In the case where the first instant or sorcery spell you cast in a turn does cost some {U} but you control more lands with flood counters on them than the number of {U} in that spell’s mana cost, all of the {U} in the cost are reduced first before moving on to the generic mana. For example, if you control four lands with flood counters on them and you cast Season of Weaving (a sorcery with mana cost {4}{U}{U}) as the first instant or sorcery spell of your turn, its cost will be reduced by {2}{U}{U}, meaning its total cost will be {2}.