Cooking Up A Storm (A Recipe)

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Cooking Up A Storm (A Recipe)

Postby Tim S » Wed Jan 31, 2007 11:05 am

I thought I would start a topic about what to look for in a Skew-T or Sounding if you want to know if thunderstorms are likely or not, mainly because I have a bit to learn but hopefully so others can learn a bit too. :)

Now there are several ingredients needed to make a thunderstorm and if you make an anology to making a cake, if you leave out one ingredient the cake, or in this case the thunderstorm wont come out like you expected ie. it wont be a thunderstorm but a mess of cloud! :)

Now the 4 general ingredients are, mositure, heating, instability and sheer, you dont have one of these you dont have a very good storm, if one at all.

Now the things you look for on a skew T or sounding are quite varied and take a trained eye to pick the little things that say yah or nah to thunderstorms, but as a guide some of the things you look for are Lifting Index - LI (instability), Precipatle(sp?) water PW (moisture), wind direction and speed at certain levels (sheer) and of course the temperatures at certain levels.

Now the things to look for in these four figures are quite involved so I wont go into them, mainly as someone already has in a very good thunderstorm guide. Anthony Cornelius has a thunderstorm guide somewhere on the net, I used to have the link but can no longer find it, hopefully someone else has it and can post a link? It is for SE Qld but a lot of it can be used for Perth, the figures may just vary slightly.

Here is the sounding from Wednesday last week:
Image
Firstly the two red lines are the temperature profiles up through the atmosphere, the the bottom of the graph is ground level (~1000hpa) while the top is the tropopause, the limit of which weather can go.

Now the graph does look very complicated as they are lines everywhere but the main ones you need to look at are, the horizontal brownish lines (pressure or basically height) and the diagonal brownish lines (temperature) with these two scales you can work out the temperature of the air (right red line) and the dewpoint (left red line) at certain heights.

Also there is a wind profile of the atmosphere on the right hand vertical bar, the one with the wind bars (like arrows but with feathers only one side). This gives you an idea of sheer and steering winds, very useful to know for thunderstorm development.

Now I will leave it there for now as this is getting a lot longer that I thought, I will continue it later today. :)

If anyone has anything else to add feel free, also if I made any mistakes please point them out. :)
Kia Kaha, stay strong, live long and above all have fun. :)

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Postby radar » Wed Jan 31, 2007 12:29 pm

Can't fault anything you've written so far Tim. When looking at a Skew-T I think it helps to make things as simple as possible. If you can remember the following you'll have no problems interpreting a Skew-T to recognise the potential for storms:

1) The red line on the left is the dew point line, and it shows you the amount of moisture at different levels of the atmosphere

2) The red line on the right is the temperature line, and it shows you the temperature at different levels of the atmosphere

3) The grey line on the plot is the theoretical air parcel line and is the single most important line to look at. It describes the path which an air parcel will take as it ascends from the surface. Note that from surface there are actually two grey lines drawn - the right one originates from the expected max air temperature, the left one originates from the expected dew point. An air parcel will follow the right hand grey line up to the point where the two meet, and will follow the single grey line from that point up.

Basically what you're looking for is for the grey line to be to the right of the temperature line - if it is to the right then there is the potential for storms to occur, if it to the left then storms are unlikely to occur. If the grey line is to the right of the temperature line it means that a parcel of air rising from surface will be warmer than the surrounding air and will continue to rise leading to convection and (possibly) storms. If the grey line is to the left of the temperature line it means that a parcel of air rising from surface will be cooler than the surrounding air which means it will sink thus suppressing convection and resulting in no storms.

The Skew-T that Tim posted is interesting in that the grey line is to the left of the red line between the 800 hPa and 680 hPa levels, but is to the right above that. This essentially represents a cap - a parcel of air rising from surface (right hand grey line) will rise to the 800 hPa level but cannot rise any higher as above this level it will become cooler than the surrounding air and will therefore sink. So for storms to form you'd need something to break that cap. if the cap can be broken then the parcel of air can rise all the way to around the 230 hPa level giving rise to storms.

Now bearing in mind the thread topic - recipies for a thunderstorm, what we basically want is to shift the grey line to the right as that basically means greater instability and greater potential for storms. So what shifts that line to the right?

First is air temperature - increase the air temperature and the grey line will move to the right. If you look at where the right hand grey line originates on the plot posted by Tim it's at about 36 degC. Had it originated at 40 degC there would have been no cap and hence we would have had storms last Wednesday.

The second is moisture - increase the amount of moisture in the atmosphere and the grey line will also shift to the right, by a lot! Using last Wednesday as an example again, the left hand grey line originates at about 12 degC which was the dew point last Wednesday. Had it been about 18 degC the two grey lines drawn from surface would have met much further to the right resulting in no cap, a very unstable atmosphere, and explosive thunderstorm development.

So, as we've known all along, all you need for thunderstorms is heat and humidity. What teh Sew-T allows you to do is determine whether there is sufficient heat and humidity for stoms to occur. Unfortunately in Perth we don't often get both at the same time hence the reletive scarcity of storms compared to (say) Darwin.

Anyway, I better get back to work!

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Postby Tim S » Wed Jan 31, 2007 3:56 pm

Cool, that saves me a lot of typing, thanks Radar! :)

Though it isnt really just heat and humidity you need for a good thunderstorm, too much humidity and in the wrong levels gives you rain, or worse just a slab of cloud. Also I remember reading that in the upper levels you want it to be nice and cold and dry for good thunderstorm development, can't remember the reasoning for this but I think it has something to do with the fact you dont want upper level clouds blocking the sun, ie. the heating or maybe so that the upper levels of the storm get nice and icy, giving it that glaciated look we all love?

Though without good enough sheer, even the best heating/humidity and instability levels will go to waste as any storm that develops will quickly die as it wont be able to move on and keep 'feeding' itself the nessacary heat and moisture nessacary to sustain itself. Though saying this, these conditions can lead to Pulse storms which are very specky, though I don't think these can occur in Perth due to the fact you need bundles of moisture and heat for a storm to be severe in a very low sheer environment. Though I would love for Perth to prove me wrong! :)

I should define exactly what sheer is and what kinds of sheer helps with thunderstorm development. Wind sheer can come in two forms, directional sheer, ie. winds changing direction as you go through the levels in the atmosphere, evident in the 850-920hpa range on the above sounding. Speed Sheer refers to the significant changing of wind speed at different levels (not really evident in the above sounding) but say if you had 10knots at 900hpa, 20knots at 850hpa and 30 knots at 700hpa, you would have good speed sheer in the range 900-700hpa. Speed sheer quite regular goes hand in hand with directional sheer in a good thunderstorm environment. Now this is where I get a bit hazy, but I know that if there is 'backing' up to around 500hpa, that is the difference in directional sheer between 850hpa and 500hpa is 90degrees or more this is good for thunderstorm development, why I am not too sure, maybe you know radar?

Though as can be seen from the above sounding sheer is useless if it only exists in the stable layers, ie. where the air temp line is to the right of the grey line.
Kia Kaha, stay strong, live long and above all have fun. :)

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Postby MikeF » Thu Feb 01, 2007 7:08 am

Great descriptions from you both. Thanks for that.

I was going to add the grey line bit when reading this last night but scrolled down and radar had done all of that!

There is a couple of variables that they derive and one of them is from the area between the grey line and the temperature line.

Not sure of this variable but I just look at the graphs and work it out from there. Often when these are taken the air changes though the day so that even though the mornings sounding may indicate no storms. During the day the profile changes enough to make it quite unstable and storms will go off due to the trapped heat over the day. This can sometimes happen if the "cap" is broken during the afternoon as well. Often I have seen clouds start to form at about 4.30 in the afternoon and after that they go crazy.

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Postby radar » Thu Feb 01, 2007 8:26 am

If the grey line is to the right of the red line then the area between the two is used to derive CAPE - Convective Available Potential Energy. The greater the area between the grey line and the red line the greater the CAPE which basically means more fuel for thunderstorms. To have a large CAPE you typically either need a lot of moisture in the lower levels, or very cold mid to upper levels, or, ideally, both! :)

If the grey line is to the left of the red line then the area between the two is used to derive CIN - Convective Inhibition. In general the greater the CIN is the more stable the atmosphere is - not good. The BOM does not actually report either of the two variables, but they are used a lot in the US.

One variable that they do report on the Skew-T chart is the LI - Lifted Index. Lifted Index is calculated by subtracting the parcel air temperature (ie read off the grey line) at 500 mb level away from the actual temperature (ie read off the red line) at 500 mb level. Now this is a good indicator of instability because remember that a parcel of air rising from the surface must be warmer than the surrounding air for it to continue to rise.

If the LI is positive it means that the parcel is cooler than surrounding air and thus will fall and suppress convection. If the LI is negative it means that the parcel is warmer than the surrounding air and will continue to rise and result in convection, which is what we want. General threshold values for LI are as follows:

LI > 0 thunderstorms unlikley
0 > LI > -3 thunderstorms possible - trigger needed
-3 > LI > -5 thunderstorms probable
LI < -5 severe thunderstorms

I find LI extremly useful. Without looking at any other information the LI will give you a reasonable feel for whether storms are, or are not, likely to occur.

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Postby Lert » Thu Feb 01, 2007 9:56 am

Good information chaps.. Someone mentioned the Anthony Cornelius thunderstorm notes. They can be found here:

http://www.downunderchase.com/storminfo/stormguide/
2006 - 543.5mm, 2007 - 701.5mm, 2008 - 833.5mm, 2009 - 579mm, 2010 - 631.5mm, 2011 - 872.5mm, 2012 - 770mm

2013 Jan 5mm YTD 5mm

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Postby Tim S » Thu Feb 01, 2007 11:57 am

Thats the one Lert, good work. :)

I found it very useful, even though it is set up for SE QLD a lot of the figures and scales do apply to Perth, we just dont expect to see a day with a CAPE 2000+ anytime soon! :)

Good segment on CAPE and LI Radar, they are the two things I tend to concentrate on, though I take the LI's with a grain of salt as it is only really a measure of the atmosphere at one level, eg. if you have a LI of -5C but a massive CAP at 800hpa I know not to chase, well I do now anyway! ;) ahh it was a good drive anyway, 500kms to see storms that land up going over your house! priceless. :lol:
Kia Kaha, stay strong, live long and above all have fun. :)

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Postby squid » Sat Feb 03, 2007 6:23 am

this is the like to his site tim and also the thunderstorm guide he is put in

http://www.downunderchase.com/

http://downunderchase.com/storminfo/sto ... index.html

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Postby triplepoint » Thu Sep 20, 2007 9:43 am

Good answers. Unfortunately there's not enough data on the indices list - but you do have the lifted index of LI numbers of -4.3 which is great and the CAPE on the sounding is sufficient for TS initiation .

I use soundings a fair bit even before I chase and can really only use the 00Z sounding - mainly because it's the only one of two we get for the whole day! What you want to look for generally on the chart is the difference in dry and moist areas in the atmosphere - storms like dry in the lowers, moist in the mids and dry again in the uppers. ( i think i got that right - better check!)

Too much moisture will have the effect of suffocating the updraughts with too much precip and killing any convection or bouyancy capability - we need the parcel of air to rise and not sink! The sounding you show has steady 20kt winds from the NW, so the atmos is getting moist air from that direction, you have some low to mid wind shear and good speed shear in the uppers - that assists storms to 'suck up' more air through the towers and expel it. The blue lines are from the first sounding 12Z (11:30pm EST) and the red lines are the 00Z sounding (11:30am EST)

You want to look for high Dew Point numbers, good negative LI numbers and wind shear - at least shear from surface to 500mb range. If you have good speed shear with directional change then the better. A couple of things to note :- CAPE determines the updraught strength while the shear determines storm character (single pulse, multicell, supercell etc)

Multicells are favored in cases where good speed shear (significant change in wind spped and height) is present but the low level directional shear is weak. for the normal pulse storms we get and as you would also they favour high CAPE and low shear, supercells - which we hardly, rarely get favour conditions where CAPE and shear are in balance and significant.

Anyway - the answers others gave are correct. You can find a heap of sites to learn about sounding charts, it's not as easy as it looks and it really depends how deep you want to go into them. But for basics use the CAPE, LI and dew points. You can replot the charts with the maximum daytime expected temp and if you can get dewpoint readings at call then you can replot the sounding and that will give you some indication of what is happening in the atmos.

One sounding chart is pretty hard to go by - one example was when we had a tropical low pass over us which became severe TC George you guys had. The sounding massive amounts of moisture right through the range and from the sounding storms were very likely - I forgot to take the camera to work which ended up being a huge mistake - because initially starting out with CAPE at 2910 and LIs at -5.84C and with huge shear from 7kts to 38knts to 500mb - by lunchtime it had jumped to CAPE of 4000+, Lis of -6.9C (unheard of here!) sheasr right through from 1000 to 10,000mb of 40kts plus - well, you think i was excited! we had over 2000 lightning strikes in 3 hours and had an F2 tornado in Kakadu that afternoon!

So the moral of the story is that keep an eye on the sounding and make calculations during the day if storms are likely - it may get much better or worse as the day goes on!

Mike (sorry for it being long winded...:)

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Postby Vinny » Thu Sep 20, 2007 4:36 pm

don't mean to be a pain and tell people what is wrong :) but

tim i think you mean

recipes (correct spelling) :)

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Postby squid » Sun Oct 07, 2007 5:30 am

li of-2 or more is good for storms when you get li of -6 and above that is likely to bring severe storms of course the conditions need to be right

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Re: Recipies for a thunderstorm

Postby Pete » Sat Jun 12, 2010 10:42 am

I read in Warren Faidley's Storm Chaser that you need a dew point of at least 13c to support severe weather.
So, in theory, the higher the dew point the more severe the storm?

Does figures like 500 hPA mean the level of height in the atmosphere?

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Re: Recipies for a thunderstorm

Postby Fu Manchu » Sat Jun 12, 2010 5:36 pm

Yeah kind of mate :) Have a squiz at these charts and select different hights like 750, 800 etc.
http://forecasts.bsch.au.com/stormcast.html

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Re: Recipies for a thunderstorm

Postby Pete » Sun Jun 13, 2010 6:15 pm

Cheers, Fu.

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Re: Recipies for a thunderstorm

Postby Fu Manchu » Sun Jun 13, 2010 8:28 pm

Ya reckon? I can't get my head round yet :lol: :P :P :P :mrgreen: :mrgreen: :mrgreen: :mrgreen: :mrgreen: :mrgreen: :lol: :lol: :lol: :lol: :lol:

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Re: Recipies for a thunderstorm

Postby Pete » Mon Jun 14, 2010 8:22 pm

:lol: :lol: :lol:

Lots of instability of the northwest coast. LI of -4 in some parts.

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Re: Cooking Up A Storm (A Recipe)

Postby Fu Manchu » Mon Mar 24, 2014 10:12 pm

This was recently produced by BoM as part of their news page.
Cooking Up A Storm
How Thunderstorms Form:
http://media.bom.gov.au/social/blog/64/ ... orms-form/

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