Storm NENSW / SEQLD - Scattered Thunderstorms - 1st October 2020

Seabreezes

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Jul 4, 2019
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South West Rocks, NSW
Thunderstorms are likely across the majority of the region on Thursday (Oct 1st) as a trough moves into our region. Storm activity looks like favouring the Northern Rivers and Southeast Coast districts at this stage.

Already been some discussion about Thursday's potential in the day-to-day thread, e.g.:
La La La-nina :emoji_musical_score: Bring it on :emoji_cloud_lightning::emoji_thunder_cloud_rain::emoji_cloud_rain::emoji_dash::emoji_zap::emoji_cloud_lightning::emoji_cloud_tornado::emoji_camera::emoji_tada::woohoo:
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Thurs does look interesting, & the Higher Alt steering winds are forcast very favourable, in-turn bringing action this way & possibly further if satorms intitally form in the Nthrn reaches of Nsw.

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I agree @PlumbBob I had a quick look this morning and I actually couldn't find too many things that were horrible with the setup.
Good shear, stiff SW steering winds aloft (which increases the likelihood of some storms reaching the coast even though instability doesn't look great near the coast), lobe of cold air aloft, no southerly or westerly changes coming through at 7am, fairly good lapse rates aloft and best of all, the upper trough is exiting out to sea (for once) rather than approaching us from the inland which would otherwise normally cause a widespread thick morning overcast and rainfall.

The two things I'm not as keen on is the modest nature of the instability and somewhat marginal moisture in the lower levels. So this would be more conducive to any threats being mainly restricted to strong wind gusts although couldn't rule out some hail under a few of the storms. But I think there should still be activity developing.

Definitely not a widespread rain event by any means with only modest rainfall amounts overall but for the luckier ones who do get under a heavier storm, couldn't rule out a decent downpour and localised higher amounts despite how fast any storms will be moving.
Of course, still a couple of days to go so not completely immune just yet from the dreaded SEQ Factor downgrading things.
 

PlumbBob

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Good one SeaBreeze.
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Quite some activity out west today (Weds) See two pics below for midday today - with 2nd of lightning count, both highlighted box count near 3,500 each - not massive but good action none-the-less.

01 Radar nsw.JPG
02 Lghtning count.JPG

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These next two pics show potential stormage for midday today and midday tommorow
03 Storm Potential Weds.JPG

04 Storm Potential Thurs.JPG

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Next two from BSCH Storm Cast : Today and tommorow
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And a couple of only Tommorows SuperCell Potential,,, 1pm & 4pm
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PlumbBob

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One key point Iv'e investigated about tommorow relating the Scew-'T' diag's on bsch's stormcast, is being the slight/moderate 'Capping' which is evident from the NE onshore's but have noticed on the Skew T is that the correlating temps they have are only 20-21 degrees - after 'bumping them up to 23 and 24 degs then 'replotting' the capping is gone, CAPE doubles if not Tripples & Li's double ..
I mention this because also checking the 'Cloud forcasts' - looks to be quite clear most of the morning and into early afternnoon ahead of any action that gets going, so temps could be a couple higher again ?

Guess all comes down to how the morning looks/feels, Clear skies could produce some Nasty Cells - then again, always the dreaded Flip Side (touch wood)

Temp.JPG
wCloud.JPG
 

Ken Kato

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I mentioned some of these the other day as welll but there are a few aspects of Thursday's setup which are worth noting.

Firstly, the greater amplitude part of the upper trough will be in NSW rather than approaching us from inland QLD (which would otherwise often cause excessive morning cloud cover or precip). But that same upper trough is likely to still be close enough for its much colder air aloft to help with instability.

Secondly, the strong SW steering winds increase the chances of some of the inland showers and storms to quickly sweep right through to some sections of the coast, despite the somewhat less favourable environment there (shear is strong as well which may encourage severity). Shear is also strong so a small portion of the activity could become severe with the main hazards being brief damaging winds or some small or large hail).

This is where it becomes a bit more complicated though – one one hand, if storms can reach near-coastal areas, they could suddenly intensify for a short time due to interaction with the seabreeze front/dryline and moister air but on the other hand, if they can't reach the coast, that effect would be wasted.

There are some limitations – instability is reasonable but not at the upper end of the scale, depth of good moisture is also somewhat marginal in some areas, westerly winds should invade the area almost to near-coastal areas (although they should then get pushed back a bit further inland in the late afternoon by the seabreeze). There's also just the slight chance of inland activity getting dragged onto the coast in the morning due to the strong steering winds aloft which wouldn't help for the afternoon although we'd have to be a bit unlucky for this to happen.

In any case, I'm still of the view that any rainfall amounts will probably be fairly low overall (albeit with a downpour that might look and sound heavy)..... but for those who manage to get under any heavier activity, could be some half decent totals.
Any individual cells will move at a fast rate of knots also.

P.S. @PlumbBob I'd be cautious about replotting soundings with forecast surface temps especially from GFS which has a long known tendency to often be a bit too moist near the surface. In reality, the lower atmosphere typically becomes pretty well mixed over land on warm days by afternoon so air parcels tend to rise from this mixed layer rather than only from the surface. That's why replotting soundings using only surface temps will often yield much greater instability than from a mixed layer.

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Stormwalker

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Jul 11, 2019
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4 days ago WATL had between 5-10mm of rain for Brisbane area tomorrow... as of last night and this mornings updates, its down to 0. Each day WATL has plotted the rain shifting to the south.

I'm just going to go ahead and say that tomorrow will be a fizzer for SEQ. Hopefully, now I've committed it to writing I'll be proven wrong (and will be happier for it).
 

Ken Kato

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I wouldn't treat forecast rainfall amounts on storm potential days too literally @Stormwalker . Unlike widespread steady rain events, rainfall amounts from purely convective shower/storm setups are notoriously hard to predict accurately and the correlation between forecast amounts and thunderstorm potential is very weak (unless there's widespread high amounts or heavy storms). Even a forecast for only a mm or two can end up with a good scattering of storms if the probability of precip is high.
Also, WATL doesn't show anything below a threshold of 1mm so there'll often be times when the probability of any precip is fairly high (like tomorrow where 75 to 100% of models are currently forecasting 0.2mm or more of precip) but hardly anything will show up on the WATL maps.
Having said that though, I'd expect rainfall amounts for tomorrow to be pretty low overall except for those who get heavier activity.
 

Tsunami

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I wouldn't treat forecast rainfall amounts on storm potential days too literally @Stormwalker . Unlike widespread steady rain events, rainfall amounts from purely convective shower/storm setups are notoriously hard to predict accurately and the correlation between forecast amounts and thunderstorm potential is very weak (unless there's widespread high amounts or heavy storms). Even a forecast for only a mm or two can end up with a good scattering of storms if the probability of precip is high.
Also, WATL doesn't show anything below a threshold of 1mm so there'll often be times when the probability of any precip is fairly high (like tomorrow where 75 to 100% of models are currently forecasting 0.2mm or more of precip) but hardly anything will show up on the WATL maps.
Having said that though, I'd expect rainfall amounts for tomorrow to be pretty low overall except for those who get heavier activity.
Thanks Ken. The 420pm Bom forcast does appear to be playing down the idea of storms tomorrow
 

PlumbBob

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Hi plumb bob. Just looked at bsch and was going to post. You beat me to it lol
I dont think ive seen bsch that excited about SE qld for a couple of years. Definitely didnt last year
Fingys crossed
Ooops, I can sympathize @Tsunami, tho don't let it stop you from posting your input/version mate, its all good :thumbs: & I agree with bsch's strong colors etc you mention, hope it's not an 'omen' as have seen it time and again resulting in the Dreaded Word 'FIZZERRR' o_O but yes -hope- for some camera action
Also agree, Bom not showing much at all, even out west and below border, eg, Warwick and Lismore = clear all day with a slight chance of shower or storm miday or mid arvo,, !! funny that those storms the other day had bom having nothing untill it happened and I seen all their forecast changed after the fact :whistle:
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Thnx @Ken Kato for your further insight & tip on 'replotting' = valuable learning for this casual duck, cheers !
 

Ken Kato

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I feel showers/storms will probably start off as popcorn high based convection over inland parts in the drier flow out there rather than a big line or merged mass (although couldn't rule out the outside chance of a narrow line trying to get advected through in the morning).......... before the ~35kt SW steering flow will be strong enough to advect some of that activity right through to the coast.
If they do get close enough to the coast, they could get a second wind and suddenly intensify after interacting with the seabreeze front and moister air while developing some better low level structures...... but the modest dewpoints near the coast make me nervous about whether this traditional phenomenon will happen or not.

I'm hoping the strong steering winds don't advect an inland line onto the coast TOO early in the day or the outflow from inland activity doesn't wash over the coast in the morning... otherwise, it'd seriously hamper any storm potential in the afternoon.

Of course being the weather black hole this place is, it could be a total fizzer or tomorrrow morning's runs could feature a downgrade, but would be nice to think that the strong steering winds and cold air aloft will win out in the end, even if rainfall amounts will be low and activity fast-moving.
 

Flowin

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I am quite borderline on the potential for this event.
CAPE on EC is very ordinary, and good old Access model does not give us CAPE (great to see our taxpayers $ Not delivering).
This morning’s access C 18z run looks ok for total totals.
Question for those with more knowledge- can modest or severe storms occur in low CAPE environments if the shear is good?

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Ken Kato

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I am quite borderline on the potential for this event.
CAPE on EC is very ordinary, and good old Access model does not give us CAPE (great to see our taxpayers $ Not delivering).
This morning’s access C 18z run looks ok for total totals.
Question for those with more knowledge- can modest or severe storms occur in low CAPE environments if the shear is good?

805C8E95-41FF-4B59-AB0F-D80984C5F9E5.jpeg
Yep they can but you have to have at least some true CAPE and the lower the CAPE, the lower the risk of severe storms.
Strong shear increases the vertical pressure gradients in the immediate environment of storms which in turn enhances updrafts and rotation. It also helps separate the rain-cooled downdrafts from the updrafts by leaning storms over.
Not enough shear = short lived pulse storms.

ACCESS-C and G do output CAPE as one of their many fields but unfortunately I don’t know of any freely available websites that display it.
 

Ken Kato

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First three images above via WZ are the last 3 runs of ACCESS-C for 24hr precip up to 4am Friday - can see how 18z's gotten more excited.

I'd also encourage people to download the WeatheX app. I've been using it since 2018 and it was recently upgraded to allow notifications of nearby severe weather reports submitted by people. It's a cool app which allows you to view severe weather reports made by the community over the past 24hrs and updated in realtime. It also allows you to submit your own. Reports are anonymous. Also helps the Bureau and other organisations to get extra situational awareness of any severe weather occurring in realtime. Above is a screenshot I took of it in 2018.
 

Gleno71

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First three images above via WZ are the last 3 runs of ACCESS-C for 24hr precip up to midnight tonight - can see how 18z's gotten more excited.

I'd also encourage people to download the WeatheX app. I've been using it since 2018 and it was recently upgraded to allow notifications of nearby severe weather reports submitted by people. It's a cool app which allows you to view severe weather reports made by the community over the past 24hrs and updated in realtime. It also allows you to submit your own. Reports are anonymous. Also helps the Bureau and other organisations to get extra situational awareness of any severe weather occurring in realtime. Above is a screenshot I took of it in 2018.


Cheers Ken ill download it today
 

Ken Kato

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The Bureau has some internal products that do that @Gleno71 i.e. uses algorithms to combine various parameters that have a known correlation with storms to spit out maps of general storm activity, large hail, tornadoes, etc. Some OLD examples above. One of the products has undergone calibration to past lightning data.

The thunderstorm script output I post on here sometimes uses an ingredients-based principle as well although the main thing it uses is convective cold cloud depth (the depth of convective cloud above the freezing level) because it correlates very well with past lightning data.
The US has a huge range of such products calibrated to past severe storm reports and lightning data.
 

Ken Kato

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The main advantage of CAPE is that it looks at the entire atmosphere to calculate the amount of instability whereas LI only uses a single fixed height (nominally the 500hpa level).
The only real advantage of LI is that if the equilibrium level (the height at which the air stops becoming unstable) is fairly low like it is in areas which get low-topped cold air convection in winter, the LI value won't be that exciting even though there may be a reasonable amount of CAPE below that level. In these situations, there may be some thunder but genuinely big Cb's are unlikely due to the low EL. So in that way, it takes the fairly low equilibrium level into account by proxy.
There's a number of different variations of CAPE such as downdraft CAPE (used to estimate the potential for damaging winds from storms), hail CAPE (used to calculate the amount of hail in the primary hail growth zone aloft), etc, effective layer CAPE (which reflects the true CAPE in a more realistic way), most unstable CAPE (using the most unstable parcel), etc as well. The latter is especially handy because the simple surface-based version of CAPE as shown by default on sites like BSCH can sometimes be quite misleading if there's lots of dry stable air just above a shallow layer of moist air at the surface. Surface based CAPE is still useful for stock standard warm season storm setups where there's no big variations of moisture or temps with height in the lower atmosphere though.
 

PlumbBob

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Temperatures are reaching high 20s, but dewpoints are trash at the moment. I'm not even sure a seabreeze will be enough.
I'll go along with that @Nature's Fury - Looks and feels rather dry 'ere on the GCoast.
Seeing some spot clouds here as in @WarwickEye2Sky 's pic. rather high bases with nothing much above them, yet,
Humidity here is only in the mid 40's and the NE seebreeze is very clear, can not feel much moisture in it at all !

I think if anything forms, would most likely be a couple/few 'little' rain/shower cells, far and few between.
Also, what potential ther is drops right off around late arvo ?
Not holding my breath for Gold Coast at least, unfortunately !

There is some good deeper cloud formation near Casino nsw on the Sat Imagery at-the-moment (12:30) might develop into something and worth keeping an eye on at this stage ?

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Ken Kato

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Pretty sure most of them will be going out to sea by tonight, if not earlier.

Meanwhile here's some images below. 1st and 2nd images = my views of the big mammatus and virga from the approaching cells. 3rd image = view from near Rocklea taken by Irish Cloherty. 4th image = Weatherzone radar/lightning/current wind streamlines/dew point obs. 5th image = Good storm near Lismore taken by Michael Bath:

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