Storm Australian Severe Weather Forum

Severe Weather Discussion => General Weather - all topics not current severe weather. => Topic started by: Gambit on 22 December 2007, 03:57:35 PM

Title: GFS and Soundings for storm forecasting
Post by: Gambit on 22 December 2007, 03:57:35 PM
Michael please or anyone really who can answer, if i can ask a few questions about the charts that you have posted links to in the SE Aust severe weather thread (

most of the questions are based on my own research and limited understanding so if I'm wrong please point it out.

CAPE - this is the potential energy that a storm has to create convection with?  And if I'm reading it correctly it was quite (almost freakishly) high over Melb?

Lifted index: this is a measure of instability: some parts were at -6 I’m not sure what constitutes high instability or very unstable atmospheric conditions.

I still haven’t read enough about Humidity and Temp in the atmosphere and the effects it has to be able to comment.

Winds: At the different heights it was blowing at markedly different directions and at speed. is this what creates shear and gives us the anvils and overall creates turbulence that feeds or powers the storm?

And as for the SKEW/t yeah well I’m not sure if I'll ever get that one lol, but I have found a few sites that give a great explanation and am learning slowly.

Overall it seems that the atmosphere over Melbourne and other parts of VIC were almost volatile (highly unstable) hence the "significant" weather events that have been experienced.

Cheers and thanks in advance

Title: RE: GFS and Soundings for storm forecasting
Post by: Mike on 22 December 2007, 10:00:34 PM
Gambit - here's a layman's answer to your questions:

CAPE - Is the measure of convective available potential energy in the atmosphere for storms to develop.  It determines and/or represents the amount of buoyant energy available to lift a parcel vertically. Storms require high CAPE values, the higher the value the more energy is available for the storm for growth.

LI or lifted index is the atmospheric instability.  Negative values such as the one you posted as -6 show high instability and the more unstable air values are the stronger the updraughts are with any developing storms.

Shear is very important in determining what type of storms will develop.  I think you are referring to speed and directional shear with this storm - ie; wind speed increasing with height and/or changes in direction with height.  Anvils are the by-products of the updraught when it reaches the tropopause and spreads out - reaching the lid so to speak, but with strong storms they can keep overshooting as it were.  The wind shear will create the turbulance within the storm - a bit like a washing machine mixing everything up.  The more mixing with height the stronger it will be and it determines what type of storm it forms.  The nature of this storm represents all the high instability and favourable winds shear properties of severe storms!

I think there's a thread on skew-t plots in the can do a search and different threads will come up, i know i asked a lot of questions re this in particular in the early days!

Hope this helps.


Title: RE: GFS and Soundings for storm forecasting
Post by: Gambit on 23 December 2007, 02:13:35 AM
Thanks Mike,

I more wanted to make sure that I was on the right track  :) and reading the data correctly. From what I can tell it was very turbulent and unstable in the lower and mid atmosphere and then tended to calm down as it got up to 300mb & 200mb.

I did also read that Ewan saw small vorticies this would further enforce the instability?


Title: RE: GFS and Soundings for storm forecasting
Post by: Mike on 23 December 2007, 09:02:29 PM
Vortices are a result, not a cause.  Vorticity is a whole different ball game and gets complicated.  Vortices are caused by several factors which include updraughts, rotation (if any)/speed/direction of the winds/storm and the earths rotation and inlcude wind shear - but yes, it shows very unstable conditions but the vortices did not enhance it...that's a visual sign :)  You could include the GFS graphics if possible so we can see what you are viewing and it would help!

Title: RE: GFS and Soundings for storm forecasting
Post by: Gambit on 24 December 2007, 02:32:44 AM
There are quite a few graphics but here are a few that I was looking at

Title: RE: GFS and Soundings for storm forecasting
Post by: Mike on 24 December 2007, 08:12:29 AM
Yeah you really need to look at the wind charts also.  Don't be too convinced when you see CAPE and LI numbers that are either high or low or whatever - storms will propogate in small CAPE envionments also!  The trigger needs to be there and it could be air temps, surface temps, wind direction any enhancing/inhibiting factors such as troughs at different levels - the list goes on!  I've experienced days here with CAPE at over 3500 and LI's at -6 and yet little cumulous with towering characteristics.  Something was missing.  even with the sounding you posted, you have very moist air in the lowers, an inversion and then drier air in the mids to uppers with some speed shear and then strong winds in the uppers which would help blow the anvil muck away - but noticing the low CAPE and low LI at -3 you still got strong storms on that day - so it proves that the mixing was right, the temps were right and whatever atmosphereic conditions prevailed were favourable.

I, myself don't know enough about mixing ratios and the like to explain everything and in fact it gets very detailed and the best thing is to do more reading!  I have to do it all the time considering my memory is akin to a goldfish.  In reality the storms you get in the eastern states are different to what we get here in Darwin - totally different environments - so I'm learning as much as you re storms there as to low CAPE etc etc....:)

You can view reports from past storm events in the storm news and chasing area  There you'll find everything re wind, skew-t, cloud structure - everything.  That's the best way to compare events, from what the reports say.

Hope i've been some help with my limited knowledge !


Title: RE: GFS and Soundings for storm forecasting
Post by: Jimmy Deguara on 24 December 2007, 12:18:56 PM

It has been documented that surface lifted indicies can be locally much higher along and in colse proximity to a boundary.


Jimmy Deguara
Title: RE: GFS and Soundings for storm forecasting
Post by: Mike on 24 December 2007, 07:37:58 PM
No arguments with that Jimmy, thanks for clearing that up.  I have read that when storms form on a boundary, indeed surface indices would certainly be enhanced in such a local environment.  There's just so many variables and that's what I was trying to convey to Gambit, if we had to list all the different atmospheric conditions associated with LI's and CAPE, instability - well we'd be here forever.  :)

Title: RE: GFS and Soundings for storm forecasting
Post by: Gambit on 02 January 2008, 03:33:27 PM
Three questions please learned friends;

With SKEW/t readings, my closest points for readings are Brisbane: approx 150km's North and Moree: 344km's
West Southwest.
How relevant will these reading be to the atmospheric conditions here in Lismore?

Making the assumption that they are going to have no bearing on the conditions here, what can I do?

& Finally

What area does a weather balloon cover when it is released to get the reading?

Regards all

Mat :)
Title: RE: GFS and Soundings for storm forecasting
Post by: Mike on 02 January 2008, 05:06:26 PM

What you have to remember is that the sounding is taken at certain times during the night and day depending on how many times the balloon is sent up.  Some stations only send up two per day, 11:30pmCST and 11:30amCST - some met offices send up more regular balloons but this data is not always available online.

The link for BSCH (brissy storm chasers) gives you a SKEW-T links and a map of Aust and you can use your mouse to click on any area to get a given 'sounding' for that area.

The assumption you made is correct.  There's nothing you can do.  Even Meteorologists can't forecast 100%.  Soundings are purely the atmospheric conditions at THAT time it is sent up and the data collected by the radiosondes is then transferred to laptops etc for the officers to calculate what might potentially be available or not available at the time re CAPE, winds, LI, temps etc, etc.  The sounding heads straight up until the balloon pops - the data is gathered during its ascent and descent.

What you have to do is interpret what the sounding is giving and then do the rest re looking at wind profiles, temps, freezing levels and the list goes on.  If purely reading a sounding would allow us to say 'yep' there will be storms' it would be too easy!  The sounding allows chasers to geta measure of what 'may' happen and even then it's up to Mother Nature to provide the ingredients as the day goes on.  Daytime heating, cooling and all the rest of it has to be done by the chaser to pinpoint where storms may develop - even then we get it wrong. 

That's why storm chasing is a very touchy thing re trying to forecast storms - you have to gain a lot of experience both by reading, research, observation and testing your theories, taking note of what conditions were seen on the skew-t and then do the homework!

There's no magic numbers and in reality you have to educate yourself till you pass out and then even then you'll be scratching your head!

Title: RE: GFS and Soundings for storm forecasting
Post by: Jimmy Deguara on 03 January 2008, 01:01:27 AM
Mike approaches this well in his explanation.

Soundings are always used as a guide. You can use the soundings to pick up specific features such as capping inversions, moisture profiles, dry slots but in particular lapse rates and the real measure of temperatures and moisture in the atmosphere. These are the advantages of soundings.

I have long argued the disadvantages of soundings. They are often difficult to master for most beginner chasers. Their timing of release and also locations are quite useless. I also do not like the fact that they can often exhibit great moisture profiles and then simply dry out rapidly which means modifying profiles can be difficult and almost unrealistic at times.

In the end, you are left with just trying to pick out features and relate to the models which do use previous soundings and other sources of data on a global scale. The advantages of models are they are in 2D and 3D and can make it easier to pick features.

So a combination of using both is what is the most ideal.


Jimmy Deguara
Title: RE: GFS and Soundings for storm forecasting
Post by: Michael Bath on 03 January 2008, 02:09:43 AM
Mat - for your first question they are both relevant depending on the situation. If you have a weather system moving in fairly quickly from the W or SW then Moree may give a good idea of what the atmosphere over Lismore will be like later in the afternoon or evening.

Generally the Brisbane one is a better idea of conditions. I have noted over the years that mid (500hPa) and particularly upper (300hPa) level temps can be slightly cooler over the Northern Rivers than the Brisbane sounding indicates (as you'd expect as you move south). Surface moisture is often higher in many storm setups too.

Title: Re: GFS and Soundings for storm forecasting
Post by: Mike on 03 January 2008, 08:04:06 AM
When i initially started chasing all my theories revolved around the sounding as every site i visited mentioned the skew-t plots.  Over the short time I've been busting chases I came to understand that now the first thing I do is look at the sounding, then the satpics, then the radar and then use the GFS variables then go outside and take a short drive to get some observations (or walking down the road to an open area is just as good) to see what the clouds are doing.

Jimmy and Michael are quite correct in their comments.  Soundings as I've found out are ruthless in their ability to trick you into thinking 'there's got to be storms'!  Use the skew-t as a tool first up and then follow through with all the other things after viewing it.  Always keep in mind that conditions will change hour by hour and atmospheric changes occur constantly.

Mat, keep at it.  All you have to do is get a handle on the basics of a skew-t to assist your chasing.  The rest will follow.