Evaluating the CAP for chasing ?

Feb 5, 2004
Celina, Ohio
With much talk these past few chase weeks about the "gremlin" CAP - was wondering what your method is for evaluating the cap in chase planning stratagies ?
Jon Davies has a great explanation of the cap and estimating its strength against potential for tornadoes:


Acceptable temperatures vary according to the time of year - in general, anything over the temperatures listed on his chart here become questionable for supercell development. But as Jon says:

the guideline 700 mb temperature values shouldn't be applied as \"thresholds\"... common sense and other information are required.

A lot of variables come into play when determining whether the cap is a true threat or not, one of which being the way the models have been estimating the cap strength - as well as other considerations he explains.
Also, for a quick and dirty snapshot of forecast cap strength, Earl's situational severe awareness maps includes the Lid Strength Index:


This index is based on a calculation of the capping inversion. Although I don't have any sources as far as the meaning of absolute values of the lid strength index, at least the map gives you some idea of where the cap may be expected stronger relative to other areas when you are making your targeting decisions.

As far as considering the cap in chase strategy, my own approach is to weigh this factor relative to the overall setup. If my potential chase target is in proximity to a well-defined surface convergence feature (which provides lift), I tend to worry less about the cap. Signs of well-defined surface convergence include sharp wind shift lines, tight surface temperature gradients, and moisture convergence. On the other hand, if I am considering an area well out in the warm sector and/or if the surface convergence line is very ill-defined, then I pay closer attention to the cap strength. If the cap is likely to make convective initiation a 50/50 proposition or less, then I would tend to make less of an investment in time and distance to chase, but this decision gets back to personal preference and philosophy. I'm sure others may take a different approach to considering the cap in their chase strategies.