Analysis of potential vorticity (PV) distribution and of its evolution is used for studying the dynamic forcing of convective processes responsible for the flash flood in Gdańsk on 9th July 2001.

The dynamic forcing has a form of a mesoscale ascent and is usually related to the presence of PV anomalies located at the tropopause. Their presence is detectable in satellite images in WV emission channel. Such anomalies were present in the vicinity of Gdańsk during that day, but their evolution indicates an additional PV anomaly influencing their motion. Further analysis suggests that it has a form of diabatically-produced tropospheric PV anomaly related to the prior distant convective activity. The anomaly's presence was not detected by operational data analysis, but its dynamic effect was to enhance the dynamic ascent over Gdańsk by about 50%.