains in our backyard! I'm referring to the NW Slopes and Plains as a tornado alley to the extent that is may be a region in Australia where tornadoes occur relatively frequently - where meteorological conditions are most favourable. It must be remembered that it is a large area and quite diverse in many respects. Compare say Inverell / Elsemore (F2 / F3 tornado in 1989), which is on the eastern extreme of the region with somewhere like Coonamble out there on the plains.

Good points Jimmy regarding the predominantly nocturnal low-level jet that can develop out there, it is a somewhat regular feature during the warm season. The thing is, such a jet is only as good as the moisture it brings in, and this related back to climate cycles. The other issue is that without a reliable EML, convection does tend to break out during the afternoon over here on your typical high CAPE day, and things can stabilise or become cluttered by the time the low-level flow is cranking. You really need convection to be isolated and then take in the low-level jet (LLJ) as the temperature cools. Of course you need a theta-rich LLJ strengthening to at least maintain dewpoints / boundary layer moisture in general -- in this situation you have several things working together for tornadogenesis (ie lower LCL and more favourable kinematic fields).
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Offline David Brodrick

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Re: Mythbusters: Northwest Slopes and Plains - a tornado alley?
« Reply #5 on: 09 November 2007, 08:51:58 AM »
Rob McNaught passed some research on to me which turned up at least three likely tornadoes that have come through Narrabri township over the last 125 years. These were events were listed as tornadoes in the BOMs record and/or historical news clippings and exclude microbursts and other destructive events such as "cyclones" which were reported more frequently in the records.

1892 Nov 13
1902 Jan 07 "The storm in all its fury burst upon us, first a rushing wind, the sound of which could be heard for miles, then a hail storm and heavy rain"
1944 Feb "A terrifying tornado ripped through town..Within minutes trees were uprooted, tank stands blown away and fences were flattened"

There were also records of tornadoes in Burren Junction and other nearby locations (Merah North?). In addition to the events already mentioned in this thread, a tornado was reported 20km from Moree fairly recently (late 1990's?).

I haven't read the book and don't have any information about other areas in Australia so I'm not sure about the relative frequency here and elsewhere, but there are certainly a good number of tornadoes on the records out this way.

It is very informative and interesting to read all your comments in this thread... gets me excited :-)
Cheers,
  Dave

Edit: While it may or may not have been a tornado I couldn't help but relate a 'candidate' people have told me about. In January 1990 intense winds struck the radio observatory out of Narrabri. The path of destruction was very narrow but extended for ~10km. The only person on the site at the time hid in a bathtub she was so worried by the storm, so we'll never know whether or not there was a funnel. Radio telescope antennas which had been standing for 30 years were blown to pieces in the wind. There was massive damage to large trees. But I think the best part of the story is that the anenometer was blown from its mount and landed 200m away! Unfortunately we no longer have the records of what the wind peaked at before it blew down.
« Last Edit: 09 November 2007, 09:32:00 AM by David Brodrick »

Offline Michael Bath

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Re: Mythbusters: Northwest Slopes and Plains - a tornado alley?
« Reply #6 on: 09 November 2007, 11:42:56 AM »
In addition to the events already mentioned in this thread, a tornado was reported 20km from Moree fairly recently (late 1990's?).

I found 20th November 1995 in the database. Event occurred at 0150Z (12.50pm local) 20km east of Moree - no damage reported.
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Offline Jimmy Deguara

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Re: Mythbusters: Northwest Slopes and Plains - a tornado alley?
« Reply #7 on: 10 November 2007, 02:09:58 PM »
David B. et al,

Not to lesson the importance of the researched findings with regards to the observations you listed. However, we do have to consider that at least beyond say 30 years or so, the ocurrences of microbursts surely must be incorrectly reported as tornadoes. So in some cases, unless a funnel/tornado has been reported, we have to carefully analyse the type of damage reported as well as the associated observations before concluding such. I guess we have to approach reports from the past carefully.

In general, you would rate each event on a confidence scale say 1 to 5 as to being a tornado against the prospect of being a microburst.

Regards,

Jimmy Deguara
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