Nonparametric discriminant analysis for an antisymmetric average of the vertical vorticity, , in filter TD4, in the same format as Figure 1. Left: 24.5 hr before the emergence time, the density estimate for NE regions has a peak and a mean value that are slightly positive, whereas the peak and mean value for the PE distribution are at negative values. Right: given the uncertainties in the skill score, the performance of this variable does not clearly evolve with time.