Daily reports on weather conditions regarding major italian cities are daily published on the “Protezione Civile” web site (the website is in Italian). Local city administrations receive updated weather informations and are in charge to spread possible weather alerts.
You can find this information on the official Venice Municipality web site or by calling the dedicated number 041-2411996. One can also call the Official Sea Water Weather Condition information number 041-2748787.
More info: High water in Venice: frequently asked questions (FAQ)