Effective SRH (Storm Relative Helicity) is based on threshold values of lifted parcel CAPE (100 J kg^-1) and CIN (-250 J kg^-1). These parcel constraints are meant to confine the SRH layer calculation to the part of a sounding where lifted parcels are buoyant, but not strongly capped. For example, a supercell forms or moves over an area where the most unstable parcels are located a couple of thousand feet above the ground, and stable air is located at ground level. The question then becomes "how much of the cool air can the supercell ingest and still survive?" Our estimate is to start with the surface parcel level, and work upward until a lifted parcel's CAPE value increases to 100 Jkg^-1 or more, with an associated CIN greater than -250 Jkg^-1. From the level meeting the constraints (the "effective surface"), we continue to look upward in the sounding until a lifted parcel has a CAPE less than 100 Jkg^-1 OR a CIN less than -250 J kg^-1. Of the three SRH calculations displayed on the SPC mesoanalysis page, effective SRH discriminates the best between significant tornadic and nontornadic supercells.
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