By Kacper Pempel WROCLAW, Poland (Reuters) -Volunteers and emergency personnel worked through the night to fortify the Polish city of Wroclaw against approaching flood waters, while Hungary opened a dam as the prime minister warned a "crucial period" was approaching.

A massive flood wave moving across Central Europe is threatening new areas and raising concerns among residents and officials.

The storm that hit central Europe — which unleashed several months’ worth of rain in just a few days — was spawned by a rare combination of meteorological factors.