Topic: Conditions Resulting in Thunderstorms

[Mike]

  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

[Jimmy Deguara]

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

[Mike]

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

[David Brodrick]

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

[Michael Bath]

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

[David Brodrick]

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

[Michael Bath]

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.

[Mike]

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....!)

[Mike]

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

[Michael Bath]

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

[Mike]

In relation to occlusion.  When a thunderstorm tower - either supercellular or 'standard' is described as having an occluded tower, what is meant by this?  Is there a pic that can be shown to display this if it is something not common per se?

Also in relation to supercells generally within the eastern states: what percentage of these are right movers and how many encountered were left movers?  A simply ratio or percentage figure will suffice thanks gentlemen !

Mike

[Jimmy Deguara]

Hi Mike,

Try compaing there two images of the same tornado:





You can see the top image is in its reasonably strong mature stage - well it took out a house causing F2 damage at the time and the second image shows the process of the occlusion. The process of occlusion sees the gradual separation from the supercell of an updraft as the fuel or inflow gradually gets cut off. This is when you normally see roping out of the tornado - sometimes in sunshine!

For cyclic supercells, this is not a problem as usually a second mesocyclone is in operation and ready to drop another tornado or one is on the ground. The supercell above produced 5 tornadoes during the afternoon and evening.

Regards,

Jimmy Deguara

[Mike]

Hi Jimmy, 

So as far as this scenario : observing a developing wall cloud with good lowering and rotation and subsequent funnel emerged, why would the tower be in the stages of occulsion at the same time?  If occlusion is the separation or lifting mechanism of different air temp masses/boundaries etc, would not the supercell be dissipating and tornadogensis weaken if this was happening?

So occlusion does not necesarily mean dissipation if the updraught is strong and there is a strong middle or high level jet to discard any precip that may impede inlfow and meso formation - hence the funnel formed even though the tower was occluding?  It's just that the storm was strong enough to keep the inflow warm by actually inhibiting the precip and cold pool from impeding its progress?

As your photos depicted and you mentioned that the same cell produced more tornadoes, if it's cyclic then my comment above is correct?

I think i'm on the right track - I'm picturing it in my head...and probably answered it myself but you can sympathize with me as I try to get the terminology right with what's in my head!

Mike

[Mike]

If you had easterly winds at 20kts from 750 to 500mb constant and above that they dropped off and below that they started at 10kts, that would not be classified as significant shear would it?  As it's not increasing or changing direction with height it would not be classed as 'shear' could it?  Or is shear in this sense a strong wind profile within a certain Mb that is unchanging with height?

Mike

[Mike]

I was watching some weak convection today over the Islands to our north - they tend to spark the onset of buildup convectioin this time of year - i was observing a line of clouds and it occurred to me that do these small scale CU clouds exhibit the same properties of maturing thunderstorms?

 I observed what looked like an inflow tail and a wall cloud underneath on one in particular - it certainly was a textbook shape and fanned out slightly at the lower edges and then lifted back up into the cloud.  There were strong winds in the uppers as the domes were being pushed out to the side in opposition to the direction the thick clouds were travelling.  Now is this possible to observe this or was i just seeing what i thought looked like a small wall cloud. It was not scud and definitely had the lowering shape and structure - if this Cu was an actual thunderstorm i'd almost be convinced.

Mike