Topic: Sydney / Illawarra Storms: 29 March 2008

[TroyVR]

absolutely brilliant work for a first time Michael, What kind of camera do you use?

[Richary]

One I forgot to mention was early on Saturday morning. I had to work which is not usual for a Saturday, and woke about 6:15am to see a nice thunderhead out to the east (well off the coast looking at the radar). It was lighting up quite nicely inside against the dawn.

[David C]

Hi,

Here is at least 15 minutes of the storms life cycle in timelapse at 40 times speed. People can judge for themselves as to whether there is a sustained deep rotating updraft:

 Afternoon Sydney Storm timelapse 29th March 2008

This is the Camden LP supercell 2003 as a comparison from a slightly different perspective:

Camden LP supercell 12th February 2003

I would also like to draw attention to this cell from September 19th 2004

http://www.australiasevereweather.com/video/stills/2004/20040919.html


Regards,

Jimmy Deguara

There was definitely rotation in the updraft - I could see it at the time quite clearly and mentioned this to Paul who was on a shift at TWC at the time. As to whether this rotation was sufficiently deep, persistent and of sufficient magnitude to be considered a supercell by definition....who knows.

I saw lightning from the cell down near Wollongong as I drove home. What amazed me, it seemed little more than  a towering cumulus as it put out the first CC flashes after sunset. Spectacular stuff!

[Jimmy Deguara]

David,

You called it an LP storm and that was my point of call. Definitely not a severe storm and the updrafts were not persistent in my opinion.

Regards,

Jimmy Deguara

[David C]

Hi guys,
Just going to weigh in on this issue, im with you Jimmy, definitely not a true supercell: my reasoning for this is simple:
1. Weak updraft that was almost like one of those US 'mini' supercells in nature, no real anvil or depth to convection.
2. Rotation is slight enough that im going to say there was sufficient low level shear to induce some organisation of the updraft. Definitely insufficient deep shear both visible and sounding base: any sort of decent shear would pull a storm like that apart.

Definitely a LP type storm, in fact this is probably due to a moisture starved enviroment characterised by the base, and a distinct lack of energy in the updraft....just not severe or supercellular: very photogenic though.

Hi John, Jimmy:

John, correct me if happen to be misinterpreting you re. point 1  -- this would seem a contradiction of sorts IF you are arguing, by inductive reasoning, to the extent that the similarity of this storm, visually, with 'anvil-less' mini-supercells (which I assuming that you don't consider a true supercell?) can be used to support the argument that this storm is not a supercell.

Mini-supercell:
http://frozone.itsc.uah.edu:8080/LEAD_Glossary/m.jsp >>
"Mini supercell- Convective storm that contains similar radar characteristics to those of a supercell (e.g., hook echo, WER, BWER), but is significantly smaller in height and width. The diameter of the radar detected rotation is 1 to 8 km. This is a relatively new storm type, the existence of which has been confirmed by data from the recently installed WSR 88D radars in the United States. Mini supercells occur in areas where the height of the equilibrium level is low, most often in the northern United States, but possibly under certain weather conditions in any area of the world. They are sometimes found in landfalling tropical cyclones."

If we take the above definition, which is by no means set in stone, then these are 'mini-supercells' or 'low-topped' supercell storms are indeed a special class of supercell storm. 

Jimmy - I am assuming that you consider low-topped supercells as true supercells based on the following quote taken from: http://www.australiasevereweather.com/forum/index.php?topic=421.20  ::

"24th April 2007 Low topped supercell develops on triple point in central Kansas producing two tornadoes - (one of the tornadoes produced multiple spin ups)."

In that case what are the differences between this event and the following events, that allows you to draw a line somewhere in between?
 
eg2:
Sculptured Low Precipitation Supercell NSW North Coast: Monday 20th October 2003:
http://australiasevereweather.com/storm_news/2003/docs/200310-04.htm

eg3:
The Cut-off Low September 1st 2001 - What a start to Spring: confirmed LP Supercell April 2002:
http://australiasevereweather.com/storm_news/2001/docs/200109-02.htm (yes, this produced 3 cm hail)

I think we have to be careful when we start imposing somewhat arbitrary boundaries on what is in essence a natural continuum of events. My own opinion is that there is no question that this storm had rotation. It did not appear terribly deep although i can see form your time-lapse that there is rotation in the updraft. I doubt it was persistent enough (20 minutes or 30 minutes) by the generally accepted criteria since the parent updrafts were not long lived. That is why i would consider this Sydney storm the be a rotating storm but not a supercell by popular definition. I also doubt that some of the above storms, esp October 20, would meet these criteria either.

What does everyone else think?

[David C]

What I was suggesting by point 1 was not in fact a contradiction, rather a statement as to a possible explanation for the appearance of this storm. Bluntly i think in no way we can be definitive on these things: IF and only IF this storm did display rotation then a possible classification could be as a low-topped 'minature' supercell.

As for my feel on this matter, I dont believe that it was very strong in any case, and I believe this to be a symptom of moisture starvation and a lack of deep layered shear.

I think you are clutching at straws in order to draw comparison to the the 20th of October storm, clearly this storm had very evident rotation, was highly organised and sculpted, together with displaying a deep convective updraft and anvil. To suggest this was not a supercell is quite bold, given the obvious rotation and base structure together with a rotating updraft that infer beyond doubt that this was a supercell. I believe you may have confused this storm with the cut-off low event, which was far weaker and less supercellular in appearance

The second storm however I can see the similarities which you have identified. Rotation in this current case appears to be low level, somewhat synonomous with this cut-off low event. Consideration of the synoptic condition may lead to explanation, I have a feeling that this may explain this low-topped nature evident in both storms.

Rotation of updraft has been dismissed often and regularly particular for any storm occuring to the south of the border, and I actually find the weak rotation and structure present to be less than that visible in at least two storms i have chased down here, even though they were dismissed on such grounds.

An important thing to remember here is that criteria are just that: criteria arbitrarily placed on a storm. There is no way to actually assess these systems unless they are subjected to dopplar radar scans to determine the presence of updrafts, and hence we can only comment on the similarity.

I apologise if there was ranting in this post, I always get annoyed with the seeming opinion that nothing decent happens south of the border (Well when is the last time anyone looked at the frequency of tornadoes and supercells in the region near Bendigo?), and find it hard to not be sceptical of systems that are margnial at best to the north.

There are no arguments about what happens south of the border! I suspect the prolonged drought and dry conditions over the last decade or so in these parts don't reflect what might happens during periods where moisture return is better, and given the not infrequent strong baroclinic systems that impact the region. But that's another story.

On supercells, I am always cautious in labeling a storm a supercell (and unless there are several lines of circumstantial evidence I am a nay-sayer). I labeled this as an LP storm, As I posted in this case. I also think there was rotation. In this case, without knowing for sure, I don't think the updrafts were sufficiently long-lived and, with less certainty, I don't think, based on the timelapse, that rotation was deep enough either. I also think the storms in the links I posted above are also questionable as supercells for various reasons and to varying extents, depending on how you strictly you apply the definition. If you start relaxing criteria for rotating storms (ie rotational velocity and / or depth and / or duration) at what point do you stop? In terms of operational forecasting, that line is drawn taking into account hazardous weather.

Just on those links, and me ''clutching at straws", I suggest you have a good, long look at that video from Oct 20. I am very skeptical of the 20th of October! Shallow rotation in the 'sculpted' base displaced by a seemingly large distance from the parent storm (and I suspect independent of it with a possible triple point producing a shallow circulation).

http://australiasevereweather.com/photography/photos/2003/1020rm02.jpg

This is not powerful looking convection to my eyes, I'm surprised 4cm hail was reported.

[Jimmy Deguara]

David,

Can you just clarify your points in response to the September 1 2001 event NOT being a supercell or doubtful to being a supercell? This was confirmed by the Bureau as a supercell and it produced hail to golf ball size - the only issue at the time - was it a classic supercell or LP supercell. The picture sent to me in my opinion with circular base suggest it was and high based as well. The updrafts did also have cork screw. The radar analysis also suggest deviation from the mean level flow as it neared the coast. Yes I know this could be the interaction with the sea breeze and backed winds but this would have improved the hodographs sufficiently and with strong 500hPa level flow and destabilisation, we are talking strong asruments that lead to high confidence of being a supercell.

With the 20th October event, at first I fell into the trap and I guess took the word of those who observed it. I can tell you, when I looked at the clear video, I could instantly see the updraft was relatively weak and did not clearly indicate persistent deep rotation - well insufficient to be labelled at high confidence as a supercell. Certainly there is reasoning as to why it should nto have been labelled as one.

As I indicated, the other similar structured storm that could be dubious and questionable as a supercell is this event:

http://www.australiasevereweather.com/video/stills/2004/20040919.html

Yes it had the classic supercell characteristics but then as I drew near, it seemed the structure was not as interesting as first thought - I recall David you thought the same initially until close examination.

On the subject of rotation, take a look at this example of a rotating cumulus:

http://www.australiasevereweather.com/photography/photos/2001/jd20011223.html

This cumulus could not break the cap unfortunately and I think drier air mixing did not assist. But I can verify the large cumulus did rotate for some time perhaps up to an hour! So level hodographs were favourable for low level rotation -  the upper levels did not complete the puzzle I guess.

The 29th March 2008 event showed low level rotation but insufficient rotation to be quantified as a supercell which has already been labelled on another forum. So I agree with your point, scientifically, we must be careful in making claims that:

a) are not backed up with reasoning
b) that can confuse readers of the reports that label other storms incorrectly

Of course in saying this, there is a lot of skill and experience with identifying things 'visually'. And we know that really, the true verification of such systems are doppler radar and other conventional radar characteristics in the absense of doppler.

What we are trying to achieve here in the forum is not creating discomfort for those who observed storms and 'down grading' its significance - just trying to promote accuracy and correct reporting prcedures. As I have suggested in the past using words such as possible and probable introduces the probabilistic components in terms of confidence for whatever reasoning it may have been attributed.

The storms in all cases above were spectacular and very interesting and all developed and traversed along boundaries. Just because they may not quantify within the acceptable criteria to be supercells does not make them less important. There seems to be this prestige attributed to success in intercepting supercells as compared to other storm types. I can tell you a well structured multicell can easily be more appealing than an outflow dominant supercell but that is besides the point.

Regards,

Jimmy Deguara

[David C]

David,

Can you just clarify your points in response to the September 1 2001 event NOT being a supercell or doubtful to being a supercell? This was confirmed by the Bureau as a supercell and it produced hail to golf ball size - the only issue at the time - was it a classic supercell or LP supercell. The picture sent to me in my opinion with circular base suggest it was and high based as well. The updrafts did also have cork screw. The radar analysis also suggest deviation from the mean level flow as it neared the coast. Yes I know this could be the interaction with the sea breeze and backed winds but this would have improved the hodographs sufficiently and with strong 500hPa level flow and destabilisation, we are talking strong asruments that lead to high confidence of being a supercell.

With the 20th October event, at first I fell into the trap and I guess took the word of those who observed it.

Hi Jimmy

Let's backtrack slightly. I think we are in agreement that due to the short-lived nature of each updraft, this storm, Saturday's storm was almost certainly not a supercell proper. I am of the opinion that it may have been a mini-supercell, depending on how that is defined given the small scale of the updraft and duration of the rotation. I don't think one can be supremely confident of supercell status in the absence of what one might call major criteria (well, only one and that is doppler radar) or at least several minor criteria (eg visual, storm motion in some cases, base reflectivity and RHI morhphology, storm environment etc).

On top of that we relax the typical mesocylcone recognition criteria for a special class (or the small end of the size spectrum) of rotating storms known as mini-supercells. How deep does the rotation have to be, or for how long.

With this said going back to September 1. I do in fact suspect that this storm was a supercell but I am not certain of it. You list some of the 'minor criteria' that support the idea of this being an LP supercell (I used this example to illustrate anvil structure relating to Johns original post).

The updraft is fairly emaciated looking to me in this pic and the storm looks shallow: http://australiasevereweather.com/photography/photos/2001/0901bw03.jpg
although the towers look nice and solid from the back. Also  http://australiasevereweather.com/photography/photos/2001/0901jd11.jpg
Where's the anvil? Yes there was an anvil, as there was on Saturday, not evident due to the updraft structure and flanking line. I'd dispute any BoM confirmation of a mesocyclone at this distance from Kurnell too?. Also, corkscrew is synonymous with barber pole and usually refers to the striated appearance of an updraft. The shots taken in high contrast show no such striations - I don't see any evidence of a corkscrew. At the end we are left with somewhat favourable environment, a rain free base, an unusual rain free structure the first of its kind seen in Australia, billowing towers and hail maybe to golfball size as criteria to claim this as a supercell. I don't think this is a clear cut example of a supercell, although I'll agree that the weight of evidence suggests that it probably was (quite long lived).

Regarding October 20, we are in agreement, although clearly that is a nice flank and there was hail to golfball size reported.

One can look at this from, as I said previously, an operational forecast perspective - ie it was too short loved to produce severe weather of significance (note the BoM put a warning probably due to forecast instability rather than what they were seeing on radar, interesting nonetheless). Or look at it from the science and consider why such a small storm like this (and another HP version that you mentioned) produce features commonly seen in supercells.

[Jimmy Deguara]

David,

How can you say the storm on the 29th March was short lived?

It lasted about 2 hours? The updrafts would go up for about 20 minutes or so and fuzz out in my opinion and then other towers would go up.

In regard to the 1st September 2001, I clearly disagree strongly. You failed to indicate the deviatiton I refer to. The core on radar of this cell shows a reasonable core with persistent intensity. I think the Bureau did pick up specific features including radar characteristics at the time and the wind shear profile as well as the rapid growth in such a stong wind shear profile is more suggestive of a far more powerful system than the one on Saturday.

You mentione the first picture:

http://australiasevereweather.com/photography/photos/2001/0901bw03.jpg

This was well to the west before the storm really took hold and exploded much closer than that

http://australiasevereweather.com/photography/photos/2001/0901bw01.jpg

http://australiasevereweather.com/photography/photos/2001/0901bw02.jpg

Regards,

Jimmy Deguara

[Macca]

Interesting discussion guys.  I'd really like to put some time into a decent reply but its nearly midnight and I have a 6am start in the morning.  Just to note, I was in Sydney on Saturday and watched this storm from birth to death (from Parramatta Lake and then just a few km up the road for the sunset show).

Two things about the storm on Saturday.

1.  The timelapse video you posted Jimmy shows the initial strong updraft tilting WAY over as the second big pulse starts to go up but I think you can still align the intial strong updraft with the base. Sure it is tilted at a 45 degree angle but it is still attached, thus it may've actually lasted longer than 20 mins (although the video starts some time into the mature phase of this updraft so how long the intial part went for is a little unknown).  This is partly owing to the base being near stationary but the upper parts (top 10k ft) of the storm being dragged off by 50knt WNW'lys.  With 15knts in the low levels from the ENE and 25-35knts in the 600-400mb layer from the WNW, that puts shear in to the "sufficient" level for supercells and the turning of the winds in the lower levels is something we often only dream of here in Australia (shame we couldn't add 15-20ktns to each level).  The second updraft isn't too bad either (shows very similar characteristics to the first (to be expected since it occurred in the same environment).  Curiously, the base structure didn't seem to change overly much (although it was noticeable from a few km away where we were that it weakened) between the two main updraft pulses.  Interesting question...what difference is there between this (new updraft phase, little change to supercell-like base/features in the low levels) and a cyclical supercell?  Surely the updraft completely occludes in a cyclical supercell and a whole new updraft then takes over...does this not then classify it as a super multicell?   Does two main pulses which both exhibited rotating updrafts (and bases) then classify as cyclical?  How many cycles does it need to have?  (Opening cans of worms all over the shop here). 

2.  In terms of instability, there was not much wrong with those updrafts in my opinion.  They were pretty solid.  Sure its been a while since I have seen a storm (no jokes please...the last decent storm was way back in early November), but they were definitely crisp and quite explosive.  The sounding Michael Bath posted shows instability was quite good at 4z with LI's of -6C once the updrafts got above the capping inversion at 750mb.  Guestimating the sounding, I'd say CAPE was in the 1200-1500j/kg vicinity.  Certainly not massive CAPE numbers but not too bad either. 

If you put those parametres in front of me on the morning of a storm day, I'd be pretty happy about at least the chance of a severe storm/possible supercell.

In terms of classifying this event, I would have to sit on the fence.  I think it is borderline as to whether this was a supercell or not.  Rotation, yes.  Parametres favourable for supercell development, yes (albeit marginal but they were there).  The main question seems to come down to duration of rotation and updrafts.  I think the updraft may've persisted for long enough (and rotated for long enough) for it to get over the 20 minute line.  In terms of depth of rotation, hard to tell (really) from visual aspect.  How deep is deep enough?   Like I said, I think it was borderline.  It had several multi-cell characteristics but it also had some supercell characteristics. 

Truthfully, I was surprised the BoM had forecast "fine with a mix of sun and cloud" for Sydney for Saturday (even in their late morning update) as it looked like there was a pretty decent chance of storms even from first thing in the morning.  And then during the late morning and early afternoon, there was some intense convection under the capping inversion on the ranges with the odd pulse busting through the cap before the hole "sealed over" again and the updraft would die.  MesoLAPS and GFS were going for the surface winds to swing from NW'ly to NE'ly/E'ly between 3z and 6z (which they did) and drag in additional moisture (which it did).  Additionally, the upper trough sitting to the SW kicked a bit further N during Saturday, allowing the upper level temperatures to remain in the -19C vicinity (500mb).  This was also forecast by MesoLAPS and GFS.  To the trained eye (ie ours...hehe), it was at LEAST worth a "possible shower" in the forecast. 

Anyway - so much for the short post.  Interesting discussion.  Sometimes difficult to compare these events to others as there is always slightly different conditions to work with. 

Macca

[Jimmy Deguara]

Macca,

The question I now post is how high did this storm go given the temperatures were cold in the upper atmosphere. It just seems to reach the level where glaciation occurs and does not go further.

To me this is a low topped event by mid-latitude standards. As to the stength of the updrafts, they were going up perhaps rapidly for certain stage and then seem to tilt over rather quickly - not being able to go further. Crisp of course they were given the cooler temperatures and drier air aloft that would have enhanced some cooling with height. However, the updraft strength were nowhere near those that would allow such tilt in the wind shear environment. Remember the backing below would have kept the base stationary and winds moderate in strength - 20 to 30 knots at 500hPa.

Definitely this little storm got some deserved discussion.

Regards,

Jimmy Deguara

[Michael Bath]

Does anyone know if any hail or other severe weather was reported from this storm ? It certainly passed over a few suburbs so you'd expect something.

[Jimmy Deguara]

Michael,

Dave Nelson in his first post indicates that small hail did occur from the storm and he observed the storm for about 2 hour?

I had not doubt that from the updraft and structure, the storm had tiny hail but definitely was nowhere near the severe criteria. I could tell this from the structure. Had it had larger hail, I would have driven closer to it and under it no worries.

Regards,

Jimmy Deguara

[David C]

David,

How can you say the storm on the 29th March was short lived?

It lasted about 2 hours? The updrafts would go up for about 2o minutes or so and fuzz out in my opinion and then other towers would go up.

Jimmy Deguara

Hi Jimmy, correct me if I made an error I thought I made it clear that I was referring to updrafts (as in updraft pulses) not the storm as a multi-cell complex itself.

[Dave Nelson]

Michael,
Dave Nelson in his first post indicates that small hail did occur from the storm and he observed the storm for about 2 hour?
I had not doubt that from the updraft and structure, the storm had tiny hail but definitely was nowhere near the severe criteria. I could tell this from the structure. Had it had larger hail, I would have driven closer to it and under it no worries.
Regards,
Jimmy Deguara

 Agreed Jimmy,
                          I would not class it as severe either not compared to some of the storms I chased/observed this season
 The hail was pea sized and very minimal ... in fact... I could almost count them as they fell around me 
Although the storm lasted ~ 1.5 - 2 hr, it didnt really move very far across the city before it dissapated maybe only a few
5 - 10km at the most.  During the 45 mins that I did intermittant videoing (during its peak) from the wharf, I didn't even
need to move the camera (repoint it).  It was significant that the upper section of the updraught was getting blown off
to the ~ east very quickly but the base was relatively stationary. The effects of the stronger upper winds can be clearly
seen in Jimmy's video from his perspective.

Dave N