Author Topic: Conditions Resulting in Thunderstorms  (Read 36456 times)

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Offline orage

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Conditions Resulting in Thunderstorms
« on: 31 December 2006, 09:24:14 AM »
On this site, and in other places, I have seen terms such as CAPE, lifted index and wind shear etc.

Firstly, could anyone tell me whether there are any other measures of conditions which may effect the potential for a severe storm, and what do they mean.

Secondly, does anyone know how I might be able to find current figures, what charts I could use to use to work it out (and possibly how they should be read), or how I can observe it myself?

Offline David Brodrick

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Re: Conditions Resulting in Thunderstorms
« Reply #1 on: 08 January 2007, 11:24:37 AM »
Nice post thanks John.

I've got a GFS forecast table on our weather site that attempts to show the convection around my home town, eg 'isolated showers' 'severe thunderstorms' etc, but at the moment it only looks at CAPE, so it is obviously somewhat deficient.

Does anyone have suggestions on combining some of these other parameters, such as shear and maybe LI, CIN, cloud cover, to come up with a useful forecast of the storm activity?

Best wishes,
  Dave

Offline Jimmy Deguara

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Re: Conditions Resulting in Thunderstorms
« Reply #2 on: 08 January 2007, 12:44:48 PM »
Hi David,

Hopefully there is something you are able to find.

I guess though that is the fun of forecasting - to combine your own experiences as well as current conditions and compare with forecast models. Even if there was something that combined all parameters, how accurate are they?

This year, more than any other recent years at least, the models have on the whole been disappointing in pinpointing where the best area of convection will occur. Success at forecasting and storm chasing has been attributed to past experiences in my case at least.

Regards,

Jimmy Deguara
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Offline David Brodrick

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Re: Conditions Resulting in Thunderstorms
« Reply #3 on: 08 January 2007, 01:19:15 PM »
Cheers Jimmy. I guess my question was more on the best way to interpret the models rather than the usefulness of the models themselves.

I was thinking of something grossly along the lines of:
potential=CAPE/500
if ( shear(850,600) > 20 ) potential=potential + 1
if ( CIN > 200 ) potential=potential - 1

etc.

While no algorithm will encapsulate what's really happen, at the moment I rely on CAPE alone and there has to be something better than that. Maybe I should just use SWEAT instead?

Cheers,
  Dave

Offline Jimmy Deguara

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Re: Conditions Resulting in Thunderstorms
« Reply #4 on: 08 January 2007, 01:40:48 PM »
Hi David,

I appreciate what you are saying and had thought about this concept about 2 years ago. If only I was a good programmer, I would have come up with a solution years back.

When I came across the GFS (I guess then the AVN models) back in 1998, it was a gold mine. With these models alone we were able to confidently expand our storm chasing beyond the horizon.

CAPE is a pretty useful variable as well as is surface lifted index. The sensitivity of CAPE though with only slight changes in temperature and moisture should not be underestimated. But using it as a guide is still helpful.

So any programmers?

Regards,

Jimmy Deguara
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Offline Jimmy Deguara

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Re: Conditions Resulting in Thunderstorms
« Reply #5 on: 08 January 2007, 01:51:29 PM »
Hi John,

If you are searching for surface lifted index values off the models of the order of -6 and lower, you won't be doing much chasing in Australia! Particularly this year.

I tend to look for values of less than -2 depending of course on available moisture and sunshine. This will often determine whether the models have underestimated or overestimated the specific variables. And of course, it depends on the distance travelled and the risk for failure.

One has to be extremely careful though. On the days leading up to the Muswellbrook hailstorm, LI values of -10C were forecast for the north east NSW region. The Bureau even had hail and squally winds in the forecast up there. In the region from Mudgee to Muswellbrook, Taree to just south of Sydney, conditions indicated less forecast CAPE and lower LI values. We opted for the region near Mudgee and Gulgong. The result was a long lived supercell that tracked for nine hours. The north coast struggled to break the cap with only late night lightning in the distance as a storm(s) tracked up the coast.

Regards,

Jimmy Deguara
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Offline David Brodrick

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Re: Conditions Resulting in Thunderstorms
« Reply #6 on: 08 January 2007, 03:30:17 PM »
I'm a programmer and I'm very keen to code up something like this.

I've already got some python code that rips down bits and pieces of the NOMADS GFS data, the formatted output for my site is here:
http://narrabriweather.net/forecast.shtml

It's just that I'm new to the storm game and need some advice about the right way to put the different parameters together. Some of the suggestions already made by John are useful.

Cheers for the encouragement,
  Dave

Offline Jimmy Deguara

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Re: Conditions Resulting in Thunderstorms
« Reply #7 on: 09 January 2007, 12:34:10 AM »
John,

No apologies required. Perhaps there could be a an indication for supercells as well givent eh complex nature of the program.

This is an important topic.

Regards,

Jimmy Deguara
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Offline Michael Bath

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Re: Conditions Resulting in Thunderstorms
« Reply #8 on: 09 January 2007, 03:01:20 AM »

5) Ill look at 500mb wind charts to see what the propagation of storm direction is going to be like over the target area.(steering flow is usually near the top of the troposphere)


I would say you need to look lower than that for steering. I tend to use the average from about 850 to 500 depending of how strong the winds are at various levels in that range. No set rules of course.

regards, Michael
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Offline David Brodrick

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Re: Conditions Resulting in Thunderstorms
« Reply #9 on: 09 January 2007, 05:29:49 AM »
John why do find LI and DP more useful than CAPE? I see the input parameters are different but isn't CAPE already an attempt to combine the convective potential of a parcel and the DP? Do you see much difference between the two approaches?

I could use a combination of exisiting indicies, such as LI or CAPE, as a general convective indicator like I currently am, but then also look at the ratio of that and something that incorporates shear, such SWEAT, as the trigger for warning of severe storms. Is it also worth looking at CIN?

Although if I go back and look at the input parameters that make these different quotients there's probably an even simpler implementation of what I'm seeking. If anyone wants to suggest an interesting combination of the GFS forecast parameters I'll code it up and put it on my site with a few of our locations so we can assess it. My current code can't generate maps, just forecasts for specific locations.

Even though they're for specific locations, I do need to change my algorithm to look at the surrounding 0.5 degree forecast points as well. There's been a few times where the reported convection was fairly low, because a front has gone through faster than the 3 hour forecast interval of the GFS, such that you don't see the storms if you just look at a single forecast point.

Regards,
  Dave

Offline David Brodrick

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Re: Conditions Resulting in Thunderstorms
« Reply #10 on: 09 January 2007, 05:33:56 AM »
This page describes how the existing stability indicies are calculated:
http://weather.cod.edu/sirvatka/si.html

Offline David Brodrick

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Re: Conditions Resulting in Thunderstorms
« Reply #11 on: 10 January 2007, 11:18:10 AM »
I'm with you RE atmospheric soundings happening at the 'wrong' time of day. My site's 'thunderstorm potential' was initially based on the sounding data from Moree (100km North), but I found it essentially useless since it runs at 00z, so switched to using the GFS forecast data instead.

I may have this wrong, but isn't LI calculated by calculating the temperature of a parcel of air from the bottom 1km of the atmosphere, lifted to 500mb, and subtracting this from the actual temperature at 500mb? In which case you still need soundings (or forecasts of soundings) in order to know the actual temperature at that altitude?

Thanks,
  Dave


Offline Mike

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Re: Conditions Resulting in Thunderstorms
« Reply #12 on: 20 January 2007, 09:38:10 AM »
Orage, myself being a novice also, the best advice I can give you from what I've learnt over the past months is two things:  Ask questions and research.  Even if your question is answered and you still don't understand the terminology - ask again for more clarification in layman's terms.  One can't learn if one can't understand the terminology used and then apply it.  Never think your questions may be irrelevant because they're not - because what you ask may be something that someone else is thinking.

 Research as much as you can and take notes, copy pages and make up booklets on different subjects on storms/chasing/structure etc.  It's what I've done and you can go back and reread as much as you want!

For the best info on Skew-T charts and explanations go to http://www.downunderchase.com - you'll find modules on every part of interpreting the sounding charts and also Jimmy's observation technique articles on observing clouds on this very site.

 Get yourself a storm term glossary list also, this will enable you to know straight away when forum members are talking about CAPE, CIN, LI etc and will lessen the headache i assure you.

Hope this helps.  Even a dummy like me has worked out the Skew-T soundings enough for me to know if there's potential for instability!

Mike

« Last Edit: 21 January 2007, 01:06:30 AM by Jimmy Deguara »
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Offline Mike

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Re: Conditions Resulting in Thunderstorms
« Reply #13 on: 25 January 2007, 10:05:00 AM »
Hey there.  Okay, i've got some figures for you mathematicians and experienced sounding observers. I've been reading the posts on this topic and getting some education on the sounding chart through the answers given.  There's a host of questions so bear with me:

(1) At which level do winds steer storms so i can get an idea of where they're heading?  At each level (as far as mb level on the sounding diagram) what wind speed am i looking for on the sounding that would help the generation/maturity of a/or storm(s)?

(2) Jimmy mentioned that LIs less than -6 or lower there would not be much storm potential, yet other forum members said they go by LI of -3 at times?  I don't get it.  Is it the higher the LI the better potential for storm activity, is that it?

(3) As far as the wind barbs - for good storm potential is it better to have fewer barbs in all the levels?  For example fewer barbs in the lower up to 900mb, fewer but stronger wind speed barbs in the middle to say 500mb and then how many aloft and at what knots?  What wind speeds in knots should i be looking for on the sounding charts in these levels mentioned?

I have some figures for you from our sounding here - perhaps you can interpret.  This would be of great help to me so that i can get an understanding of what ingredients i should be looking for when viewing the sounding. I've got the CAPE and some other aspects understood, there's just a bit more i need.

DP 27.68 (dew point?)
LI -6.28c (that must be average, no?)
CAP 0.17c
CIN 17j/kg
PS 1005 (what's that and is it important?)
CAPE 4791j/kg ( i know that's good)
SWEAT - what is that??

winds on the sounding were: surface to 900mb 7-11knots from SW, up to 500mb 16kt from SE, from there aloft they went  NE at 9-16kt then E 14-30kt.  The air was moist in below 600mb then nice and dry right up through the middle range and then moist again in the upper - that's a good sign no?

Sorry to be long-winded but this is quite important and i really do appreciate you guys helping me out.

Mike
Darwin, Northern Territory.
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Lightning Research 2010/14

Offline Jimmy Deguara

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Re: Conditions Resulting in Thunderstorms
« Reply #14 on: 25 January 2007, 01:31:26 PM »
Mike,

Can you tell me where I mentioned the following:

Quote
(2) Jimmy mentioned that LIs less than -6 or lower there would not be much storm potential, yet other forum members said they go by LI of -3 at times?  I don't get it.  Is it the higher the LI the better potential for storm activity, is that it?

Regards,

Jimmy Deguara
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