Favored severe weather factors/indices

[Ed Lizard]

On the subforum, I was wondering why SPC actually expanded the SLIGHT RISK tomorrow when the 2 main US operationals showed it dry here locally.

I guessed it was something in the SREFs, and found the Craven-Brooks index >20,000 m^3/s^3 over 20,000 on 90% of the SREF members.

Looked it up, never heard it before, it seems like a non-tornado specific analog of EHI, a product of CAPE and deep shear. Now, the majority of the SREFS may say we're good to go (20,000 apparently being a lower bound for severe), but C-B doesn't have capping, which is more common than not down here. Same deal with EHI. SWEAT doesn't seem to do any better as far as a dealing with capping inversions. The fact the GFS and NAM are dry for areas in a risk area, well...

Anywhere, short of actually looking at a forecast skew-T or CINH map, does anyone have a preferred severe index that does factor in CINH and/or capping?

How about an index that doesn't consider CAP.

[thewxmann]

Capping is one of the most important factors in determining tornadic potential, and often one of the most overlooked. I would love to see an index that takes into account capping (remember all those thermonuke IA cap busts with EHI's up to 18? Yaa...). Unfortunately, the only way to determine the strength of the cap is by looking at a sounding, and even then since the sites are far apart the cap can be missed or inaccurately depicted.

On the other hand, multiple indexes factor in CINH, which is easier to calculate since it's an integrated quantity. SPC has a parameter called "effective SRH" which takes into account whether the parcel is sfc-based or not. SCP and STP take into account the effective and not the gross SRH.

[mnchaser]

Capping is one of the most important factors in determining tornadic potential, and often one of the most overlooked. I would love to see an index that takes into account capping (remember all those thermonuke IA cap busts with EHI's up to 18?

Ahhhh 2009 was a great year for Iowa busts! We probably had one once every week it seemed like!! Watching the towers under the CU fields get demolished as fast they developed, one of the most depressing sights to see haha.

[brettjrob]

I'm no expert forecaster, but I actually prefer to look at capping separately from other parameters. I'm not sure I like the idea of trying to factor it into indices like STP or EHI, because unlike other ingredients (instability, moisture, shear) that might scale well with increasing tornado potential, it's an all-or-nothing outcome: either storms form, or they don't. If you have an EHI of 10, I don't think reducing it to 5 because SBCINH is -50 J/kg and the cap "might not break" makes sense. Instead, just look at the EHI, recognize that the environment is ripe for significant tornadoes should a surface-based supercell develop, and then move on to assessing the capping situation via simpler, traditional parameters and/or soundings.

[AtticaFanatica]

I'm no expert forecaster, but I actually prefer to look at capping separately from other parameters. I'm not sure I like the idea of trying to factor it into indices like STP or EHI, because unlike other ingredients (instability, moisture, shear) that might scale well with increasing tornado potential, it's an all-or-nothing outcome: either storms form, or they don't. If you have an EHI of 10, I don't think reducing it to 5 because SBCINH is -50 J/kg and the cap "might not break" makes sense. Instead, just look at the EHI, recognize that the environment is ripe for significant tornadoes should a surface-based supercell develop, and then move on to assessing the capping situation via simpler, traditional parameters and/or soundings.

agree 100%

[HSVWx]

agree 100%

Could you explain the CINH on SPC's site a little more please? I have the darnest time figuring out if like is a -50 better than -100 or is a +100 worst for convection to begin? The area of "shading" is what cofuses me on some of the parameters they use. What again is a CAP that is just too strong to be overcome by anything? 3.0 or higher?

[wi_fl_wx]

The severe indices that I prefer depend on many factors--region, time of year, and storm system evolution. Usually the idea is to find the limiting factor(s) and then apply the incides accordingly.

The composite indices can be useful sometimes but they really are just mathematical combination of more basic indices. If you want to evaluate the situation in 5 minutes you can use the bulk richardson #, EHI, and/or Supercell index to get the general picture. But any really insightful analysis comes from looking at the actual core parameters because there are an infinite number of ways to achieve a composite parameter of a certain number. I use 0-6 km bulk shear, helicity and ml cape the most--and generally all the indices are just various mathematical combinations of those 3.

If its spring at the indices are too low, then there is probably too much shear and too little instability. If its August then it's the reverse. But you already knew that if you looked at the shear and instability first.

The parameters are useless for CIN/cap breaking/busts. I like the 700 mb wind and temperature forecasts along with eyeballing a few soundings upstream of the target area. For nowcasting the water vapor imagery helps too. But sometimes you just don't know.

It also helps you understand what is really going on to keep it basic.

[L.B. LaForce]

Could you explain the CINH on SPC's site a little more please? I have the darnest time figuring out if like is a -50 better than -100 or is a +100 worst for convection to begin? The area of "shading" is what cofuses me on some of the parameters they use. What again is a CAP that is just too strong to be overcome by anything? 3.0 or higher?

On the SPC site, I like to look at the MLCAPE vs SFC based CAPE.

The red contours on the following image are lines of constant positive buoyancy, or CAPE.

The dashed blue contours that is lightly shaded is referred to as CIN, or CINH (Convective Inhibition). This is also called negative buoyancy. The greater the negative number, the stronger the "cap" is. For example, CIN values of -100 J/kg is pretty significant, but -25 J/kg is fairly weak.

The second image is of a graphical representation of positive and negative buoyancy on a skew-t. The blue shaded region is CIN (negative buoyancy), and the CAPE (positive buoyancy) is the region shaded red. As an added bonus, the intersection point of the positive and negative buoyancy (approx 670mb) is called the Level of Free Convection (LFC). This is the level where a parcel can accelerate freely all the way to the equilibrium level.

[HSVWx]

On the SPC site, I like to look at the MLCAPE vs SFC based CAPE.

The red contours on the following image are lines of constant positive buoyancy, or CAPE.

The dashed blue contours that is lightly shaded is referred to as CIN, or CINH (Convective Inhibition). This is also called negative buoyancy. The greater the negative number, the stronger the "cap" is. For example, CIN values of -100 J/kg is pretty significant, but -25 J/kg is fairly weak.

The second image is of a graphical representation of positive and negative buoyancy on a skew-t. The blue shaded region is CIN (negative buoyancy), and the CAPE (positive buoyancy) is the region shaded red. As an added bonus, the intersection point of the positive and negative buoyancy (approx 670mb) is called the Level of Free Convection (LFC). This is the level where a parcel can accelerate freely all the way to the equilibrium level.

Fantastic write up! Thank you for your timely and indepth explanation