Why "few" is used instead of "little"? Because "heat wave" and "flood" are countable nouns?
Yes, you are correct. "Little" is used when dealing with non-countable nouns, and is usually used with the singular form to show that something exists only in a small amount or to a slight degree. "Few" is used when dealing with countable nouns, and is used with the plural form to show not many persons or things. Since "heatwave" and "flood" are countable nouns "few" is the correct phrase to use with them. Ex: I’ve got little money in my wallet. (Non-countable) Ex: Tara has few friends in New York. (Countable)