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

[Jeff Duda]

In support of the "high CAPE compensates for low shear" idea, check out tonight's tornadic events in SW KS. Here is the 00Z sounding for Dodge City.

In terms of helicity, not terrible, but not something you'd look at and say "TORNADO!" However, there sure is plenty of instability. One supercell spawned three tornadoes southwest of Dodge City. This supercell didn't move for about an hour. It would've been a dream to chase (I wonder if anyone was out on that). Yet there is no component of wind greater than 20 kts anywhere at or below 500 mb! Amazing.

 

[Bob Hartig]

The 1965 Palm Sunday Outbreak has been an area of longstanding fascination for me. In this event, CAPE was between 500-1,000 j/kg and mostly on the low end of that spectrum, but shear was unbelievable. I just looked at some maps last night which indicate wind speeds over central Indiana of 135 kts--at 500 mb! Presumably the winds over northern Indiana weren't much weaker. The 0-6 km bulk shear in that region was something like 100 kts. I'd say that fits Simon's description of weak but adequate surface parcels interacting with insane shear, and you can't argue with the results.

Still, given my druthers, at this point I'll take high CAPE over high shear. But I'll take either over nothing.

 

[rsheeler]

Sorry for reviving an old thread (I am new here and was looking over the old ones). Has there ever been a tornado outbreak that has had both high CAPE values and high shear both? Didn't the 1974 outbreak have both? And also the 2011 outbreak?

Conversely, I would assume that there has also been instances of both high CAPE and shear that have produced nothing or very little in the way of tornado events or severe storms?

 

[Ethan Schisler]

There have been plenty of events with both high shear and high CAPE that did absolutely nothing. I can think of numerous during June and July in the Midwest and Northern Plains. One of the bigger ones I can think of is June 18 2009 in Iowa. 15% hatched region for significant tornadoes along a warm front, insane shear/CAPE combination with maxed out parameters and nothing formed except a nocturnal supercell in Southwest Wisconsin. It gave for a great sunburn though! Here is the DVN sounding from that "event". Capping is usually a major concern with these setups. On the other spectrum, I've seen setups with maxed out soundings produce violent tornadoes (4/27/11 and 6/16/14 come to mind).

 

[Dean Baron]

Sorry for reviving an old thread (I am new here and was looking over the old ones). Has there ever been a tornado outbreak that has had both high CAPE values and high shear both? Didn't the 1974 outbreak have both? And also the 2011 outbreak?

Conversely, I would assume that there has also been instances of both high CAPE and shear that have produced nothing or very little in the way of tornado events or severe storms?

I believe the Pilger twin EF4s formed in an environment with high CAPE and high shear.

From my experience, high shear low instability produces more often than high instability low shear. From a chasing perspective, high shear low instability set ups suck to chase because high shear usually means upper level winds are really strong and cause the storms to move faster than you can keep up with them. On the opposite end, high instability low shear storms are the slow moving storms that you can sit and watch without having to move much. Problem is the low shear can cause the downdraft to be too close to the updraft and the storm kills itself before it has a chance to do anything. Even if the storm is able to sustain itself, its usually in the form of an HP supercell because the upper level winds aren't strong enough to push the downdraft far enough downwind of the updraft.

 

[JamesCaruso]

It is the balance that is important and, of course, ideally want optimal levels of both, but anecdotally I think high CAPE days have more potential for a mesoscale accident.

I remember seeing a great chart somewhere that plotted historical events that showed the correlation of CAPE/shear, but I can’t remember where. Perhaps one of the links earlier in this thread were to that chart, but both links are no longer active.

 

[Ethan Schisler]

It is the balance that is important and, of course, ideally want optimal levels of both, but anecdotally I think high CAPE days have more potential for a mesoscale accident.

I remember seeing a great chart somewhere that plotted historical events that showed the correlation of CAPE/shear, but I can’t remember where. Perhaps one of the links earlier in this thread were to that chart, but both links are no longer active.

Is this the image that you are referencing? Its from Jon Davies' website. I corrected my previous post because I found an updated plot that he created this year.

 

[Stan Rose]

I'd take high instability alone over high shear every time. Shear can't create instability, but instability can create shear, in the form of SRH. I remember chasing a storm about 20 years ago in southern Nebraska where the winds were below 20 kts from 500mb down, but CAPE was huge. And it produced a bunch of funnels and golf ball hail. Regardless of tornado potential, the high CAPE storms are almost always more visually impressive than the high shear ones. your mileage may vary.

 

[JamesCaruso]

@Ethan Schisler Yes I think that's the one, thanks!

 

[rsheeler]

The Jarrell TX 1997 event seems to be one of those strange cases of a setup that had huge CAPE instability with relatively low shear as mentioned on this forum before. Of course the stalled front and gravity wave movement definitely played into that. (btw, is the gravity wave the same as an outflow boundary in the nomenclature?). All in all, it seems like there are any number of possible setup scenarios that have led to tornado formation over the years.

 

[Jeff Duda]

(btw, is the gravity wave the same as an outflow boundary in the nomenclature?).

Apologies for going off-topic, but no, outflow boundaries and gravity waves are very much separate entities (although large thunderstorm outflows can generate gravity waves, but the causal relationship only goes in that direction).

 

[rsheeler]

Apologies for going off-topic, but no, outflow boundaries and gravity waves are very much separate entities (although large thunderstorm outflows can generate gravity waves, but the causal relationship only goes in that direction).

thanks for that. I had often wondered about that.

 

[Dean Baron]

Apologies for going off-topic, but no, outflow boundaries and gravity waves are very much separate entities (although large thunderstorm outflows can generate gravity waves, but the causal relationship only goes in that direction).

Sorry to continue going off topic with this but to build off your point gravity waves can also generate additional lift. Some of the stronger tornadoes were the result of a storm firing from a gravity wave in an otherwise capped environment. I believe the Joplin and Parkersburg, IA EF5's are a couple of examples of storms firing from a gravity wave, though I could be wrong about that.