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How deep is too deep for surface lows and significant severe wx/tornadoes ??

With yesterday's event still fresh in the mind, I was wondering if there had ever been any studies done in which depth of surface lows was analyzed for corresponding severe wx that occurred. In most of the better events I have witnessed, the surface lows were in the 990-996mb range, with the exception of the 4/8/99 IA event. Past experiences with sub 990mb surface lows have often lead to frustratingly low to zero tornado counts. Anybody else of that same opinion? I know that other parameters/factors work into the equation, but give me a solid 991mb surface low and I'll be happy!!
I'm not aware of any studies relating sfc low depth to severe weather, but I've had two "tornado days" with a sub-990mb surface low: 04/06/06 and 09/21/06. Both events had a surface low of 986mb.

As for yesterday, the sfc dewpoints rapidly mixed out into the 30s from the southwest.. IMO this left a very narrow juxtoposition of sufficiently low LCL heights and marginal 3km CAPE in the vicinity of the occluded front. The behavior of the storms seemed to support this theory. Not to mention, 1km SRH wasn't impressive due to the highly occluded nature of the low (though this naturally isn't a requirement in environments with high ambient vorticity), and instability was way overforecast due to the moisture mixing out. My 2 cents.

I agree with Chris that moisture can make or break the very intense systems/deep surface lows that are more common in the cool season. For instance, I can only assume the super outbreak (which had a sfc low in the low 980s I believe) was preceded by many days of return flow to get widespread rich BL moisture in place... possibly induced by a lead shortwave or two ejecting out of the mean trough position. This is an extreme example, of course.

Maybe it would be better to look at the deep surface low cases where BL moisture is sufficient but tornadogenesis fails?
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I guess this question is in relation to the CONUS, but we have had some fairly significant (well, by UK standards) tornadoes with lows of sub 970 hPa, and sometimes lower. The intensity of the low rather than the min pressure will determine the kinetics around the low - too much shear compared with the instability can ruin storm chances.
Another thought to add... a lot of these stronger systems occur in the spring and fall when moisture is limited (therefore CAPE is also limited). With this in mind, a strong sfc low usually means strong wind shear for a vertically moving parcel to rise through. If the CAPE is too low and the shear too strong, a parcel can be sheared into oblivion. I think the best tornadic days happen when there is a balance between the vertical wind shear and convective energy. If shear is too strong, what I just mentioned will happen. If CAPE is too high and shear too low then there will not be enough tilting on the updraft to induce rotation. I should also mention that when I refer to shear in these regards, its speed shear with height. From what I can understand, directional shear should be relatively unchanged with regards to time of year.