Águas verdadeiras... ('true waters...')

I drove over the serra this morning to see a large black cloud hanging over the plains to the north. My first thought was a change in the weather but I later leant that this came from the fires that have been burning all night in the Beiras.  We desperately need some rain to dampen things down and the weather is forecast to change this weekend. Saturday is Dia de São Mateus (St Mathew’s Day) and there is a saying round here that goes águas verdadeiras, São Mateus as primeiras ('the first true rains fall on St Mathew’s day'). It looks as though there might be some truth in this.