Potential Vorticity (PV)
It is now well known that the combination of low-level warm advection and an upper level PV anomaly can result in strong cyclogenesis, with the magnitude of the effect depending on the relative positions of the anomalies. If the upper-level PV anomaly stays near the rear of the low-level warm anomaly (usually with a large positive low-level PV value), the wind field induced by it can result in mutual intensification. Low-level PV is often used as a ССsurrogateТТ for the low-level warm anomaly (Lackmann et al. 1996). It is helpful to look at the relationship between the upper- and lower-level PV anomalies in order to study Alex's re-intensification.
The upper anomaly elongates and intensifies, and a family of low level PV anomalies, aligned southwest to northeast ahead of the trough, suggests convective instability in the region