Originally posted by Mike Johnston+--><div class='quotetop'>QUOTE(Mike Johnston)</div>
<!--QuoteBegin-Jeff Snyder
The upper-level dynamics were strong, but there was very little in the way of surface-based instability.
Jeff, something I didn't quite understand last night in the SPC's forecasting, though. It seems like, in prior situations where convection has already initiated at night, and the storm motions will take the storms into a very, very strong LLJ, it has been almost automatic that the forecasters note this, with the realization that strengthening will occur regardless of the subsiding instability. Once initiated, doesn't CIN become less of a factor? I just remember several situations last spring where a strengthening nocturnal LLJ seemed to be such a key factor in the SPC's discussions, and - frankly - just sat there last night staring at what the RUC was saying, staring at the incredible radar echoes and wondering, is anyone else seeing this? As usual, your feedback is appreciated from one still learning.[/b]
Mike,
It's easier for a supercell to sustain itself in a given environment with moderate or high CINH than it is to initiate. We've all seen this in action -- many storms develop in the late afternoon or evening, but only a few sustain themselves into the night. Once a storm is going, there is increased convergence at the base of the updraft, so it usually takes stronger environmental CINH to 'cut off' the updraft given the stronger convergence. Now, the rotation of the updraft (which defines a mesocyclone in a supercell) induces vertical pressure perturbation gradients, which further enhances the updraft velocity and the convergence at the base of the updraft.
In regards to the LLJ, the nocturnal strengthening of the LLJ is often a result of boundary layer decoupling (which results in a transition from subgeostrophic flow common in the low-levels to supergeostrophic flow). Now, in times of rapid surface cyclogenesis, the strengthening of the LLJ may not be enhanced as much by boundary layer decoupling as by the increase in the pressure gradient as the cyclone develops.
So, IF a supercell can persist into the night, as remain surface-based, the increased shear resulting from the stregntehnign of the LLJ can create a very favorable environment for tornadogenesis. However, surface-based instability wasn't all that strong during the daylight hours, much less after dark. As I said before, the strong gradient wind may have prevented boundary-layer decoupling to a degree, so the surface-based instability probably didn't decrease as much in this case as is typically experienced near and afters sunset. These high-shear cases are relatively common during the cool season (late fall through early spring), and very few yield a strong tornadic supercell. I haven't looked at a whole lot of data from last night, but I assume that the supercell ingested a rather localized area of strong instability that allowed it to either strengthen more for a while, or allowed it to become entirely surface-based.
EDIT: Just saw Nick's answer above, which explains this well. Also, I remember several cases this past spring which were high-shear, low instability in the warm sector. Unfortunately (or fortunately), we saw a very different result in these cases this spring, with the only tornadoes in the cool sector beneath the upper-level/mid-level low.
