I guess I’m just a slow learner when it comes to technical stuff that involves linear thinking. Sooner or later, though, if I stick with something long enough, I usually emerge more knowledgeable for having done so. Nowhere has this been more true than in storm chasing, an activity which obviously depends heavily on figuring out if and where there will be decent storms to chase.

Seems like I’m constantly being confronted with some new aspect of the atmosphere that I haven’t factored into my forecasting, or that I haven’t factored in as effectively as I needed to. The upside of that, though, is that I wouldn’t even be aware of what I don’t know if I hadn’t learned enough to at least recognize my areas of ignorance. If my forecasts aren’t as expert as those of a trained meteorologist, they’re nonetheless a seven-league bound beyond when I was just beginning to grapple with all those arcane terms and acronyms of convective weather such as SBCAPE, CIN, 0-6km bulk shear, LIs, helicity, and lapse rates, and when the only thing I could do with a skew-T or a hodograph was shake my head in bewilderment.

This has been the year of discovering the 700 mb/12C limit. By “discovering,” I mean through empirical experience, and by “empirical experience,” I mean cap busts. Of course, I’ve endured plenty of cap busts in my development as a storm chaser; I just didn’t understand exactly what was going on, or why the high risk area I was sitting in was producing nothing more than smug blue skies rather than carnivorous supercells.

One memorable day in Iowa drove home the lesson perfectly. MLCIN was supposed to erode by later in the afternoon, and it got to a point where it was eroding, at least according the RUC. With SBCAPE at some ridiculous figure like 7,000 j/kg, I figured that at some point a convective tower would punch through the cap and go absolutely gonzo. Instead, the clouds kept firing up into the nicely sheared environment and then dying, firing and dying, firing and dying. The reason? A 700 mb temperature of around 14C, possibly considerably higher. Lesson learned: it doesn’t much matter what the models have to say about the CIN eroding when you’ve got mid-level temperatures like that.

I experienced another cap bust yesterday, though I can’t feel too bad about it since I had no compelling reason to head out in the first place, the conditions were so marginal. It was interesting to notice that in this situation, the circumstances were reversed: RUC showing my area under very breakable 700 mb temps of around 10C, but with MLCIN creating some concern. However, the CIN appeared to be eroding, and when an SPC mesoscale discussion spotlighted the area I was in, I started feeling happy about having made the drive down to west-central Illinois.

But the CIN started building back in, and by 00Z I found myself socked in under values around -300 j/kg. Not much a parcel of air can do with that, I guess, no matter how big the CAPE is. I turned around and headed home.

A paper by Bunker, Wetencamp, and Schild of the NWS in Rapid City, South Dakota, explores the ins and outs of the 700 mb/12C limit and concludes that it has a limited, conditional application. However, as my buddy Mike Kovalchick pointed out to me, the paper also reveals that only 5 percent of tornadoes within the study period formed when H7 temps exceeded 12C, and virtually no violent (EF4 and EF5) tornadoes occurred above that threshold.

So for practical use in storm chasing, the 12C limit appears to be a very useful rule of thumb. The issue for me then becomes a matter of refining my ability to know when cold air advection will lower the 700 mb temps. But that’s a subject for another blog. I’m tired of thinking. It’s time to go meet my buddy Dewey down in Plainwell and grab a brew at Arie’s. Ciao!