Which is more important....High Instability or Strong Shear?

[Ryan Wichman]

A large problem with this past Sunday's event (4-25-09) was the lack of clearing ahead of the dry line. I realize a lot of other factors were in place this Sunday which prevented a major outbreak but this made me wonder; would you rather have a lot of instability with lower shear values or stronger shear values with limited instability when forecasting a chase?

Personally, I am a big believer in strong directional shear when forecasting severe weather events. Obviously, having instability in combination with strong directional shear would be ideal but for arguments sake which would you prefer?

 

[Jason Boggs]

I would much prefer high CAPE over high shear. High shear with little CAPE= storms that get sheared apart.

 

[Jay McCoy]

I will take instability over shear. Like Jason said if you have lots of shear with little instability storms tend to get leaned over and torn apart. CAPE makes up for lack of shear anytime. Just look at Jarrel, Tx.

 

[Connor McCrorey]

You will hear different opinions. I think instability is more important, for the same reasons Jason and Jay just said. Just toss in a weak boundary or wave in a high CAPE/low shear environment and you get Jarrell.

 

[Tim Vasquez]

Yeah, I agree this is certainly a matter of personal opinion and experience. For some solid information, here's a great diagram to look at -- see Figure 9 in:

"A Baseline Climatology of Sounding-Derived Supercell and Tornado Forecast Parameters" (Rasmussen & Blanchard 1998)
http://wdtb.noaa.gov/courses/dloc/svrparams/rasmussenAndBlanchard1998.pdf

This diagram suggests that tornadic storms are forgiving of low CAPE, but not so much with low shear. It also suggests when you get into high CAPE days, usually there is enough shear to get something going, perhaps introducing some confirmation bias, then again there's not a whole lot of data in that part of the chart.

Back in the 1990s I was a believer in high CAPE, which was an easy thing to experiment with as I did a lot of chasing out of Abilene far down the dryline, but often I found myself too far south of the action and I went through all of 1993 and all of 1994 (except one day) without seeing a tornado. After seeing many chase days where a little shear squeaked out something in KS but not in TX or OK I've come around to being more interested in heading to where the shear is and will often bias myself further north than south.

Also be aware that CAPE and shear can change significantly as a storm moves from one air mass regime to another. The quantitative output fields (SPC experimental, etc) don't capture this very well and it does require some subjective diagnosis of what's going on.

Tim

 

[dmckemy]

According to this (http://www.crh.noaa.gov/crh/?n=tsp10-supcel-torn), vertical wind shear is more important than CAPE ... but I guess it really just depends on the situation and conditions.

 

[Derek Weston]

I confess to being more obsessive over perfect shear than amazing CAPE.

Being from the upper midwest, I'm used to summers with 70 degree dewpoints and lots of cap busts. Most of the time you don't even have shear supportive of tornadoes in the first place, though... despite the big CAPE.

But, apparently, there is a give and take to the game, and different combinations of each that will yield positive results... so I try to concern myself first with finding these combinations first.

 

[Derek Weston]

Also be aware that CAPE and shear can change significantly as a storm moves from one air mass regime to another. The quantitative output fields (SPC experimental, etc) don't capture this very well and it does require some subjective diagnosis of what's going on.

Tim

Curious, does this comment include data found in the SPC's mesoanalysis product?

 

[Simon Brewer]

It really depends on the definition of 'high shear'. Are you talking about high 0-1 km, 0-3 km, or 0-6 km helicity values or are you merely refering to relatively fast mid-level winds, or are you refering to relatively fast storm motions. I'll take high shear (0-1km) with weak or marginal instability any day over high CAPE with weak shear, but that can be achieved along an outflow or warm frontal boundary in late June with 6,000+ CAPE. Just because storm motions are fast or mid-level winds are fast doesn't imply 'high shear'. One can find higher storm relative shear sometimes on days with 500mb winds at 30 kts over days with 500mb winds at 60 kts.

I'm not sure who started the whole "...updraft getting knocked over..." or "...updraft ripped apart..." myths, but that doesn't actually happen: if an updraft 'rips apart' it's most likely due to the updraft moving over an area with little or no surface-based instability or a stronger inversion that 'cuts-off' the updraft and then the visually identified cloud remnants of the updraft appear to get 'knocked over" or "ripped apart". I don't care if the 700mb wind are 200 kts and the updraft is completely horizontal above the boundary layer as long as surface-based air parcels are feeding the updraft there will most likely be an insanely violent tornado.

One problem that usually occurs on a 'high shear' day or day with relatively fast mid-level winds is the storm motion might be fast and an updraft might cover a great distance in a short period of time moving in and out of favorable environments along the way. All it takes to 'kill' an updraft is a small region of a locally 'stronger' inversion or a localized area of little to no surface-based instability. And if that would occur the updraft would then appear to 'knock over' or 'rip apart'.

Simon

 

[afischer]

Tough question... I think it was asked a few years back and I said I prefered low CAPE-high shear... I think I've changed my mind though lol.

It might be good to specify what type of vertical shear you mean, since near-surface shear (e.g. 0-1 km SRH) has been shown to discriminate much more strongly between sig tornadic and non tornadic storms than deep layer shear. I assume that by shear you generally meant a measure of SRH in your original question, or shear near the surface.

Vertical pressure perturbations associated with strong vertical shear can in some cases seem to go a long way to enhance supercell updraft strength/accelerations, allowing for tornadoes in environments of marginal total instability... or even in environments with relatively stable low levels (i.e. significant CIN).

Regarding the general "low CAPE high shear" environment alluded to in your question... I tend to loathe these environments if they are characterized by very strong deep layer shear (e.g. 0-6 km bulk shear > 80 kts) but there isn't strong low-level shear present (e.g. 0-1 km SRH > 250 m2/s2) as well; storms tend to be strung out with tornadoes not nearly as common. Meanwhile, if you've got fairly strong SRH coupled with moderate to strong deep layer shear... supercells seem to thrive far better within marginal total instability environments and more productive w.r.t. tornadoes.

I'd probably generally prefer high CAPE-low shear rather than low CAPE-high shear, as long as low-level stability (CIN) isn't out of control (refer to the mention of Midwestern cap busts above... those have bitten me more than once and are no fun at all). Same as with shear, in a lot of cases CAPE in the lowest few km can be as important as total CAPE... illustrated pretty well by the typical Davies cold core setup with weak CAPE but a good chunk of it rooted below 3 km.

The other good thing about high CAPE-low shear environments is, forcing mechanisms tend to be more subtle and there is far less competition among storms and less rarely a question of convective mode... not to mention fewer storms to choose from.

 

[Simon Brewer]

Another thing to consider is 'very high CAPE' environments like environments with 5000 or 6000+ SBCAPE: storms in these environments can completely alter regional low-level wind field and produce high 0-1 km SRHEL values like Jarrel, TX in 1997, Central IL in June of 2004, Southcentral KS on May 12, 2004, etc...

I said above that I usually prefer to chase high shear low instability areas, but I, also, like to chase regions of forecasted relatively weak shear with insane instability.

 

[Justin Turcotte]

If I want to just see a modest storm in North Dakota then give me instability over shear. If I want a bona fide supercell in North Dakota and chance at a tornado then give me shear over instability. Obviously you will need some combination of instability and shear to get a rotating updraft. Shear seemingly wins nearly every time in North Dakota. High CAPE days with light synoptic shear will yield some towers and hail but very few tornadoes. The exceptions producing tornadoes are almost always along some preexisting OFB from morning convection or a storm ahead of the cell eventually producing the tornado. The former is easy to target but is usually boom or bust as the cap is stronger behind the morning crud. The latter is nearly impossible to target and is really only conducive for a local chase. I've been burned several times heading for the higher CAPE/lower shear environment in, say, southeast ND when the EF2 is grinding away 150 miles north in the lower CAPE/higher shear environment. The tail-end Charlie storms here tend to be more isolated, offer better visibility, are nice to look at but also seem to produce few tornadoes perhaps because the best shear is up the road.

 

[Jay McCoy]

I'm not sure who started the whole "...updraft getting knocked over..." or "...updraft ripped apart..." myths, but that doesn't actually happen: if an updraft 'rips apart' it's most likely due to the updraft moving over an area with little or no surface-based instability or a stronger inversion that 'cuts-off' the updraft and then the visually identified cloud remnants of the updraft appear to get 'knocked over" or "ripped apart". I don't care if the 700mb wind are 200 kts and the updraft is completely horizontal above the boundary layer as long as surface-based air parcels are feeding the updraft there will most likely be an insanely violent tornado.
Simon

If your saying an updraft wont be leaned over and ripped apart by strong shear vs weak instability then your nuts. I have seen it hundreds of times. Ofcourse its about lack of instability..hence the comment "strong shear vs low cape". I have seen so many storms start out looking great but once the shear takes effect it leans over and get stretched out and torn apart. The instability isnt strong enough to overcome the upper level winds and keep the updraft upright.

 

[Chris Hayes]

I'm not an expert, but I'll agree with some of the concensus here. I'll take high cape over low cape and high shear. I've seen many a high shear days with low cape that turn to crap real quick. You'll either have storms that dont form because of lack of instability, or they form and they're tore apart because the instability isnt high enough to keep the updraft towers in the vertical.

 

[Paul Knightley]

Putting personal preferences for chasing aside, technically speaking strong shear through the part of the atmospheric sounding with the most CAPE will cause the most pronounced dynamic enhancement of the updraughts.

Most winter tornadoes here in the UK occur with very modest CAPE (<500 J/Kg) but high shear. And last summer, an EF4 tornado occurred in NE France overnight on a shallow frontal wave, with just a few hundred J/Kg of CAPE, but very high low-level directional and speed shear.