A diagnostic study of persistent intense jet stream events in the Pacific has been carried out with a 15 winter NMC dataset to assess the relevance of the weakly nonlinear model recently proposed by Haines and Malanotte-Rizzoli. Composited data from 14 episodes of persistent intensification anomalies in the central and eastern Pacific have been analyzed with scatter diagrams of potential vorticity q plotted against geopotential Φ on the 300-mb surface. The slope of the functional relationship gives a measure of the wavelength independent component of the refractive index (n 2 = −Λ0 = −dq 0/dψ 0). The theoretical model suggests that if dq 0/dψ 0 is more negative on the northern and southern flanks, a local intense region within the jet stream may be abnormally persistent. The composited dataset shows that this condition is satisfied during the postonset period as defined by Dole. In contrast, the climatology and the mean flow before onset does not show much variation in Λ0 across the jet. Results are encouraging, but higher-resolution data is needed to draw firm conclusions.