Australian Severe Weather Forum

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

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

Sorry I didnt make it clear: there are alternative methods(somewhat more numerical) for calculating LI: Easiest to use Lapse rate in my opinion.

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

Mike,

Can you tell me where I mentioned the following:


Regards,

Jimmy Deguara

  Here it is - i was paraphrasing your sentence and put the word 'storm' in because that's what I believed you were referring to - but you did mention lifted index - apologies if misquoted, not being picky, just inquiring!  Were you referring to something else again? 

You said: 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.

All good

Mike,

That's better. Now that makes a fair bit of difference from what was paraphrased as I think my friends even will be confused if I did not chase -6LI:) My reference was to John's message if I recall referring to -6LI as a base point. He has corrected it in a subsequent message.

Regards,

Jimmy Deguara

Certainly - sometimes I have a bad habit of paraphrasing!  comes with the employment i do!  But all good, pretty sure your friends will be relieved!

Mike

I finally got around to coding up something that is (somewhat) superior to just relying on the GFS forecast for CAPE. It's very piece-wise, but if anyone can follow it I'd appreciate any suggestions or comments on the 'magic numbers'. Obviously it will never be perfect but I'd like something that 'kinda works' to use on my weather site.

shear=wind speed at 2700m (~720mb) in km/h

if cape<=120: No Storms
elif cape<=400: Cumulus Cloud
else (cape is >400):
  if cin>-100:
      if cape<600: Isolated Storms/Showers
      elif cape<1200: Storms Possible
      elif cape<1900: Storms
      else (cape is >1900):
          if shear>80: Severe Storms
          elif shear>60: Severe Storms Possible
          else (shear is <60): Storms

  elif cin>-220:
      if cape<900: No Storms
      elif cape<1600: Cap but Storms Possible
      elif cape<2000 or shear<70: Cap but Storms Likely
      else (cape is >2000 and shear is >70): Cap but Severe Storms Possible

  else (cin is <-220): Strong Cap - Storms Unlikely

Thanks!
  Dave

Hi David,

Are you able to include shear at more than the one level ?   It may be more useful in helping to identify areas where severe storms are possible, and also where turning is sufficient or not for supercells.

regards, Michael

The models produce velocity and direction numbers for the winds at 4572m, 3658m, 2743m, 1829m, 914m, 610m, 457m, 305m and 10m. I just picked the 2743m shear to start off as it's middle of the range. We could use more than two of the levels. But I'm not sure of the best way to incorporate multiple shear measurements into the forecast.

Perhaps we just add them, eg:
shear1 = vector_shear(ground, 914m)
shear2 = vector_shear(914m, 2743m)
total_shear = magnitude(shear1) + magnitude(shear2)

and then use total_shear in a similar way to the shear index in my previous post? Or is there a better way to integrate the different levels into the forecast rather than simply add their magnitudes together? Any suggestions which levels would be most useful to incorporate?

Many thanks,
  Dave

Yes - it is going to be difficult and I'm not sure which levels amongst those would be most suitable for your program. It really depends if the shear is fairly linear or turning. If mainly linear you would need to average the speeds. Others on this forum may be able to explain all this : )

One thing though - I am of the understanding that so long as you have 40 knots of shear in total between the surface and 500hPa, it is sufficient for supercells given instability, humidity and temperature profile support storms. Eg. 15 knots NE at the surface and 25 kts SW at 500 is sufficient.

I like our sounding indices for today.  Have not seen the skew-t but the numbers aren't too bad!  CAPE 2902, CAPE with virtual temps 3161.  TTs at 41 - but you'd never know looking out the window now at 4pm NT time.  Don't know what the winds are doing as i can't bring up the sounding chart to view them so can't determine what type they'll be....;(

Mike.

(not relying on radar today - i'm just getting out there....!)

In relation to supercells/severe weather that NSW get; I'd like to know what specific conditions result in those storms.  The storms that i have noticed have a lot of shear properties influencing them probably because of the number of spouts etc, but I'm not familiar with the 'conditions' that form the storms in that part of the woods.  Could the members enlighten me with some info - please if you could include some indices for me.  I've been focussing so much on tropical conditions that I wanted to expand the horizons.


Mike

At the end of most Storm News reports up until the end of the 2005/06 season I have included soundings and GFS run output. Might be a good idea to go through some of those and have a look at values and compare them to tropical setups. Last season many of the chases were discussed on this forum so there might be soundings and GFS output there as well.

http://australiasevereweather.com/storm_news/news2005.htm
etc

Certainly the temperature, humidity and wind shear profile can be quite different. Notice the cooler upper temps, drier upper levels, and the windshear contrast on the big days.

17 December 2005
http://australiasevereweather.com/storm_news/2005/docs/200512-04.htm

26 - 27 November 2005
http://australiasevereweather.com/storm_news/2005/docs/200511-06.htm