Severe weather indexes

[Adam Simone]

This is my first post to the forum. I've been reading it for several weeks but just now became a member. I have a question I couldn't find anywhere else. I know for severe weather there is no 1 map or model that will tell you exactly where they will occur but do you guys use one of the indexes more then the other or do you even use them at all? I know there is the Lifted Index, K-index, EHI, Sig Tornado Parameter, Stensrud Tornado Risk, SWEAT index, Showalter index, Total-Totals, Bulk Rich number, Craven Sig severe, and probably many others. I've been practicing some dry runs and it just seems like I'm being overloaded with information and models and was trying to simplify it. I know looking at the moisture, instability, lifting, and shear are all very important and I'm sure there is importance in the other indexes as well I just didn't know if you guys have found one or two to be most accurate or not.

Also I have talked to a couple really experienced chasers in the past, one mentioned ML Cape and the other SB cape. Which CAPE value is best?

Thanks

 

[Robert Edmonds]

First start with the basics... learn to read a skew-t /t-p diagram, hodograph, and surface map. You'll learn about how these indices are made. For example as you trace out an air parcel being lifted in in a skew-t, hopefully you learn about what CAPE actually means.

 

[rdale]

http://www.cimms.ou.edu/~doswell/OZtrip/STSConf/Indices.html

 

[Robert Edmonds]

Here's a sounding from near where I live, nothing special.

Some simple questions to lead you in the right direction...
What does that yellow line represent?
Why does the yellow have a kink in it (and why there)?
What is the white line to right of the yellow line?
What is the white line to the left of the yellow line?
What do the green lines represent?
What do the dashed lines represent?

If you can answer these questions. Then, find a sounding which doesn't provide the yellowline (or whatever color they use), and draw in what it should be.

 

[Jim Parsons]

Everything you wanted to know about weather ;-) This is a great learning site for everything...
http://www.theweatherprediction.com/

 

[Greg Blumberg]

Adam,

I used to use indices quite a bit, but since I've grown in knowledge, I've used less and less of them.

http://www.theweatherprediction.com/severe/indices/
http://www.spc.noaa.gov/sfctest/help/begin.html

Any algorithm or simplified indice in meteorology that I've come across exists to be an "check this out here this is interesting" type of alert. One example is the tornado vortex signature utilized by the WSR-88D software the National Weather Services uses. It's there to draw your attention to the data, where you can further analyze it. I typically use CAPE indices as they provide a good number to analyze the potential energy of the atmosphere. This number does has it's issues because soundings that display different updraft accelerations can exhibit the same CAPE number. I'll also use helicity, but since the helicity that is ingested into the storm is dependent on the storm motion, I'm moving away from that number. As for MLCAPE and SBCAPE, there is no best calculation because every situation is different, and that usually takes experience to see. Most people I know look at MLCAPE, but I tend to look at both for a comparison rather than an exact number.

I've slowly been changing over to more of a lapse rate analysis of soundings. Environmental lapse rates can give you a better idea of parcel accelerations and can help assist the CAPE calculation in producing a better environmental analysis. Looking at them and the parcel trace can help you see the limitations of the CAPE calculation.

I also used to use SWEAT, which I always found to be a good number that told me a lot about a sounding. I've found that the supercell composite and significant tornado parameter have flaws in them as forecasting and analysis tools. Sometimes I've seen a large number for the supercell composite parameter, but nothing happened. Sometimes I've seen a small number, but something has happened. Still, they're fun to look at.

In general indicies have roots in the data and that's what you should be returning to when you use these indices. I believe that's what a lot of the other posters have been hinting at, and I agree. I recommend the COMET MetEd program, where they have a fantastic module called Skew-T Mastery. It's completely free, and you will need to sign up for it.

http://www.meted.ucar.edu/mesoprim/skewt/

 

[Adam Simone]

Thanks for all the help guys. I'm excited to learn more and more about forecasting the weather and getting out there and storm chasing this spring.

 

[Adam Simone]

So guys am I understanding this right? When the lifted parcel line is to the right of the temperature line you have CAPE and when it's to the left, like it is in your example then tha'ts negative energy or CIN? Also when your looking at a SKEW T do you guys basically just look for CAPE/CIN or are you looking at everything. I think I can look at it and see if there is CAPE or CIN but what other important things does it show? Obviously winds at different levels and dew point levels. Sorry if my questions are stupid. I've been doing a lot of studying this winter and the Skew T is what I've struggled with most.

I also posted a attachment of a skew t I copied from F5data for Thursday. Is that a good area of cap?

 

[Robert Edmonds]

When the lifted parcel is warmer than the 'surrounding' air (the parcel temp. line is to the right of the observed temp.) the parcel is buoyant and will tend to rise. If the parcel is cooler than the 'surrounding' air then the parcel will tend to fall. So yes the area between the observed temperature line, and lifted parcel temperature line (when to the right) is the CAPE (for the location you lift the parcel from). You could lift the parcel from other locations too (i.e. not just the surface [see other thread presently ongoing]). I would say the cap is pretty much gone in the plot (image is kinda small).

Side note, I don't see much turning in the winds in that plot (hard to make out the lowest two barbs).

 

[Ryan Wichman]

Adam, there have been a number of great posts in here so far but I thought I would add my 2 cents.

Last year I did a few blog posts on how to read and interpret skew-t's. They can be found near the bottom of this page.

http://convectiveaddiction.com/education-2/

Below is a quick sample:

Here is a break down of what each line is:

1) ISOBARS: Vertical lines of equal pressure and are put 50mb apart. Spacing decreases as pressure decreases, similar to the actual atmosphere.

2)ISOTHERMS: Lines of equal temperature in Celsius that start in the bottom left corner and run to the top right.

3)DRY ADIABAT: These represent an unsaturated parcels accent in the atmosphere of 10 degrees Celsius per kilometer.

4)MOIST ADIABAT: Once a parcel of air has been saturated, it will follow this line. The moist adiabat moves to the left as the parcel decreases temperature since colder air cannot hold as much moisture.

5)DEWPOINT: The left of the two roughly vertical lines on the Skew-t is the dewpoint of the air at that level.

6) TEMPERATURE: The right line is the environmental temperature, the dew point and temperature are derived from the actual sounding.

7) On the right side of nearly every sounding are wind barbs which show the wind speed and direction at the different levels of the atmosphere.

 

[Rob H]

First start with the basics... learn to read a skew-t /t-p diagram, hodograph, and surface map. You'll learn about how these indices are made. For example as you trace out an air parcel being lifted in in a skew-t, hopefully you learn about what CAPE actually means.

Awesome advice.. Skew-Ts didn't make sense to me until I printed off 10 blank sheets and plotted out my own. You might learn differently than I did, but that was the *ding, lightbulb* moment for me.

Any algorithm or simplified indice in meteorology that I've come across exists to be an "check this out here this is interesting" type of alert

Another piece of awesome advice. Take a popular parameter like EHI, which only uses two components: CAPE and helicity. Looking at your target area, you have an EHI of 5. That's a good sign that you'll see some tornadoes in your area at a quick glance, but do you have high CAPE, high helicity, or a moderate amount of both? You can't tell from looking at the index. For some events it might not matter, but for most events, you'll want to know the difference. The 2009 Kirksville, MO tornado is a favorite example of mine. Two cells right next to each other - one is in an environment with higher CAPE and lower helicity, and the other has higher helicity and lower CAPE. The people that picked the one with more helicity got to see the Kirksville tornado. If you had just looked at an index like EHI or STP, it might have been like flipping a coin.

You can't always pick helicity over CAPE as with this example, but when you look at the individual parameters you at least have the knowledge to make an informed decision, and if you make the wrong one, you can learn from it.